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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href=https://json.nlohmann.me/features/arbitrary_types/ rel=canonical><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.4.2, mkdocs-material-8.5.10"><title>Arbitrary Type Conversions - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.975780f9.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.2505c338.min.css><link rel=preconnect href=https://fonts.gstatic.com crossorigin><link rel=stylesheet href="https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,700,700i%7CJetBrains+Mono:400,400i,700,700i&display=fallback"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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<span class=w> </span><span class=c1>// a simple struct to model a person</span>
<span class=w> </span><span class=k>struct</span><span class=w> </span><span class=nc>person</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>name</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>address</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=kt>int</span><span class=w> </span><span class=n>age</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>};</span><span class=w></span>
<span class=p>}</span><span class=w> </span><span class=c1>// namespace ns</span>
<span class=n>ns</span><span class=o>::</span><span class=n>person</span><span class=w> </span><span class=n>p</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=s>&quot;Ned Flanders&quot;</span><span class=p>,</span><span class=w> </span><span class=s>&quot;744 Evergreen Terrace&quot;</span><span class=p>,</span><span class=w> </span><span class=mi>60</span><span class=p>};</span><span class=w></span>
<span class=c1>// convert to JSON: copy each value into the JSON object</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;name&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>p</span><span class=p>.</span><span class=n>name</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;address&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>p</span><span class=p>.</span><span class=n>address</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;age&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>p</span><span class=p>.</span><span class=n>age</span><span class=p>;</span><span class=w></span>
<span class=c1>// ...</span>
<span class=c1>// convert from JSON: copy each value from the JSON object</span>
<span class=n>ns</span><span class=o>::</span><span class=n>person</span><span class=w> </span><span class=n>p</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>&quot;name&quot;</span><span class=p>].</span><span class=n>get</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>&gt;</span><span class=p>(),</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>&quot;address&quot;</span><span class=p>].</span><span class=n>get</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>&gt;</span><span class=p>(),</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>&quot;age&quot;</span><span class=p>].</span><span class=n>get</span><span class=o>&lt;</span><span class=kt>int</span><span class=o>&gt;</span><span class=p>()</span><span class=w></span>
<span class=p>};</span><span class=w></span>
</code></pre></div> <p>It works, but that's quite a lot of boilerplate... Fortunately, there's a better way:</p> <div class=highlight><pre><span></span><code><span class=c1>// create a person</span>
<span class=n>ns</span><span class=o>::</span><span class=n>person</span><span class=w> </span><span class=n>p</span><span class=w> </span><span class=p>{</span><span class=s>&quot;Ned Flanders&quot;</span><span class=p>,</span><span class=w> </span><span class=s>&quot;744 Evergreen Terrace&quot;</span><span class=p>,</span><span class=w> </span><span class=mi>60</span><span class=p>};</span><span class=w></span>
<span class=c1>// conversion: person -&gt; json</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>p</span><span class=p>;</span><span class=w></span>
<span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=c1>// {&quot;address&quot;:&quot;744 Evergreen Terrace&quot;,&quot;age&quot;:60,&quot;name&quot;:&quot;Ned Flanders&quot;}</span>
<span class=c1>// conversion: json -&gt; person</span>
<span class=k>auto</span><span class=w> </span><span class=n>p2</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o>&lt;</span><span class=n>ns</span><span class=o>::</span><span class=n>person</span><span class=o>&gt;</span><span class=p>();</span><span class=w></span>
<span class=c1>// that&#39;s it</span>
<span class=n>assert</span><span class=p>(</span><span class=n>p</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>p2</span><span class=p>);</span><span class=w></span>
</code></pre></div> <h2 id=basic-usage>Basic usage<a class=headerlink href=#basic-usage title="Permanent link">&para;</a></h2> <p>To make this work with one of your types, you only need to provide two functions:</p> <div class=highlight><pre><span></span><code><span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=k>namespace</span><span class=w> </span><span class=nn>ns</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=kt>void</span><span class=w> </span><span class=nf>to_json</span><span class=p>(</span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>person</span><span class=o>&amp;</span><span class=w> </span><span class=n>p</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=p>{</span><span class=w> </span><span class=p>{</span><span class=s>&quot;name&quot;</span><span class=p>,</span><span class=w> </span><span class=n>p</span><span class=p>.</span><span class=n>name</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>&quot;address&quot;</span><span class=p>,</span><span class=w> </span><span class=n>p</span><span class=p>.</span><span class=n>address</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>&quot;age&quot;</span><span class=p>,</span><span class=w> </span><span class=n>p</span><span class=p>.</span><span class=n>age</span><span class=p>}</span><span class=w> </span><span class=p>};</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=kt>void</span><span class=w> </span><span class=nf>from_json</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=n>person</span><span class=o>&amp;</span><span class=w> </span><span class=n>p</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>&quot;name&quot;</span><span class=p>).</span><span class=n>get_to</span><span class=p>(</span><span class=n>p</span><span class=p>.</span><span class=n>name</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>&quot;address&quot;</span><span class=p>).</span><span class=n>get_to</span><span class=p>(</span><span class=n>p</span><span class=p>.</span><span class=n>address</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>&quot;age&quot;</span><span class=p>).</span><span class=n>get_to</span><span class=p>(</span><span class=n>p</span><span class=p>.</span><span class=n>age</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>}</span><span class=w> </span><span class=c1>// namespace ns</span>
</code></pre></div> <p>That's all! When calling the <code>json</code> constructor with your type, your custom <code>to_json</code> method will be automatically called. Likewise, when calling <code>get&lt;your_type&gt;()</code> or <code>get_to(your_type&amp;)</code>, the <code>from_json</code> method will be called.</p> <p>Some important things:</p> <ul> <li>Those methods <strong>MUST</strong> be in your type's namespace (which can be the global namespace), or the library will not be able to locate them (in this example, they are in namespace <code>ns</code>, where <code>person</code> is defined).</li> <li>Those methods <strong>MUST</strong> be available (e.g., proper headers must be included) everywhere you use these conversions. Look at <a href=https://github.com/nlohmann/json/issues/1108>issue 1108</a> for errors that may occur otherwise.</li> <li>When using <code>get&lt;your_type&gt;()</code>, <code>your_type</code> <strong>MUST</strong> be <a href=https://en.cppreference.com/w/cpp/named_req/DefaultConstructible>DefaultConstructible</a>. (There is a way to bypass this requirement described later.)</li> <li>In function <code>from_json</code>, use function <a href=../../api/basic_json/at/ ><code>at()</code></a> to access the object values rather than <code>operator[]</code>. In case a key does not exist, <code>at</code> throws an exception that you can handle, whereas <code>operator[]</code> exhibits undefined behavior.</li> <li>You do not need to add serializers or deserializers for STL types like <code>std::vector</code>: the library already implements these.</li> </ul> <h2 id=simplify-your-life-with-macros>Simplify your life with macros<a class=headerlink href=#simplify-your-life-with-macros title="Permanent link">&para;</a></h2> <p>If you just want to serialize/deserialize some structs, the <code>to_json</code>/<code>from_json</code> functions can be a lot of boilerplate.</p> <p>There are four macros to make your life easier as long as you (1) want to use a JSON object as serialization and (2) want to use the member variable names as object keys in that object:</p> <ul> <li><a href=../../api/macros/nlohmann_define_type_non_intrusive/ ><code>NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(name, member1, member2, ...)</code></a> is to be defined inside the namespace of the class/struct to create code for. It will throw an exception in <code>from_json()</code> due to a missing value in the JSON object.</li> <li><a href=../../api/macros/nlohmann_define_type_non_intrusive/ ><code>NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(name, member1, member2, ...)</code></a> is to be defined inside the namespace of the class/struct to create code for. It will not throw an exception in <code>from_json()</code> due to a missing value in the JSON object, but fills in values from object which is default-constructed by the type.</li> <li><a href=../../api/macros/nlohmann_define_type_intrusive/ ><code>NLOHMANN_DEFINE_TYPE_INTRUSIVE(name, member1, member2, ...)</code></a> is to be defined inside the class/struct to create code for. This macro can also access private members. It will throw an exception in <code>from_json()</code> due to a missing value in the JSON object.</li> <li><a href=../../api/macros/nlohmann_define_type_intrusive/ ><code>NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(name, member1, member2, ...)</code></a> is to be defined inside the class/struct to create code for. This macro can also access private members. It will not throw an exception in <code>from_json()</code> due to a missing value in the JSON object, but fills in values from object which is default-constructed by the type.</li> </ul> <p>In all macros, the first parameter is the name of the class/struct, and all remaining parameters name the members. You can read more docs about them starting from <a href=../macros/#nlohmann_define_type_intrusivetype-member>here</a>.</p> <div class="admonition info"> <p class=admonition-title>Implementation limits</p> <ul> <li>The current macro implementations are limited to at most 64 member variables. If you want to serialize/deserialize types with more than 64 member variables, you need to define the <code>to_json</code>/<code>from_json</code> functions manually.</li> <li>The macros only work for the <a href=../../api/json/ ><code>nlohmann::json</code></a> type; other specializations such as <a href=../../api/ordered_json/ ><code>nlohmann::ordered_json</code></a> are currently unsupported.</li> </ul> </div> <details class=example> <summary>Example</summary> <p>The <code>to_json</code>/<code>from_json</code> functions for the <code>person</code> struct above can be created with:</p> <div class=highlight><pre><span></span><code><span class=k>namespace</span><span class=w> </span><span class=nn>ns</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE</span><span class=p>(</span><span class=n>person</span><span class=p>,</span><span class=w> </span><span class=n>name</span><span class=p>,</span><span class=w> </span><span class=n>address</span><span class=p>,</span><span class=w> </span><span class=n>age</span><span class=p>)</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Here is an example with private members, where <code>NLOHMANN_DEFINE_TYPE_INTRUSIVE</code> is needed:</p> <div class=highlight><pre><span></span><code><span class=k>namespace</span><span class=w> </span><span class=nn>ns</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>class</span><span class=w> </span><span class=nc>address</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>private</span><span class=o>:</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>street</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=kt>int</span><span class=w> </span><span class=n>housenumber</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=kt>int</span><span class=w> </span><span class=n>postcode</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=k>public</span><span class=o>:</span><span class=w></span>
<span class=w> </span><span class=n>NLOHMANN_DEFINE_TYPE_INTRUSIVE</span><span class=p>(</span><span class=n>address</span><span class=p>,</span><span class=w> </span><span class=n>street</span><span class=p>,</span><span class=w> </span><span class=n>housenumber</span><span class=p>,</span><span class=w> </span><span class=n>postcode</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>};</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <h2 id=how-do-i-convert-third-party-types>How do I convert third-party types?<a class=headerlink href=#how-do-i-convert-third-party-types title="Permanent link">&para;</a></h2> <p>This requires a bit more advanced technique. But first, let's see how this conversion mechanism works:</p> <p>The library uses <strong>JSON Serializers</strong> to convert types to json. The default serializer for <code>nlohmann::json</code> is <code>nlohmann::adl_serializer</code> (ADL means <a href=https://en.cppreference.com/w/cpp/language/adl>Argument-Dependent Lookup</a>).</p> <p>It is implemented like this (simplified):</p> <div class=highlight><pre><span></span><code><span class=k>template</span><span class=w> </span><span class=o>&lt;</span><span class=k>typename</span><span class=w> </span><span class=nc>T</span><span class=o>&gt;</span><span class=w></span>
<span class=k>struct</span><span class=w> </span><span class=nc>adl_serializer</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>static</span><span class=w> </span><span class=kt>void</span><span class=w> </span><span class=nf>to_json</span><span class=p>(</span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>T</span><span class=o>&amp;</span><span class=w> </span><span class=n>value</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// calls the &quot;to_json&quot; method in T&#39;s namespace</span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=k>static</span><span class=w> </span><span class=kt>void</span><span class=w> </span><span class=nf>from_json</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=n>T</span><span class=o>&amp;</span><span class=w> </span><span class=n>value</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// same thing, but with the &quot;from_json&quot; method</span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>};</span><span class=w></span>
</code></pre></div> <p>This serializer works fine when you have control over the type's namespace. However, what about <code>boost::optional</code> or <code>std::filesystem::path</code> (C++17)? Hijacking the <code>boost</code> namespace is pretty bad, and it's illegal to add something other than template specializations to <code>std</code>...</p> <p>To solve this, you need to add a specialization of <code>adl_serializer</code> to the <code>nlohmann</code> namespace, here's an example:</p> <div class=highlight><pre><span></span><code><span class=c1>// partial specialization (full specialization works too)</span>
<span class=n>NLOHMANN_JSON_NAMESPACE_BEGIN</span><span class=w></span>
<span class=k>template</span><span class=w> </span><span class=o>&lt;</span><span class=k>typename</span><span class=w> </span><span class=nc>T</span><span class=o>&gt;</span><span class=w></span>
<span class=k>struct</span><span class=w> </span><span class=nc>adl_serializer</span><span class=o>&lt;</span><span class=n>boost</span><span class=o>::</span><span class=n>optional</span><span class=o>&lt;</span><span class=n>T</span><span class=o>&gt;&gt;</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>static</span><span class=w> </span><span class=kt>void</span><span class=w> </span><span class=nf>to_json</span><span class=p>(</span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>boost</span><span class=o>::</span><span class=n>optional</span><span class=o>&lt;</span><span class=n>T</span><span class=o>&gt;&amp;</span><span class=w> </span><span class=n>opt</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>opt</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>boost</span><span class=o>::</span><span class=n>none</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=k>nullptr</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w> </span><span class=k>else</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=o>*</span><span class=n>opt</span><span class=p>;</span><span class=w> </span><span class=c1>// this will call adl_serializer&lt;T&gt;::to_json which will</span>
<span class=w> </span><span class=c1>// find the free function to_json in T&#39;s namespace!</span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=k>static</span><span class=w> </span><span class=kt>void</span><span class=w> </span><span class=nf>from_json</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=n>boost</span><span class=o>::</span><span class=n>optional</span><span class=o>&lt;</span><span class=n>T</span><span class=o>&gt;&amp;</span><span class=w> </span><span class=n>opt</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>j</span><span class=p>.</span><span class=n>is_null</span><span class=p>())</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>opt</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>boost</span><span class=o>::</span><span class=n>none</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w> </span><span class=k>else</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>opt</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o>&lt;</span><span class=n>T</span><span class=o>&gt;</span><span class=p>();</span><span class=w> </span><span class=c1>// same as above, but with</span>
<span class=w> </span><span class=c1>// adl_serializer&lt;T&gt;::from_json</span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>};</span><span class=w></span>
<span class=n>NLOHMANN_JSON_NAMESPACE_END</span><span class=w></span>
</code></pre></div> <div class="admonition note"> <p class=admonition-title>ABI compatibility</p> <p>Use <a href=../../api/macros/nlohmann_json_namespace_begin/ ><code>NLOHMANN_JSON_NAMESPACE_BEGIN</code></a> and <code>NLOHMANN_JSON_NAMESPACE_END</code> instead of <code class=highlight><span class=k>namespace</span><span class=w> </span><span class=nn>nlohmann</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=p>}</span><span class=w></span></code> in code which may be linked with different versions of this library.</p> </div> <h2 id=how-can-i-use-get-for-non-default-constructiblenon-copyable-types>How can I use <code>get()</code> for non-default constructible/non-copyable types?<a class=headerlink href=#how-can-i-use-get-for-non-default-constructiblenon-copyable-types title="Permanent link">&para;</a></h2> <p>There is a way, if your type is <a href=https://en.cppreference.com/w/cpp/named_req/MoveConstructible>MoveConstructible</a>. You will need to specialize the <code>adl_serializer</code> as well, but with a special <code>from_json</code> overload:</p> <div class=highlight><pre><span></span><code><span class=k>struct</span><span class=w> </span><span class=nc>move_only_type</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>move_only_type</span><span class=p>()</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=k>delete</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>move_only_type</span><span class=p>(</span><span class=kt>int</span><span class=w> </span><span class=n>ii</span><span class=p>)</span><span class=o>:</span><span class=w> </span><span class=n>i</span><span class=p>(</span><span class=n>ii</span><span class=p>)</span><span class=w> </span><span class=p>{}</span><span class=w></span>
<span class=w> </span><span class=n>move_only_type</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>move_only_type</span><span class=o>&amp;</span><span class=p>)</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=k>delete</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>move_only_type</span><span class=p>(</span><span class=n>move_only_type</span><span class=o>&amp;&amp;</span><span class=p>)</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=k>default</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=kt>int</span><span class=w> </span><span class=n>i</span><span class=p>;</span><span class=w></span>
<span class=p>};</span><span class=w></span>
<span class=k>namespace</span><span class=w> </span><span class=nn>nlohmann</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>template</span><span class=w> </span><span class=o>&lt;&gt;</span><span class=w></span>
<span class=w> </span><span class=k>struct</span><span class=w> </span><span class=nc>adl_serializer</span><span class=o>&lt;</span><span class=n>move_only_type</span><span class=o>&gt;</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// note: the return type is no longer &#39;void&#39;, and the method only takes</span>
<span class=w> </span><span class=c1>// one argument</span>
<span class=w> </span><span class=k>static</span><span class=w> </span><span class=n>move_only_type</span><span class=w> </span><span class=nf>from_json</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=p>{</span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o>&lt;</span><span class=kt>int</span><span class=o>&gt;</span><span class=p>()};</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=c1>// Here&#39;s the catch! You must provide a to_json method! Otherwise, you</span>
<span class=w> </span><span class=c1>// will not be able to convert move_only_type to json, since you fully</span>
<span class=w> </span><span class=c1>// specialized adl_serializer on that type</span>
<span class=w> </span><span class=k>static</span><span class=w> </span><span class=kt>void</span><span class=w> </span><span class=nf>to_json</span><span class=p>(</span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=n>move_only_type</span><span class=w> </span><span class=n>t</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>t</span><span class=p>.</span><span class=n>i</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=p>};</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <h2 id=can-i-write-my-own-serializer-advanced-use>Can I write my own serializer? (Advanced use)<a class=headerlink href=#can-i-write-my-own-serializer-advanced-use title="Permanent link">&para;</a></h2> <p>Yes. You might want to take a look at <a href=https://github.com/nlohmann/json/blob/develop/tests/src/unit-udt.cpp><code>unit-udt.cpp</code></a> in the test suite, to see a few examples.</p> <p>If you write your own serializer, you'll need to do a few things:</p> <ul> <li>use a different <code>basic_json</code> alias than <code>nlohmann::json</code> (the last template parameter of <code>basic_json</code> is the <code>JSONSerializer</code>)</li> <li>use your <code>basic_json</code> alias (or a template parameter) in all your <code>to_json</code>/<code>from_json</code> methods</li> <li>use <code>nlohmann::to_json</code> and <code>nlohmann::from_json</code> when you need ADL</li> </ul> <p>Here is an example, without simplifications, that only accepts types with a size &lt;= 32, and uses ADL.</p> <div class=highlight><pre><span></span><code><span class=c1>// You should use void as a second template argument</span>
<span class=c1>// if you don&#39;t need compile-time checks on T</span>
<span class=k>template</span><span class=o>&lt;</span><span class=k>typename</span><span class=w> </span><span class=nc>T</span><span class=p>,</span><span class=w> </span><span class=k>typename</span><span class=w> </span><span class=nc>SFINAE</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=k>typename</span><span class=w> </span><span class=nc>std</span><span class=o>::</span><span class=n>enable_if</span><span class=o>&lt;</span><span class=k>sizeof</span><span class=p>(</span><span class=n>T</span><span class=p>)</span><span class=w> </span><span class=o>&lt;=</span><span class=w> </span><span class=mi>32</span><span class=o>&gt;::</span><span class=n>type</span><span class=o>&gt;</span><span class=w></span>
<span class=k>struct</span><span class=w> </span><span class=nc>less_than_32_serializer</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>template</span><span class=w> </span><span class=o>&lt;</span><span class=k>typename</span><span class=w> </span><span class=nc>BasicJsonType</span><span class=o>&gt;</span><span class=w></span>
<span class=w> </span><span class=k>static</span><span class=w> </span><span class=kt>void</span><span class=w> </span><span class=n>to_json</span><span class=p>(</span><span class=n>BasicJsonType</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=n>T</span><span class=w> </span><span class=n>value</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// we want to use ADL, and call the correct to_json overload</span>
<span class=w> </span><span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>to_json</span><span class=p>;</span><span class=w> </span><span class=c1>// this method is called by adl_serializer,</span>
<span class=w> </span><span class=c1>// this is where the magic happens</span>
<span class=w> </span><span class=n>to_json</span><span class=p>(</span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=n>value</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=k>template</span><span class=w> </span><span class=o>&lt;</span><span class=k>typename</span><span class=w> </span><span class=nc>BasicJsonType</span><span class=o>&gt;</span><span class=w></span>
<span class=w> </span><span class=k>static</span><span class=w> </span><span class=kt>void</span><span class=w> </span><span class=n>from_json</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>BasicJsonType</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=n>T</span><span class=o>&amp;</span><span class=w> </span><span class=n>value</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// same thing here</span>
<span class=w> </span><span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>from_json</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>from_json</span><span class=p>(</span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=n>value</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>};</span><span class=w></span>
</code></pre></div> <p>Be <strong>very</strong> careful when reimplementing your serializer, you can stack overflow if you don't pay attention:</p> <div class=highlight><pre><span></span><code><span class=k>template</span><span class=w> </span><span class=o>&lt;</span><span class=k>typename</span><span class=w> </span><span class=nc>T</span><span class=p>,</span><span class=w> </span><span class=kt>void</span><span class=o>&gt;</span><span class=w></span>
<span class=k>struct</span><span class=w> </span><span class=nc>bad_serializer</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>template</span><span class=w> </span><span class=o>&lt;</span><span class=k>typename</span><span class=w> </span><span class=nc>BasicJsonType</span><span class=o>&gt;</span><span class=w></span>
<span class=w> </span><span class=k>static</span><span class=w> </span><span class=kt>void</span><span class=w> </span><span class=n>to_json</span><span class=p>(</span><span class=n>BasicJsonType</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>T</span><span class=o>&amp;</span><span class=w> </span><span class=n>value</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// this calls BasicJsonType::json_serializer&lt;T&gt;::to_json(j, value);</span>
<span class=w> </span><span class=c1>// if BasicJsonType::json_serializer == bad_serializer ... oops!</span>
<span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>value</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=k>template</span><span class=w> </span><span class=o>&lt;</span><span class=k>typename</span><span class=w> </span><span class=nc>BasicJsonType</span><span class=o>&gt;</span><span class=w></span>
<span class=w> </span><span class=k>static</span><span class=w> </span><span class=kt>void</span><span class=w> </span><span class=n>to_json</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>BasicJsonType</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=n>T</span><span class=o>&amp;</span><span class=w> </span><span class=n>value</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// this calls BasicJsonType::json_serializer&lt;T&gt;::from_json(j, value);</span>
<span class=w> </span><span class=c1>// if BasicJsonType::json_serializer == bad_serializer ... oops!</span>
<span class=w> </span><span class=n>value</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=k>template</span><span class=w> </span><span class=n>get</span><span class=o>&lt;</span><span class=n>T</span><span class=o>&gt;</span><span class=p>();</span><span class=w> </span><span class=c1>// oops!</span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>};</span><span class=w></span>
</code></pre></div> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../../home/sponsors/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Sponsors" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> Sponsors </div> </div> </a> <a href=../binary_formats/ class="md-footer__link md-footer__link--next" aria-label="Next: Binary Formats" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Binary Formats </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>&quot;key&quot;</span><span class=p>,</span><span class=w> </span><span class=s>&quot;value&quot;</span><span class=p>}};</span><span class=w></span>
<span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>&quot;missing&quot;</span><span class=p>];</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>Assertion failed: (m_value.object-&gt;find(key) != m_value.object-&gt;end()), function operator[], file json.hpp, line 2144.
</code></pre></div> </details> <h3 id=constructing-from-an-uninitialized-iterator-range>Constructing from an uninitialized iterator range<a class=headerlink href=#constructing-from-an-uninitialized-iterator-range title="Permanent link">&para;</a></h3> <p>Constructing a JSON value from an iterator range (see <a href=../../api/basic_json/basic_json/ >constructor</a>) with an uninitialized iterator is undefined behavior and yields a runtime assertion.</p> <details class=example> <summary>Example 2: Uninitialized iterator range</summary> <p>The following code will trigger an assertion at runtime:</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>iterator</span><span class=w> </span><span class=n>it1</span><span class=p>,</span><span class=w> </span><span class=n>it2</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>(</span><span class=n>it1</span><span class=p>,</span><span class=w> </span><span class=n>it2</span><span class=p>);</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.
</code></pre></div> </details> <h3 id=operations-on-uninitialized-iterators>Operations on uninitialized iterators<a class=headerlink href=#operations-on-uninitialized-iterators title="Permanent link">&para;</a></h3> <p>Any operation on uninitialized iterators (i.e., iterators that are not associated with any JSON value) is undefined behavior and yields a runtime assertion.</p> <details class=example> <summary>Example 3: Uninitialized iterator</summary> <p>The following code will trigger an assertion at runtime:</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>iterator</span><span class=w> </span><span class=n>it</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=o>++</span><span class=n>it</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.
</code></pre></div> </details> <h3 id=reading-from-a-null-file-pointer>Reading from a null <code>FILE</code> pointer<a class=headerlink href=#reading-from-a-null-file-pointer title="Permanent link">&para;</a></h3> <p>Reading from a null <code class=highlight><span class=kt>FILE</span><span class=w></span></code> pointer is undefined behavior and yields a runtime assertion. This can happen when calling <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>fopen</span><span class=w></span></code> on a nonexistent file.</p> <details class=example> <summary>Example 4: Uninitialized iterator</summary> <p>The following code will trigger an assertion at runtime:</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=kt>FILE</span><span class=o>*</span><span class=w> </span><span class=n>f</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>fopen</span><span class=p>(</span><span class=s>&quot;nonexistent_file.json&quot;</span><span class=p>,</span><span class=w> </span><span class=s>&quot;r&quot;</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>f</span><span class=p>);</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>Assertion failed: (m_file != nullptr), function file_input_adapter, file input_adapters.hpp, line 55.
</code></pre></div> </details> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../parsing/sax_interface/ class="md-footer__link md-footer__link--prev" aria-label="Previous: SAX Interface" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> SAX Interface </div> </div> </a> <a href=../enum_conversion/ class="md-footer__link md-footer__link--next" aria-label="Next: Specializing enum conversion" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Specializing enum conversion </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<span class=w> </span><span class=nt>&quot;_ArrayType_&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;uint8&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;_ArraySize_&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>2</span><span class=p>,</span><span class=mi>3</span><span class=p>],</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;_ArrayData_&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>1</span><span class=p>,</span><span class=mi>2</span><span class=p>,</span><span class=mi>3</span><span class=p>,</span><span class=mi>4</span><span class=p>,</span><span class=mi>5</span><span class=p>,</span><span class=mi>6</span><span class=p>]</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Likewise, when a JSON object in the above form is serialzed using <a href=../../../api/basic_json/to_bjdata/ ><code>to_bjdata</code></a>, it is automatically converted into a compact BJData ND-array. The only exception is, that when the 1-dimensional vector stored in <code>"_ArraySize_"</code> contains a single integer or two integers with one being 1, a regular 1-D optimized array is generated.</p> <p>The current version of this library does not yet support automatic detection of and conversion from a nested JSON array input to a BJData ND-array.</p> </div> <div class="admonition info"> <p class=admonition-title>Restrictions in optimized data types for arrays and objects</p> <p>Due to diminished space saving, hampered readability, and increased security risks, in BJData, the allowed data types following the <code>$</code> marker in an optimized array and object container are restricted to <strong>non-zero-fixed-length</strong> data types. Therefore, the valid optimized type markers can only be one of <code>UiuImlMLhdDC</code>. This also means other variable (<code>[{SH</code>) or zero-length types (<code>TFN</code>) can not be used in an optimized array or object in BJData.</p> </div> <div class="admonition info"> <p class=admonition-title>Binary values</p> <p>If the JSON data contains the binary type, the value stored is a list of integers, as suggested by the BJData documentation. In particular, this means that the serialization and the deserialization of JSON containing binary values into BJData and back will result in a different JSON object.</p> </div> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=k>namespace</span><span class=w> </span><span class=nn>nlohmann</span><span class=o>::</span><span class=nn>literals</span><span class=p>;</span><span class=w></span>
<span class=c1>// function to print BJData&#39;s diagnostic format</span>
<span class=kt>void</span><span class=w> </span><span class=nf>print_byte</span><span class=p>(</span><span class=kt>uint8_t</span><span class=w> </span><span class=n>byte</span><span class=p>)</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=mi>32</span><span class=w> </span><span class=o>&lt;</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=n>and</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=o>&lt;</span><span class=w> </span><span class=mi>128</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=p>(</span><span class=kt>char</span><span class=p>)</span><span class=n>byte</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=k>else</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=p>)</span><span class=n>byte</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>}</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// create a JSON value</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{&quot;compact&quot;: true, &quot;schema&quot;: false}</span><span class=dl>)</span><span class=s>&quot;</span><span class=n>_json</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// serialize it to BJData</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_bjdata</span><span class=p>(</span><span class=n>j</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// print the vector content</span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>v</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>print_byte</span><span class=p>(</span><span class=n>byte</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// create an array of numbers</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>array</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>,</span><span class=w> </span><span class=mi>4</span><span class=p>,</span><span class=w> </span><span class=mi>5</span><span class=p>,</span><span class=w> </span><span class=mi>6</span><span class=p>,</span><span class=w> </span><span class=mi>7</span><span class=p>,</span><span class=w> </span><span class=mi>8</span><span class=p>};</span><span class=w></span>
<span class=w> </span><span class=c1>// serialize it to BJData using default representation</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v_array</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_bjdata</span><span class=p>(</span><span class=n>array</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// serialize it to BJData using size optimization</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v_array_size</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_bjdata</span><span class=p>(</span><span class=n>array</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// serialize it to BJData using type optimization</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v_array_size_and_type</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_bjdata</span><span class=p>(</span><span class=n>array</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// print the vector contents</span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>v_array</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>print_byte</span><span class=p>(</span><span class=n>byte</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>v_array_size</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>print_byte</span><span class=p>(</span><span class=n>byte</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>v_array_size_and_type</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>print_byte</span><span class=p>(</span><span class=n>byte</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=n>i7compactTi6schemaF</span><span class=p>}</span><span class=w></span>
<span class=p>[</span><span class=n>i1i2i3i4i5i6i7i8</span><span class=p>]</span><span class=w></span>
<span class=p>[</span><span class=err>#</span><span class=n>i8i1i2i3i4i5i6i7i8</span><span class=w></span>
<span class=p>[</span><span class=n>$i</span><span class=err>#</span><span class=n>i812345678</span><span class=w></span>
</code></pre></div> </details> <h2 id=deserialization>Deserialization<a class=headerlink href=#deserialization title="Permanent link">&para;</a></h2> <p>The library maps BJData types to JSON value types as follows:</p> <table> <thead> <tr> <th>BJData type</th> <th>JSON value type</th> <th>marker</th> </tr> </thead> <tbody> <tr> <td>no-op</td> <td><em>no value, next value is read</em></td> <td><code>N</code></td> </tr> <tr> <td>null</td> <td><code>null</code></td> <td><code>Z</code></td> </tr> <tr> <td>false</td> <td><code>false</code></td> <td><code>F</code></td> </tr> <tr> <td>true</td> <td><code>true</code></td> <td><code>T</code></td> </tr> <tr> <td>float16</td> <td>number_float</td> <td><code>h</code></td> </tr> <tr> <td>float32</td> <td>number_float</td> <td><code>d</code></td> </tr> <tr> <td>float64</td> <td>number_float</td> <td><code>D</code></td> </tr> <tr> <td>uint8</td> <td>number_unsigned</td> <td><code>U</code></td> </tr> <tr> <td>int8</td> <td>number_integer</td> <td><code>i</code></td> </tr> <tr> <td>uint16</td> <td>number_unsigned</td> <td><code>u</code></td> </tr> <tr> <td>int16</td> <td>number_integer</td> <td><code>I</code></td> </tr> <tr> <td>uint32</td> <td>number_unsigned</td> <td><code>m</code></td> </tr> <tr> <td>int32</td> <td>number_integer</td> <td><code>l</code></td> </tr> <tr> <td>uint64</td> <td>number_unsigned</td> <td><code>M</code></td> </tr> <tr> <td>int64</td> <td>number_integer</td> <td><code>L</code></td> </tr> <tr> <td>string</td> <td>string</td> <td><code>S</code></td> </tr> <tr> <td>char</td> <td>string</td> <td><code>C</code></td> </tr> <tr> <td>array</td> <td>array (optimized values are supported)</td> <td><code>[</code></td> </tr> <tr> <td>ND-array</td> <td>object (in JData annotated array format)</td> <td><code>[$.#[.</code></td> </tr> <tr> <td>object</td> <td>object (optimized values are supported)</td> <td><code>{</code></td> </tr> </tbody> </table> <div class="admonition success"> <p class=admonition-title>Complete mapping</p> <p>The mapping is <strong>complete</strong> in the sense that any BJData value can be converted to a JSON value.</p> </div> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// create byte vector</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mh>0x7B</span><span class=p>,</span><span class=w> </span><span class=mh>0x69</span><span class=p>,</span><span class=w> </span><span class=mh>0x07</span><span class=p>,</span><span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w> </span><span class=mh>0x6F</span><span class=p>,</span><span class=w> </span><span class=mh>0x6D</span><span class=p>,</span><span class=w> </span><span class=mh>0x70</span><span class=p>,</span><span class=w> </span><span class=mh>0x61</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w> </span><span class=mh>0x74</span><span class=p>,</span><span class=w> </span><span class=mh>0x54</span><span class=p>,</span><span class=w> </span><span class=mh>0x69</span><span class=p>,</span><span class=w> </span><span class=mh>0x06</span><span class=p>,</span><span class=w> </span><span class=mh>0x73</span><span class=p>,</span><span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w> </span><span class=mh>0x68</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mh>0x65</span><span class=p>,</span><span class=w> </span><span class=mh>0x6D</span><span class=p>,</span><span class=w> </span><span class=mh>0x61</span><span class=p>,</span><span class=w> </span><span class=mh>0x69</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span><span class=p>,</span><span class=w> </span><span class=mh>0x7D</span><span class=w></span>
<span class=w> </span><span class=p>};</span><span class=w></span>
<span class=w> </span><span class=c1>// deserialize it with BJData</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>from_bjdata</span><span class=p>(</span><span class=n>v</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// print the deserialized JSON value</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;compact&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Binary Formats" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> Binary Formats </div> </div> </a> <a href=../bson/ class="md-footer__link md-footer__link--next" aria-label="Next: BSON" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> BSON </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=k>namespace</span><span class=w> </span><span class=nn>nlohmann</span><span class=o>::</span><span class=nn>literals</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// create a JSON value</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{&quot;compact&quot;: true, &quot;schema&quot;: 0}</span><span class=dl>)</span><span class=s>&quot;</span><span class=n>_json</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// serialize it to BSON</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_bson</span><span class=p>(</span><span class=n>j</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// print the vector content</span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>v</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;0x&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>hex</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setfill</span><span class=p>(</span><span class=sc>&#39;0&#39;</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=p>)</span><span class=n>byte</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot; &quot;</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=mh>0x1b</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=mh>0x08</span><span class=w> </span><span class=mh>0x63</span><span class=w> </span><span class=mh>0x6f</span><span class=w> </span><span class=mh>0x6d</span><span class=w> </span><span class=mh>0x70</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x63</span><span class=w> </span><span class=mh>0x74</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=mh>0x01</span><span class=w> </span><span class=mh>0x10</span><span class=w> </span><span class=mh>0x73</span><span class=w> </span><span class=mh>0x63</span><span class=w> </span><span class=mh>0x68</span><span class=w> </span><span class=mh>0x65</span><span class=w> </span><span class=mh>0x6d</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span>
</code></pre></div> </details> <h2 id=deserialization>Deserialization<a class=headerlink href=#deserialization title="Permanent link">&para;</a></h2> <p>The library maps BSON record types to JSON value types as follows:</p> <table> <thead> <tr> <th>BSON type</th> <th>BSON marker byte</th> <th>JSON value type</th> </tr> </thead> <tbody> <tr> <td>double</td> <td>0x01</td> <td>number_float</td> </tr> <tr> <td>string</td> <td>0x02</td> <td>string</td> </tr> <tr> <td>document</td> <td>0x03</td> <td>object</td> </tr> <tr> <td>array</td> <td>0x04</td> <td>array</td> </tr> <tr> <td>binary</td> <td>0x05</td> <td>binary</td> </tr> <tr> <td>undefined</td> <td>0x06</td> <td><em>unsupported</em></td> </tr> <tr> <td>ObjectId</td> <td>0x07</td> <td><em>unsupported</em></td> </tr> <tr> <td>boolean</td> <td>0x08</td> <td>boolean</td> </tr> <tr> <td>UTC Date-Time</td> <td>0x09</td> <td><em>unsupported</em></td> </tr> <tr> <td>null</td> <td>0x0A</td> <td>null</td> </tr> <tr> <td>Regular Expr.</td> <td>0x0B</td> <td><em>unsupported</em></td> </tr> <tr> <td>DB Pointer</td> <td>0x0C</td> <td><em>unsupported</em></td> </tr> <tr> <td>JavaScript Code</td> <td>0x0D</td> <td><em>unsupported</em></td> </tr> <tr> <td>Symbol</td> <td>0x0E</td> <td><em>unsupported</em></td> </tr> <tr> <td>JavaScript Code</td> <td>0x0F</td> <td><em>unsupported</em></td> </tr> <tr> <td>int32</td> <td>0x10</td> <td>number_integer</td> </tr> <tr> <td>Timestamp</td> <td>0x11</td> <td><em>unsupported</em></td> </tr> <tr> <td>128-bit decimal float</td> <td>0x13</td> <td><em>unsupported</em></td> </tr> <tr> <td>Max Key</td> <td>0x7F</td> <td><em>unsupported</em></td> </tr> <tr> <td>Min Key</td> <td>0xFF</td> <td><em>unsupported</em></td> </tr> </tbody> </table> <div class="admonition warning"> <p class=admonition-title>Incomplete mapping</p> <p>The mapping is <strong>incomplete</strong>. The unsupported mappings are indicated in the table above.</p> </div> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// create byte vector</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mh>0x1b</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span><span class=p>,</span><span class=w> </span><span class=mh>0x08</span><span class=p>,</span><span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w> </span><span class=mh>0x6f</span><span class=p>,</span><span class=w> </span><span class=mh>0x6d</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mh>0x70</span><span class=p>,</span><span class=w> </span><span class=mh>0x61</span><span class=p>,</span><span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w> </span><span class=mh>0x74</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span><span class=p>,</span><span class=w> </span><span class=mh>0x01</span><span class=p>,</span><span class=w> </span><span class=mh>0x10</span><span class=p>,</span><span class=w> </span><span class=mh>0x73</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w> </span><span class=mh>0x68</span><span class=p>,</span><span class=w> </span><span class=mh>0x65</span><span class=p>,</span><span class=w> </span><span class=mh>0x6d</span><span class=p>,</span><span class=w> </span><span class=mh>0x61</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mh>0x00</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span><span class=w></span>
<span class=w> </span><span class=p>};</span><span class=w></span>
<span class=w> </span><span class=c1>// deserialize it with BSON</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>from_bson</span><span class=p>(</span><span class=n>v</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// print the deserialized JSON value</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;compact&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../bjdata/ class="md-footer__link md-footer__link--prev" aria-label="Previous: BJData" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> BJData </div> </div> </a> <a href=../cbor/ class="md-footer__link md-footer__link--next" aria-label="Next: CBOR" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> CBOR </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=k>namespace</span><span class=w> </span><span class=nn>nlohmann</span><span class=o>::</span><span class=nn>literals</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// create a JSON value</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{&quot;compact&quot;: true, &quot;schema&quot;: 0}</span><span class=dl>)</span><span class=s>&quot;</span><span class=n>_json</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// serialize it to CBOR</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_cbor</span><span class=p>(</span><span class=n>j</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// print the vector content</span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>v</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;0x&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>hex</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setfill</span><span class=p>(</span><span class=sc>&#39;0&#39;</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=p>)</span><span class=n>byte</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot; &quot;</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=mh>0xa2</span><span class=w> </span><span class=mh>0x67</span><span class=w> </span><span class=mh>0x63</span><span class=w> </span><span class=mh>0x6f</span><span class=w> </span><span class=mh>0x6d</span><span class=w> </span><span class=mh>0x70</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x63</span><span class=w> </span><span class=mh>0x74</span><span class=w> </span><span class=mh>0xf5</span><span class=w> </span><span class=mh>0x66</span><span class=w> </span><span class=mh>0x73</span><span class=w> </span><span class=mh>0x63</span><span class=w> </span><span class=mh>0x68</span><span class=w> </span><span class=mh>0x65</span><span class=w> </span><span class=mh>0x6d</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span>
</code></pre></div> </details> <h2 id=deserialization>Deserialization<a class=headerlink href=#deserialization title="Permanent link">&para;</a></h2> <p>The library maps CBOR types to JSON value types as follows:</p> <table> <thead> <tr> <th>CBOR type</th> <th>JSON value type</th> <th>first byte</th> </tr> </thead> <tbody> <tr> <td>Integer</td> <td>number_unsigned</td> <td>0x00..0x17</td> </tr> <tr> <td>Unsigned integer</td> <td>number_unsigned</td> <td>0x18</td> </tr> <tr> <td>Unsigned integer</td> <td>number_unsigned</td> <td>0x19</td> </tr> <tr> <td>Unsigned integer</td> <td>number_unsigned</td> <td>0x1A</td> </tr> <tr> <td>Unsigned integer</td> <td>number_unsigned</td> <td>0x1B</td> </tr> <tr> <td>Negative integer</td> <td>number_integer</td> <td>0x20..0x37</td> </tr> <tr> <td>Negative integer</td> <td>number_integer</td> <td>0x38</td> </tr> <tr> <td>Negative integer</td> <td>number_integer</td> <td>0x39</td> </tr> <tr> <td>Negative integer</td> <td>number_integer</td> <td>0x3A</td> </tr> <tr> <td>Negative integer</td> <td>number_integer</td> <td>0x3B</td> </tr> <tr> <td>Byte string</td> <td>binary</td> <td>0x40..0x57</td> </tr> <tr> <td>Byte string</td> <td>binary</td> <td>0x58</td> </tr> <tr> <td>Byte string</td> <td>binary</td> <td>0x59</td> </tr> <tr> <td>Byte string</td> <td>binary</td> <td>0x5A</td> </tr> <tr> <td>Byte string</td> <td>binary</td> <td>0x5B</td> </tr> <tr> <td>UTF-8 string</td> <td>string</td> <td>0x60..0x77</td> </tr> <tr> <td>UTF-8 string</td> <td>string</td> <td>0x78</td> </tr> <tr> <td>UTF-8 string</td> <td>string</td> <td>0x79</td> </tr> <tr> <td>UTF-8 string</td> <td>string</td> <td>0x7A</td> </tr> <tr> <td>UTF-8 string</td> <td>string</td> <td>0x7B</td> </tr> <tr> <td>UTF-8 string</td> <td>string</td> <td>0x7F</td> </tr> <tr> <td>array</td> <td>array</td> <td>0x80..0x97</td> </tr> <tr> <td>array</td> <td>array</td> <td>0x98</td> </tr> <tr> <td>array</td> <td>array</td> <td>0x99</td> </tr> <tr> <td>array</td> <td>array</td> <td>0x9A</td> </tr> <tr> <td>array</td> <td>array</td> <td>0x9B</td> </tr> <tr> <td>array</td> <td>array</td> <td>0x9F</td> </tr> <tr> <td>map</td> <td>object</td> <td>0xA0..0xB7</td> </tr> <tr> <td>map</td> <td>object</td> <td>0xB8</td> </tr> <tr> <td>map</td> <td>object</td> <td>0xB9</td> </tr> <tr> <td>map</td> <td>object</td> <td>0xBA</td> </tr> <tr> <td>map</td> <td>object</td> <td>0xBB</td> </tr> <tr> <td>map</td> <td>object</td> <td>0xBF</td> </tr> <tr> <td>False</td> <td><code>false</code></td> <td>0xF4</td> </tr> <tr> <td>True</td> <td><code>true</code></td> <td>0xF5</td> </tr> <tr> <td>Null</td> <td><code>null</code></td> <td>0xF6</td> </tr> <tr> <td>Half-Precision Float</td> <td>number_float</td> <td>0xF9</td> </tr> <tr> <td>Single-Precision Float</td> <td>number_float</td> <td>0xFA</td> </tr> <tr> <td>Double-Precision Float</td> <td>number_float</td> <td>0xFB</td> </tr> </tbody> </table> <div class="admonition warning"> <p class=admonition-title>Incomplete mapping</p> <p>The mapping is <strong>incomplete</strong> in the sense that not all CBOR types can be converted to a JSON value. The following CBOR types are not supported and will yield parse errors:</p> <ul> <li>date/time (0xC0..0xC1)</li> <li>bignum (0xC2..0xC3)</li> <li>decimal fraction (0xC4)</li> <li>bigfloat (0xC5)</li> <li>expected conversions (0xD5..0xD7)</li> <li>simple values (0xE0..0xF3, 0xF8)</li> <li>undefined (0xF7)</li> </ul> </div> <div class="admonition warning"> <p class=admonition-title>Object keys</p> <p>CBOR allows map keys of any type, whereas JSON only allows strings as keys in object values. Therefore, CBOR maps with keys other than UTF-8 strings are rejected.</p> </div> <div class="admonition warning"> <p class=admonition-title>Tagged items</p> <p>Tagged items will throw a parse error by default. They can be ignored by passing <code>cbor_tag_handler_t::ignore</code> to function <code>from_cbor</code>. They can be stored by passing <code>cbor_tag_handler_t::store</code> to function <code>from_cbor</code>.</p> </div> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// create byte vector</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mh>0xa2</span><span class=p>,</span><span class=w> </span><span class=mh>0x67</span><span class=p>,</span><span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w> </span><span class=mh>0x6f</span><span class=p>,</span><span class=w> </span><span class=mh>0x6d</span><span class=p>,</span><span class=w> </span><span class=mh>0x70</span><span class=p>,</span><span class=w> </span><span class=mh>0x61</span><span class=p>,</span><span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mh>0x74</span><span class=p>,</span><span class=w> </span><span class=mh>0xf5</span><span class=p>,</span><span class=w> </span><span class=mh>0x66</span><span class=p>,</span><span class=w> </span><span class=mh>0x73</span><span class=p>,</span><span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w> </span><span class=mh>0x68</span><span class=p>,</span><span class=w> </span><span class=mh>0x65</span><span class=p>,</span><span class=w> </span><span class=mh>0x6d</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mh>0x61</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span><span class=w></span>
<span class=w> </span><span class=p>};</span><span class=w></span>
<span class=w> </span><span class=c1>// deserialize it with CBOR</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>from_cbor</span><span class=p>(</span><span class=n>v</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// print the deserialized JSON value</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;compact&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../bson/ class="md-footer__link md-footer__link--prev" aria-label="Previous: BSON" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> BSON </div> </div> </a> <a href=../messagepack/ class="md-footer__link md-footer__link--next" aria-label="Next: MessagePack" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> MessagePack </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54Z"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3V6m0 5h18v2H3v-2m0 5h18v2H3v-2Z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> MessagePack </span> </div> </div> </div> <form class=md-header__option 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=k>namespace</span><span class=w> </span><span class=nn>nlohmann</span><span class=o>::</span><span class=nn>literals</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// create a JSON value</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{&quot;compact&quot;: true, &quot;schema&quot;: 0}</span><span class=dl>)</span><span class=s>&quot;</span><span class=n>_json</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// serialize it to MessagePack</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_msgpack</span><span class=p>(</span><span class=n>j</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// print the vector content</span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>v</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;0x&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>hex</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setfill</span><span class=p>(</span><span class=sc>&#39;0&#39;</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=p>)</span><span class=n>byte</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot; &quot;</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=mh>0x82</span><span class=w> </span><span class=mh>0xa7</span><span class=w> </span><span class=mh>0x63</span><span class=w> </span><span class=mh>0x6f</span><span class=w> </span><span class=mh>0x6d</span><span class=w> </span><span class=mh>0x70</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x63</span><span class=w> </span><span class=mh>0x74</span><span class=w> </span><span class=mh>0xc3</span><span class=w> </span><span class=mh>0xa6</span><span class=w> </span><span class=mh>0x73</span><span class=w> </span><span class=mh>0x63</span><span class=w> </span><span class=mh>0x68</span><span class=w> </span><span class=mh>0x65</span><span class=w> </span><span class=mh>0x6d</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span>
</code></pre></div> </details> <h2 id=deserialization>Deserialization<a class=headerlink href=#deserialization title="Permanent link">&para;</a></h2> <p>The library maps MessagePack types to JSON value types as follows:</p> <table> <thead> <tr> <th>MessagePack type</th> <th>JSON value type</th> <th>first byte</th> </tr> </thead> <tbody> <tr> <td>positive fixint</td> <td>number_unsigned</td> <td>0x00..0x7F</td> </tr> <tr> <td>fixmap</td> <td>object</td> <td>0x80..0x8F</td> </tr> <tr> <td>fixarray</td> <td>array</td> <td>0x90..0x9F</td> </tr> <tr> <td>fixstr</td> <td>string</td> <td>0xA0..0xBF</td> </tr> <tr> <td>nil</td> <td><code>null</code></td> <td>0xC0</td> </tr> <tr> <td>false</td> <td><code>false</code></td> <td>0xC2</td> </tr> <tr> <td>true</td> <td><code>true</code></td> <td>0xC3</td> </tr> <tr> <td>float 32</td> <td>number_float</td> <td>0xCA</td> </tr> <tr> <td>float 64</td> <td>number_float</td> <td>0xCB</td> </tr> <tr> <td>uint 8</td> <td>number_unsigned</td> <td>0xCC</td> </tr> <tr> <td>uint 16</td> <td>number_unsigned</td> <td>0xCD</td> </tr> <tr> <td>uint 32</td> <td>number_unsigned</td> <td>0xCE</td> </tr> <tr> <td>uint 64</td> <td>number_unsigned</td> <td>0xCF</td> </tr> <tr> <td>int 8</td> <td>number_integer</td> <td>0xD0</td> </tr> <tr> <td>int 16</td> <td>number_integer</td> <td>0xD1</td> </tr> <tr> <td>int 32</td> <td>number_integer</td> <td>0xD2</td> </tr> <tr> <td>int 64</td> <td>number_integer</td> <td>0xD3</td> </tr> <tr> <td>str 8</td> <td>string</td> <td>0xD9</td> </tr> <tr> <td>str 16</td> <td>string</td> <td>0xDA</td> </tr> <tr> <td>str 32</td> <td>string</td> <td>0xDB</td> </tr> <tr> <td>array 16</td> <td>array</td> <td>0xDC</td> </tr> <tr> <td>array 32</td> <td>array</td> <td>0xDD</td> </tr> <tr> <td>map 16</td> <td>object</td> <td>0xDE</td> </tr> <tr> <td>map 32</td> <td>object</td> <td>0xDF</td> </tr> <tr> <td>bin 8</td> <td>binary</td> <td>0xC4</td> </tr> <tr> <td>bin 16</td> <td>binary</td> <td>0xC5</td> </tr> <tr> <td>bin 32</td> <td>binary</td> <td>0xC6</td> </tr> <tr> <td>ext 8</td> <td>binary</td> <td>0xC7</td> </tr> <tr> <td>ext 16</td> <td>binary</td> <td>0xC8</td> </tr> <tr> <td>ext 32</td> <td>binary</td> <td>0xC9</td> </tr> <tr> <td>fixext 1</td> <td>binary</td> <td>0xD4</td> </tr> <tr> <td>fixext 2</td> <td>binary</td> <td>0xD5</td> </tr> <tr> <td>fixext 4</td> <td>binary</td> <td>0xD6</td> </tr> <tr> <td>fixext 8</td> <td>binary</td> <td>0xD7</td> </tr> <tr> <td>fixext 16</td> <td>binary</td> <td>0xD8</td> </tr> <tr> <td>negative fixint</td> <td>number_integer</td> <td>0xE0-0xFF</td> </tr> </tbody> </table> <div class="admonition info"> <p class=admonition-title>Info</p> <p>Any MessagePack output created by <code>to_msgpack</code> can be successfully parsed by <code>from_msgpack</code>.</p> </div> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// create byte vector</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mh>0x82</span><span class=p>,</span><span class=w> </span><span class=mh>0xa7</span><span class=p>,</span><span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w> </span><span class=mh>0x6f</span><span class=p>,</span><span class=w> </span><span class=mh>0x6d</span><span class=p>,</span><span class=w> </span><span class=mh>0x70</span><span class=p>,</span><span class=w> </span><span class=mh>0x61</span><span class=p>,</span><span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mh>0x74</span><span class=p>,</span><span class=w> </span><span class=mh>0xc3</span><span class=p>,</span><span class=w> </span><span class=mh>0xa6</span><span class=p>,</span><span class=w> </span><span class=mh>0x73</span><span class=p>,</span><span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w> </span><span class=mh>0x68</span><span class=p>,</span><span class=w> </span><span class=mh>0x65</span><span class=p>,</span><span class=w> </span><span class=mh>0x6d</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mh>0x61</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span><span class=w></span>
<span class=w> </span><span class=p>};</span><span class=w></span>
<span class=w> </span><span class=c1>// deserialize it with MessagePack</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>from_msgpack</span><span class=p>(</span><span class=n>v</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// print the deserialized JSON value</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;compact&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../cbor/ class="md-footer__link md-footer__link--prev" aria-label="Previous: CBOR" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> CBOR </div> </div> </a> <a href=../ubjson/ class="md-footer__link md-footer__link--next" aria-label="Next: UBJSON" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> UBJSON </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=k>namespace</span><span class=w> </span><span class=nn>nlohmann</span><span class=o>::</span><span class=nn>literals</span><span class=p>;</span><span class=w></span>
<span class=c1>// function to print UBJSON&#39;s diagnostic format</span>
<span class=kt>void</span><span class=w> </span><span class=nf>print_byte</span><span class=p>(</span><span class=kt>uint8_t</span><span class=w> </span><span class=n>byte</span><span class=p>)</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=mi>32</span><span class=w> </span><span class=o>&lt;</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=n>and</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=o>&lt;</span><span class=w> </span><span class=mi>128</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=p>(</span><span class=kt>char</span><span class=p>)</span><span class=n>byte</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=k>else</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=p>)</span><span class=n>byte</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>}</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// create a JSON value</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{&quot;compact&quot;: true, &quot;schema&quot;: false}</span><span class=dl>)</span><span class=s>&quot;</span><span class=n>_json</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// serialize it to UBJSON</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_ubjson</span><span class=p>(</span><span class=n>j</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// print the vector content</span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>v</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>print_byte</span><span class=p>(</span><span class=n>byte</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// create an array of numbers</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>array</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>,</span><span class=w> </span><span class=mi>4</span><span class=p>,</span><span class=w> </span><span class=mi>5</span><span class=p>,</span><span class=w> </span><span class=mi>6</span><span class=p>,</span><span class=w> </span><span class=mi>7</span><span class=p>,</span><span class=w> </span><span class=mi>8</span><span class=p>};</span><span class=w></span>
<span class=w> </span><span class=c1>// serialize it to UBJSON using default representation</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v_array</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_ubjson</span><span class=p>(</span><span class=n>array</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// serialize it to UBJSON using size optimization</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v_array_size</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_ubjson</span><span class=p>(</span><span class=n>array</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// serialize it to UBJSON using type optimization</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v_array_size_and_type</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_ubjson</span><span class=p>(</span><span class=n>array</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// print the vector contents</span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>v_array</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>print_byte</span><span class=p>(</span><span class=n>byte</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>v_array_size</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>print_byte</span><span class=p>(</span><span class=n>byte</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>byte</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>v_array_size_and_type</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>print_byte</span><span class=p>(</span><span class=n>byte</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=n>i7compactTi6schemaF</span><span class=p>}</span><span class=w></span>
<span class=p>[</span><span class=n>i1i2i3i4i5i6i7i8</span><span class=p>]</span><span class=w></span>
<span class=p>[</span><span class=err>#</span><span class=n>i8i1i2i3i4i5i6i7i8</span><span class=w></span>
<span class=p>[</span><span class=n>$i</span><span class=err>#</span><span class=n>i812345678</span><span class=w></span>
</code></pre></div> </details> <h2 id=deserialization>Deserialization<a class=headerlink href=#deserialization title="Permanent link">&para;</a></h2> <p>The library maps UBJSON types to JSON value types as follows:</p> <table> <thead> <tr> <th>UBJSON type</th> <th>JSON value type</th> <th>marker</th> </tr> </thead> <tbody> <tr> <td>no-op</td> <td><em>no value, next value is read</em></td> <td><code>N</code></td> </tr> <tr> <td>null</td> <td><code>null</code></td> <td><code>Z</code></td> </tr> <tr> <td>false</td> <td><code>false</code></td> <td><code>F</code></td> </tr> <tr> <td>true</td> <td><code>true</code></td> <td><code>T</code></td> </tr> <tr> <td>float32</td> <td>number_float</td> <td><code>d</code></td> </tr> <tr> <td>float64</td> <td>number_float</td> <td><code>D</code></td> </tr> <tr> <td>uint8</td> <td>number_unsigned</td> <td><code>U</code></td> </tr> <tr> <td>int8</td> <td>number_integer</td> <td><code>i</code></td> </tr> <tr> <td>int16</td> <td>number_integer</td> <td><code>I</code></td> </tr> <tr> <td>int32</td> <td>number_integer</td> <td><code>l</code></td> </tr> <tr> <td>int64</td> <td>number_integer</td> <td><code>L</code></td> </tr> <tr> <td>string</td> <td>string</td> <td><code>S</code></td> </tr> <tr> <td>char</td> <td>string</td> <td><code>C</code></td> </tr> <tr> <td>array</td> <td>array (optimized values are supported)</td> <td><code>[</code></td> </tr> <tr> <td>object</td> <td>object (optimized values are supported)</td> <td><code>{</code></td> </tr> </tbody> </table> <div class="admonition success"> <p class=admonition-title>Complete mapping</p> <p>The mapping is <strong>complete</strong> in the sense that any UBJSON value can be converted to a JSON value.</p> </div> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// create byte vector</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mh>0x7B</span><span class=p>,</span><span class=w> </span><span class=mh>0x69</span><span class=p>,</span><span class=w> </span><span class=mh>0x07</span><span class=p>,</span><span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w> </span><span class=mh>0x6F</span><span class=p>,</span><span class=w> </span><span class=mh>0x6D</span><span class=p>,</span><span class=w> </span><span class=mh>0x70</span><span class=p>,</span><span class=w> </span><span class=mh>0x61</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w> </span><span class=mh>0x74</span><span class=p>,</span><span class=w> </span><span class=mh>0x54</span><span class=p>,</span><span class=w> </span><span class=mh>0x69</span><span class=p>,</span><span class=w> </span><span class=mh>0x06</span><span class=p>,</span><span class=w> </span><span class=mh>0x73</span><span class=p>,</span><span class=w> </span><span class=mh>0x63</span><span class=p>,</span><span class=w> </span><span class=mh>0x68</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mh>0x65</span><span class=p>,</span><span class=w> </span><span class=mh>0x6D</span><span class=p>,</span><span class=w> </span><span class=mh>0x61</span><span class=p>,</span><span class=w> </span><span class=mh>0x69</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span><span class=p>,</span><span class=w> </span><span class=mh>0x7D</span><span class=w></span>
<span class=w> </span><span class=p>};</span><span class=w></span>
<span class=w> </span><span class=c1>// deserialize it with UBJSON</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>from_ubjson</span><span class=p>(</span><span class=n>v</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// print the deserialized JSON value</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;compact&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../messagepack/ class="md-footer__link md-footer__link--prev" aria-label="Previous: MessagePack" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> MessagePack </div> </div> </a> <a href=../../binary_values/ class="md-footer__link md-footer__link--next" aria-label="Next: Binary Values" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Binary Values </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54Z"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3V6m0 5h18v2H3v-2m0 5h18v2H3v-2Z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Binary Values </span> </div> </div> </div> <form class=md-header__option 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autocomplete=off spellcheck=false data-md-component=search-query required> <label class="md-search__iconLine truncated
<span class=k>auto</span><span class=w> </span><span class=n>binary_with_subtype</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>binary_t</span><span class=p>({</span><span class=mh>0xCA</span><span class=p>,</span><span class=w> </span><span class=mh>0xFE</span><span class=p>,</span><span class=w> </span><span class=mh>0xBA</span><span class=p>,</span><span class=w> </span><span class=mh>0xBE</span><span class=p>},</span><span class=w> </span><span class=mi>42</span><span class=p>);</span><span class=w></span>
</code></pre></div> <p>There are several convenience functions to check and set the subtype:</p> <div class=highlight><pre><span></span><code><span class=n>binary</span><span class=p>.</span><span class=n>has_subtype</span><span class=p>();</span><span class=w> </span><span class=c1>// returns false</span>
<span class=n>binary_with_subtype</span><span class=p>.</span><span class=n>has_subtype</span><span class=p>();</span><span class=w> </span><span class=c1>// returns true</span>
<span class=n>binary_with_subtype</span><span class=p>.</span><span class=n>clear_subtype</span><span class=p>();</span><span class=w></span>
<span class=n>binary_with_subtype</span><span class=p>.</span><span class=n>has_subtype</span><span class=p>();</span><span class=w> </span><span class=c1>// returns true</span>
<span class=n>binary_with_subtype</span><span class=p>.</span><span class=n>set_subtype</span><span class=p>(</span><span class=mi>42</span><span class=p>);</span><span class=w></span>
<span class=n>binary</span><span class=p>.</span><span class=n>set_subtype</span><span class=p>(</span><span class=mi>23</span><span class=p>);</span><span class=w></span>
<span class=n>binary</span><span class=p>.</span><span class=n>subtype</span><span class=p>();</span><span class=w> </span><span class=c1>// returns 23</span>
</code></pre></div> <p>As <code>json::binary_t</code> is subclassing <code>std::vector&lt;std::uint8_t&gt;</code>, all member functions are available:</p> <div class=highlight><pre><span></span><code><span class=n>binary</span><span class=p>.</span><span class=n>size</span><span class=p>();</span><span class=w> </span><span class=c1>// returns 4</span>
<span class=n>binary</span><span class=p>[</span><span class=mi>1</span><span class=p>];</span><span class=w> </span><span class=c1>// returns 0xFE</span>
</code></pre></div> <p>JSON values can be constructed from <code>json::binary_t</code>:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>binary</span><span class=p>;</span><span class=w></span>
</code></pre></div> <p>Binary values are primitive values just like numbers or strings:</p> <div class=highlight><pre><span></span><code><span class=n>j</span><span class=p>.</span><span class=n>is_binary</span><span class=p>();</span><span class=w> </span><span class=c1>// returns true</span>
<span class=n>j</span><span class=p>.</span><span class=n>is_primitive</span><span class=p>();</span><span class=w> </span><span class=c1>// returns true</span>
</code></pre></div> <p>Given a binary JSON value, the <code>binary_t</code> can be accessed by reference as via <code>get_binary()</code>:</p> <div class=highlight><pre><span></span><code><span class=n>j</span><span class=p>.</span><span class=n>get_binary</span><span class=p>().</span><span class=n>has_subtype</span><span class=p>();</span><span class=w> </span><span class=c1>// returns true</span>
<span class=n>j</span><span class=p>.</span><span class=n>get_binary</span><span class=p>().</span><span class=n>size</span><span class=p>();</span><span class=w> </span><span class=c1>// returns 4</span>
</code></pre></div> <p>For convenience, binary JSON values can be constructed via <code>json::binary</code>:</p> <div class=highlight><pre><span></span><code><span class=k>auto</span><span class=w> </span><span class=n>j2</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>binary</span><span class=p>({</span><span class=mh>0xCA</span><span class=p>,</span><span class=w> </span><span class=mh>0xFE</span><span class=p>,</span><span class=w> </span><span class=mh>0xBA</span><span class=p>,</span><span class=w> </span><span class=mh>0xBE</span><span class=p>},</span><span class=w> </span><span class=mi>23</span><span class=p>);</span><span class=w></span>
<span class=k>auto</span><span class=w> </span><span class=n>j3</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>binary</span><span class=p>({</span><span class=mh>0xCA</span><span class=p>,</span><span class=w> </span><span class=mh>0xFE</span><span class=p>,</span><span class=w> </span><span class=mh>0xBA</span><span class=p>,</span><span class=w> </span><span class=mh>0xBE</span><span class=p>});</span><span class=w></span>
<span class=n>j2</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w> </span><span class=c1>// returns true</span>
<span class=n>j3</span><span class=p>.</span><span class=n>get_binary</span><span class=p>().</span><span class=n>has_subtype</span><span class=p>();</span><span class=w> </span><span class=c1>// returns false</span>
<span class=n>j3</span><span class=p>.</span><span class=n>get_binary</span><span class=p>().</span><span class=n>subtype</span><span class=p>();</span><span class=w> </span><span class=c1>// returns std::uint64_t(-1) as j3 has no subtype</span>
</code></pre></div> <h2 id=serialization>Serialization<a class=headerlink href=#serialization title="Permanent link">&para;</a></h2> <p>Binary values are serialized differently according to the formats.</p> <h3 id=json>JSON<a class=headerlink href=#json title="Permanent link">&para;</a></h3> <p>JSON does not have a binary type, and this library does not introduce a new type as this would break conformance. Instead, binary values are serialized as an object with two keys: <code>bytes</code> holds an array of integers, and <code>subtype</code> is an integer or <code>null</code>.</p> <details class=example> <summary>Example</summary> <p>Code:</p> <div class=highlight><pre><span></span><code><span class=c1>// create a binary value of subtype 42</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;binary&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>binary</span><span class=p>({</span><span class=mh>0xCA</span><span class=p>,</span><span class=w> </span><span class=mh>0xFE</span><span class=p>,</span><span class=w> </span><span class=mh>0xBA</span><span class=p>,</span><span class=w> </span><span class=mh>0xBE</span><span class=p>},</span><span class=w> </span><span class=mi>42</span><span class=p>);</span><span class=w></span>
<span class=c1>// serialize to standard output</span>
<span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;binary&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;bytes&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>202</span><span class=p>,</span><span class=w> </span><span class=mi>254</span><span class=p>,</span><span class=w> </span><span class=mi>186</span><span class=p>,</span><span class=w> </span><span class=mi>190</span><span class=p>],</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;subtype&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <div class="admonition warning"> <p class=admonition-title>No roundtrip for binary values</p> <p>The JSON parser will not parse the objects generated by binary values back to binary values. This is by design to remain standards compliant. Serializing binary values to JSON is only implemented for debugging purposes.</p> </div> <h3 id=bjdata>BJData<a class=headerlink href=#bjdata title="Permanent link">&para;</a></h3> <p><a href=../binary_formats/bjdata/ >BJData</a> neither supports binary values nor subtypes, and proposes to serialize binary values as array of uint8 values. This translation is implemented by the library.</p> <details class=example> <summary>Example</summary> <p>Code:</p> <div class=highlight><pre><span></span><code><span class=c1>// create a binary value of subtype 42 (will be ignored in BJData)</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;binary&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>binary</span><span class=p>({</span><span class=mh>0xCA</span><span class=p>,</span><span class=w> </span><span class=mh>0xFE</span><span class=p>,</span><span class=w> </span><span class=mh>0xBA</span><span class=p>,</span><span class=w> </span><span class=mh>0xBE</span><span class=p>},</span><span class=w> </span><span class=mi>42</span><span class=p>);</span><span class=w></span>
<span class=c1>// convert to BJData</span>
<span class=k>auto</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_bjdata</span><span class=p>(</span><span class=n>j</span><span class=p>);</span><span class=w> </span>
</code></pre></div> <p><code>v</code> is a <code>std::vector&lt;std::uint8t&gt;</code> with the following 20 elements:</p> <div class=highlight><pre><span></span><code><span class=mh>0x7B</span><span class=w> </span><span class=c1>// &#39;{&#39;</span>
<span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x06</span><span class=w> </span><span class=c1>// i 6 (length of the key)</span>
<span class=w> </span><span class=mh>0x62</span><span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x6E</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x72</span><span class=w> </span><span class=mh>0x79</span><span class=w> </span><span class=c1>// &quot;binary&quot;</span>
<span class=w> </span><span class=mh>0x5B</span><span class=w> </span><span class=c1>// &#39;[&#39;</span>
<span class=w> </span><span class=mh>0x55</span><span class=w> </span><span class=mh>0xCA</span><span class=w> </span><span class=mh>0x55</span><span class=w> </span><span class=mh>0xFE</span><span class=w> </span><span class=mh>0x55</span><span class=w> </span><span class=mh>0xBA</span><span class=w> </span><span class=mh>0x55</span><span class=w> </span><span class=mh>0xBE</span><span class=w> </span><span class=c1>// content (each byte prefixed with &#39;U&#39;)</span>
<span class=w> </span><span class=mh>0x5D</span><span class=w> </span><span class=c1>// &#39;]&#39;</span>
<span class=mh>0x7D</span><span class=w> </span><span class=c1>// &#39;}&#39;</span>
</code></pre></div> <p>The following code uses the type and size optimization for UBJSON:</p> <div class=highlight><pre><span></span><code><span class=c1>// convert to UBJSON using the size and type optimization</span>
<span class=k>auto</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_bjdata</span><span class=p>(</span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>);</span><span class=w></span>
</code></pre></div> <p>The resulting vector has 22 elements; the optimization is not effective for examples with few values:</p> <div class=highlight><pre><span></span><code><span class=mh>0x7B</span><span class=w> </span><span class=c1>// &#39;{&#39;</span>
<span class=w> </span><span class=mh>0x23</span><span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x01</span><span class=w> </span><span class=c1>// &#39;#&#39; &#39;i&#39; type of the array elements: unsigned integers</span>
<span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x06</span><span class=w> </span><span class=c1>// i 6 (length of the key)</span>
<span class=w> </span><span class=mh>0x62</span><span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x6E</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x72</span><span class=w> </span><span class=mh>0x79</span><span class=w> </span><span class=c1>// &quot;binary&quot;</span>
<span class=w> </span><span class=mh>0x5B</span><span class=w> </span><span class=c1>// &#39;[&#39; array</span>
<span class=w> </span><span class=mh>0x24</span><span class=w> </span><span class=mh>0x55</span><span class=w> </span><span class=c1>// &#39;$&#39; &#39;U&#39; type of the array elements: unsigned integers</span>
<span class=w> </span><span class=mh>0x23</span><span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x04</span><span class=w> </span><span class=c1>// &#39;#&#39; i 4 number of array elements</span>
<span class=w> </span><span class=mh>0xCA</span><span class=w> </span><span class=mh>0xFE</span><span class=w> </span><span class=mh>0xBA</span><span class=w> </span><span class=mh>0xBE</span><span class=w> </span><span class=c1>// content</span>
</code></pre></div> <p>Note that subtype (42) is <strong>not</strong> serialized and that UBJSON has <strong>no binary type</strong>, and deserializing <code>v</code> would yield the following value:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;binary&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>202</span><span class=p>,</span><span class=w> </span><span class=mi>254</span><span class=p>,</span><span class=w> </span><span class=mi>186</span><span class=p>,</span><span class=w> </span><span class=mi>190</span><span class=p>]</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <h3 id=bson>BSON<a class=headerlink href=#bson title="Permanent link">&para;</a></h3> <p><a href=../binary_formats/bson/ >BSON</a> supports binary values and subtypes. If a subtype is given, it is used and added as unsigned 8-bit integer. If no subtype is given, the generic binary subtype 0x00 is used.</p> <details class=example> <summary>Example</summary> <p>Code:</p> <div class=highlight><pre><span></span><code><span class=c1>// create a binary value of subtype 42</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;binary&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>binary</span><span class=p>({</span><span class=mh>0xCA</span><span class=p>,</span><span class=w> </span><span class=mh>0xFE</span><span class=p>,</span><span class=w> </span><span class=mh>0xBA</span><span class=p>,</span><span class=w> </span><span class=mh>0xBE</span><span class=p>},</span><span class=w> </span><span class=mi>42</span><span class=p>);</span><span class=w></span>
<span class=c1>// convert to BSON</span>
<span class=k>auto</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_bson</span><span class=p>(</span><span class=n>j</span><span class=p>);</span><span class=w> </span>
</code></pre></div> <p><code>v</code> is a <code>std::vector&lt;std::uint8t&gt;</code> with the following 22 elements:</p> <div class=highlight><pre><span></span><code><span class=mh>0x16</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=c1>// number of bytes in the document</span>
<span class=w> </span><span class=mh>0x05</span><span class=w> </span><span class=c1>// binary value</span>
<span class=w> </span><span class=mh>0x62</span><span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x6E</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x72</span><span class=w> </span><span class=mh>0x79</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=c1>// key &quot;binary&quot; + null byte</span>
<span class=w> </span><span class=mh>0x04</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=mh>0x00</span><span class=w> </span><span class=c1>// number of bytes</span>
<span class=w> </span><span class=mh>0x2a</span><span class=w> </span><span class=c1>// subtype</span>
<span class=w> </span><span class=mh>0xCA</span><span class=w> </span><span class=mh>0xFE</span><span class=w> </span><span class=mh>0xBA</span><span class=w> </span><span class=mh>0xBE</span><span class=w> </span><span class=c1>// content</span>
<span class=mh>0x00</span><span class=w> </span><span class=c1>// end of the document</span>
</code></pre></div> <p>Note that the serialization preserves the subtype, and deserializing <code>v</code> would yield the following value:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;binary&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;bytes&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>202</span><span class=p>,</span><span class=w> </span><span class=mi>254</span><span class=p>,</span><span class=w> </span><span class=mi>186</span><span class=p>,</span><span class=w> </span><span class=mi>190</span><span class=p>],</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;subtype&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <h3 id=cbor>CBOR<a class=headerlink href=#cbor title="Permanent link">&para;</a></h3> <p><a href=../binary_formats/cbor/ >CBOR</a> supports binary values, but no subtypes. Subtypes will be serialized as tags. Any binary value will be serialized as byte strings. The library will choose the smallest representation using the length of the byte array.</p> <details class=example> <summary>Example</summary> <p>Code:</p> <div class=highlight><pre><span></span><code><span class=c1>// create a binary value of subtype 42</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;binary&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>binary</span><span class=p>({</span><span class=mh>0xCA</span><span class=p>,</span><span class=w> </span><span class=mh>0xFE</span><span class=p>,</span><span class=w> </span><span class=mh>0xBA</span><span class=p>,</span><span class=w> </span><span class=mh>0xBE</span><span class=p>},</span><span class=w> </span><span class=mi>42</span><span class=p>);</span><span class=w></span>
<span class=c1>// convert to CBOR</span>
<span class=k>auto</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_cbor</span><span class=p>(</span><span class=n>j</span><span class=p>);</span><span class=w> </span>
</code></pre></div> <p><code>v</code> is a <code>std::vector&lt;std::uint8t&gt;</code> with the following 15 elements:</p> <div class=highlight><pre><span></span><code><span class=mh>0xA1</span><span class=w> </span><span class=c1>// map(1)</span>
<span class=w> </span><span class=mh>0x66</span><span class=w> </span><span class=c1>// text(6)</span>
<span class=w> </span><span class=mh>0x62</span><span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x6E</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x72</span><span class=w> </span><span class=mh>0x79</span><span class=w> </span><span class=c1>// &quot;binary&quot;</span>
<span class=w> </span><span class=mh>0xD8</span><span class=w> </span><span class=mh>0x2A</span><span class=w> </span><span class=c1>// tag(42)</span>
<span class=w> </span><span class=mh>0x44</span><span class=w> </span><span class=c1>// bytes(4)</span>
<span class=w> </span><span class=mh>0xCA</span><span class=w> </span><span class=mh>0xFE</span><span class=w> </span><span class=mh>0xBA</span><span class=w> </span><span class=mh>0xBE</span><span class=w> </span><span class=c1>// content</span>
</code></pre></div> <p>Note that the subtype is serialized as tag. However, parsing tagged values yield a parse error unless <code>json::cbor_tag_handler_t::ignore</code> or <code>json::cbor_tag_handler_t::store</code> is passed to <code>json::from_cbor</code>.</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;binary&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;bytes&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>202</span><span class=p>,</span><span class=w> </span><span class=mi>254</span><span class=p>,</span><span class=w> </span><span class=mi>186</span><span class=p>,</span><span class=w> </span><span class=mi>190</span><span class=p>],</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;subtype&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>null</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <h3 id=messagepack>MessagePack<a class=headerlink href=#messagepack title="Permanent link">&para;</a></h3> <p><a href=../binary_formats/messagepack/ >MessagePack</a> supports binary values and subtypes. If a subtype is given, the ext family is used. The library will choose the smallest representation among fixext1, fixext2, fixext4, fixext8, ext8, ext16, and ext32. The subtype is then added as signed 8-bit integer.</p> <p>If no subtype is given, the bin family (bin8, bin16, bin32) is used.</p> <details class=example> <summary>Example</summary> <p>Code:</p> <div class=highlight><pre><span></span><code><span class=c1>// create a binary value of subtype 42</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;binary&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>binary</span><span class=p>({</span><span class=mh>0xCA</span><span class=p>,</span><span class=w> </span><span class=mh>0xFE</span><span class=p>,</span><span class=w> </span><span class=mh>0xBA</span><span class=p>,</span><span class=w> </span><span class=mh>0xBE</span><span class=p>},</span><span class=w> </span><span class=mi>42</span><span class=p>);</span><span class=w></span>
<span class=c1>// convert to MessagePack</span>
<span class=k>auto</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_msgpack</span><span class=p>(</span><span class=n>j</span><span class=p>);</span><span class=w> </span>
</code></pre></div> <p><code>v</code> is a <code>std::vector&lt;std::uint8t&gt;</code> with the following 14 elements:</p> <div class=highlight><pre><span></span><code><span class=mh>0x81</span><span class=w> </span><span class=c1>// fixmap1</span>
<span class=w> </span><span class=mh>0xA6</span><span class=w> </span><span class=c1>// fixstr6</span>
<span class=w> </span><span class=mh>0x62</span><span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x6E</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x72</span><span class=w> </span><span class=mh>0x79</span><span class=w> </span><span class=c1>// &quot;binary&quot;</span>
<span class=w> </span><span class=mh>0xD6</span><span class=w> </span><span class=c1>// fixext4</span>
<span class=w> </span><span class=mh>0x2A</span><span class=w> </span><span class=c1>// subtype</span>
<span class=w> </span><span class=mh>0xCA</span><span class=w> </span><span class=mh>0xFE</span><span class=w> </span><span class=mh>0xBA</span><span class=w> </span><span class=mh>0xBE</span><span class=w> </span><span class=c1>// content</span>
</code></pre></div> <p>Note that the serialization preserves the subtype, and deserializing <code>v</code> would yield the following value:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;binary&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;bytes&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>202</span><span class=p>,</span><span class=w> </span><span class=mi>254</span><span class=p>,</span><span class=w> </span><span class=mi>186</span><span class=p>,</span><span class=w> </span><span class=mi>190</span><span class=p>],</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;subtype&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <h3 id=ubjson>UBJSON<a class=headerlink href=#ubjson title="Permanent link">&para;</a></h3> <p><a href=../binary_formats/ubjson/ >UBJSON</a> neither supports binary values nor subtypes, and proposes to serialize binary values as array of uint8 values. This translation is implemented by the library.</p> <details class=example> <summary>Example</summary> <p>Code:</p> <div class=highlight><pre><span></span><code><span class=c1>// create a binary value of subtype 42 (will be ignored in UBJSON)</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;binary&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>binary</span><span class=p>({</span><span class=mh>0xCA</span><span class=p>,</span><span class=w> </span><span class=mh>0xFE</span><span class=p>,</span><span class=w> </span><span class=mh>0xBA</span><span class=p>,</span><span class=w> </span><span class=mh>0xBE</span><span class=p>},</span><span class=w> </span><span class=mi>42</span><span class=p>);</span><span class=w></span>
<span class=c1>// convert to UBJSON</span>
<span class=k>auto</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_ubjson</span><span class=p>(</span><span class=n>j</span><span class=p>);</span><span class=w> </span>
</code></pre></div> <p><code>v</code> is a <code>std::vector&lt;std::uint8t&gt;</code> with the following 20 elements:</p> <div class=highlight><pre><span></span><code><span class=mh>0x7B</span><span class=w> </span><span class=c1>// &#39;{&#39;</span>
<span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x06</span><span class=w> </span><span class=c1>// i 6 (length of the key)</span>
<span class=w> </span><span class=mh>0x62</span><span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x6E</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x72</span><span class=w> </span><span class=mh>0x79</span><span class=w> </span><span class=c1>// &quot;binary&quot;</span>
<span class=w> </span><span class=mh>0x5B</span><span class=w> </span><span class=c1>// &#39;[&#39;</span>
<span class=w> </span><span class=mh>0x55</span><span class=w> </span><span class=mh>0xCA</span><span class=w> </span><span class=mh>0x55</span><span class=w> </span><span class=mh>0xFE</span><span class=w> </span><span class=mh>0x55</span><span class=w> </span><span class=mh>0xBA</span><span class=w> </span><span class=mh>0x55</span><span class=w> </span><span class=mh>0xBE</span><span class=w> </span><span class=c1>// content (each byte prefixed with &#39;U&#39;)</span>
<span class=w> </span><span class=mh>0x5D</span><span class=w> </span><span class=c1>// &#39;]&#39;</span>
<span class=mh>0x7D</span><span class=w> </span><span class=c1>// &#39;}&#39;</span>
</code></pre></div> <p>The following code uses the type and size optimization for UBJSON:</p> <div class=highlight><pre><span></span><code><span class=c1>// convert to UBJSON using the size and type optimization</span>
<span class=k>auto</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_ubjson</span><span class=p>(</span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>);</span><span class=w></span>
</code></pre></div> <p>The resulting vector has 23 elements; the optimization is not effective for examples with few values:</p> <div class=highlight><pre><span></span><code><span class=mh>0x7B</span><span class=w> </span><span class=c1>// &#39;{&#39;</span>
<span class=w> </span><span class=mh>0x24</span><span class=w> </span><span class=c1>// &#39;$&#39; type of the object elements</span>
<span class=w> </span><span class=mh>0x5B</span><span class=w> </span><span class=c1>// &#39;[&#39; array</span>
<span class=w> </span><span class=mh>0x23</span><span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x01</span><span class=w> </span><span class=c1>// &#39;#&#39; i 1 number of object elements</span>
<span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x06</span><span class=w> </span><span class=c1>// i 6 (length of the key)</span>
<span class=w> </span><span class=mh>0x62</span><span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x6E</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x72</span><span class=w> </span><span class=mh>0x79</span><span class=w> </span><span class=c1>// &quot;binary&quot;</span>
<span class=w> </span><span class=mh>0x24</span><span class=w> </span><span class=mh>0x55</span><span class=w> </span><span class=c1>// &#39;$&#39; &#39;U&#39; type of the array elements: unsigned integers</span>
<span class=w> </span><span class=mh>0x23</span><span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x04</span><span class=w> </span><span class=c1>// &#39;#&#39; i 4 number of array elements</span>
<span class=w> </span><span class=mh>0xCA</span><span class=w> </span><span class=mh>0xFE</span><span class=w> </span><span class=mh>0xBA</span><span class=w> </span><span class=mh>0xBE</span><span class=w> </span><span class=c1>// content</span>
</code></pre></div> <p>Note that subtype (42) is <strong>not</strong> serialized and that UBJSON has <strong>no binary type</strong>, and deserializing <code>v</code> would yield the following value:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;binary&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>202</span><span class=p>,</span><span class=w> </span><span class=mi>254</span><span class=p>,</span><span class=w> </span><span class=mi>186</span><span class=p>,</span><span class=w> </span><span class=mi>190</span><span class=p>]</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../binary_formats/ubjson/ class="md-footer__link md-footer__link--prev" aria-label="Previous: UBJSON" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> UBJSON </div> </div> </a> <a href=../comments/ class="md-footer__link md-footer__link--next" aria-label="Next: Comments" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Comments </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser.
</code></pre></div> </blockquote> </li> <li> <p>It is dangerous for interoperability if some libraries would add comment support while others don't. Please check <a href=https://tools.ietf.org/html/draft-iab-protocol-maintenance-01>The Harmful Consequences of the Robustness Principle</a> on this.</p> </li> </ol> <p>However, you can pass set parameter <code>ignore_comments</code> to <code class=highlight><span class=nb>true</span><span class=w></span></code> in the parse function to ignore <code>//</code> or <code>/* */</code> comments. Comments will then be treated as whitespace.</p> <div class="admonition example"> <p class=admonition-title>Example</p> <p>Consider the following JSON with comments.</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// update in 2006: removed Pluto</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;planets&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>&quot;Mercury&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;Venus&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;Earth&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;Mars&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=s2>&quot;Jupiter&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;Uranus&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;Neptune&quot;</span><span class=w> </span><span class=cm>/*, &quot;Pluto&quot; */</span><span class=p>]</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>When calling <code>parse</code> without additional argument, a parse error exception is thrown. If <code>ignore_comments</code> is set to <code>true</code>, the comments are ignored during parsing:</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&quot;json.hpp&quot;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>s</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s></span>
<span class=s> {</span>
<span class=s> // update in 2006: removed Pluto</span>
<span class=s> &quot;planets&quot;: [&quot;Mercury&quot;, &quot;Venus&quot;, &quot;Earth&quot;, &quot;Mars&quot;,</span>
<span class=s> &quot;Jupiter&quot;, &quot;Uranus&quot;, &quot;Neptune&quot; /*, &quot;Pluto&quot; */]</span>
<span class=s> }</span>
<span class=s> </span><span class=dl>)</span><span class=s>&quot;</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=k>try</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>s</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=k>catch</span><span class=w> </span><span class=p>(</span><span class=n>json</span><span class=o>::</span><span class=n>exception</span><span class=w> </span><span class=o>&amp;</span><span class=n>e</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>e</span><span class=p>.</span><span class=n>what</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>s</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=cm>/* callback */</span><span class=w> </span><span class=k>nullptr</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=cm>/* allow exceptions */</span><span class=w> </span><span class=nb>true</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=cm>/* ignore_comments */</span><span class=w> </span><span class=nb>true</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>[json.exception.parse_error.101] parse error at line 3, column 9:
syntax error while parsing object key - invalid literal;
last read: &#39;&lt;U+000A&gt; {&lt;U+000A&gt; /&#39;; expected string literal
</code></pre></div> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;planets&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=w></span>
<span class=w> </span><span class=s2>&quot;Mercury&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=s2>&quot;Venus&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=s2>&quot;Earth&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=s2>&quot;Mars&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=s2>&quot;Jupiter&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=s2>&quot;Uranus&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=s2>&quot;Neptune&quot;</span><span class=w></span>
<span class=w> </span><span class=p>]</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </div> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../binary_values/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Binary Values" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> Binary Values </div> </div> </a> <a href=../element_access/ class="md-footer__link md-footer__link--next" aria-label="Next: Element Access" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Element Access </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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autocomplete=off spellcheck=false data-md-component=searLine truncated
<span class=w> </span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Mary Smith&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;age&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;hobbies&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>&quot;hiking&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;reading&quot;</span><span class=p>]</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Assume the value is parsed to a <code>json</code> variable <code>j</code>.</p> <table> <thead> <tr> <th>expression</th> <th>value</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>j</span><span class=w></span></code></td> <td><code class=highlight><span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Mary Smith&quot;</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;age&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;hobbies&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>&quot;hiking&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;reading&quot;</span><span class=p>]}</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>&quot;name&quot;</span><span class=p>)</span><span class=w></span></code></td> <td><code class=highlight><span class=s2>&quot;Mary Smith&quot;</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>&quot;age&quot;</span><span class=p>)</span><span class=w></span></code></td> <td><code class=highlight><span class=mi>42</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>&quot;hobbies&quot;</span><span class=p>)</span><span class=w></span></code></td> <td><code class=highlight><span class=p>[</span><span class=s2>&quot;hiking&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;reading&quot;</span><span class=p>]</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>&quot;hobbies&quot;</span><span class=p>).</span><span class=n>at</span><span class=p>(</span><span class=mi>0</span><span class=p>)</span><span class=w></span></code></td> <td><code class=highlight><span class=s2>&quot;hiking&quot;</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>&quot;hobbies&quot;</span><span class=p>).</span><span class=n>at</span><span class=p>(</span><span class=mi>1</span><span class=p>)</span><span class=w></span></code></td> <td><code class=highlight><span class=s2>&quot;reading&quot;</span><span class=w></span></code></td> </tr> </tbody> </table> </details> <p>The return value is a reference, so it can be modified by the original value.</p> <details class=example> <summary>Write access</summary> <div class=highlight><pre><span></span><code><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>&quot;name&quot;</span><span class=p>)</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>&quot;John Smith&quot;</span><span class=p>;</span><span class=w></span>
</code></pre></div> <p>This code produces the following JSON value:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;John Smith&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;age&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;hobbies&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>&quot;hiking&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;reading&quot;</span><span class=p>]</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <p>When accessing an invalid index (i.e., an index greater than or equal to the array size) or the passed object key is non-existing, an exception is thrown.</p> <details class=example> <summary>Accessing via invalid index or missing key</summary> <div class=highlight><pre><span></span><code><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>&quot;hobbies&quot;</span><span class=p>).</span><span class=n>at</span><span class=p>(</span><span class=mi>3</span><span class=p>)</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>&quot;cooking&quot;</span><span class=p>;</span><span class=w></span>
</code></pre></div> <p>This code produces the following exception:</p> <div class=highlight><pre><span></span><code>[json.exception.out_of_range.401] array index 3 is out of range
</code></pre></div> <p>When you <a href=../../../home/exceptions/#extended-diagnostic-messages>extended diagnostic messages</a> are enabled by defining <a href=../../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>, the exception further gives information where the key or index is missing or out of range.</p> <div class=highlight><pre><span></span><code>[json.exception.out_of_range.401] (/hobbies) array index 3 is out of range
</code></pre></div> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <ul> <li><a href=../../../api/basic_json/at/ ><code>at</code></a> can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a <a href=../../../home/exceptions/#jsonexceptiontype_error304><code>basic_json::type_error</code></a> is thrown.</li> <li><a href=../../../home/exceptions/#out-of-range><code>basic_json::out_of_range</code> exception</a> exceptions are thrown if the provided key is not found in an object or the provided index is invalid.</li> </ul> </div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> <tr> <td>access to invalid array index</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> </tbody> </table> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../unchecked_access/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Unchecked access: operator[]" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> Unchecked access: operator[] </div> </div> </a> <a href=../default_value/ class="md-footer__link md-footer__link--next" aria-label="Next: Access with default value: value" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Access with default value: value </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013 - 2022 Niels Lohmann </div> </div> <div class=md-social> <a href=https://github.com/nlohmann target=_blank rel=noopener title=github.com class=md-social__link> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 496 512"><!-- Font Awesome Free 6.2.1 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc.--><path d="M165.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6zm-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3zm44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9zM244.8 8C106.1 8 0 113.3 0 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C428.2 457.8 496 362.9 496 252 496 113.3 383.5 8 244.8 8zM97.2 352.9c-1.3 1-1Line truncated
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<span class=w> </span><span class=nt>&quot;logOutput&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;result.log&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;append&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Assume the value is parsed to a <code>json</code> variable <code>j</code>.</p> <table> <thead> <tr> <th>expression</th> <th>value</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>j</span><span class=w></span></code></td> <td><code class=highlight><span class=p>{</span><span class=nt>&quot;logOutput&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;result.log&quot;</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;append&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>}</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>&quot;logOutput&quot;</span><span class=p>,</span><span class=w> </span><span class=s>&quot;logfile.log&quot;</span><span class=p>)</span><span class=w></span></code></td> <td><code class=highlight><span class=s2>&quot;result.log&quot;</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>&quot;append&quot;</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>)</span><span class=w></span></code></td> <td><code class=highlight><span class=kc>true</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>&quot;append&quot;</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>)</span><span class=w></span></code></td> <td><code class=highlight><span class=kc>true</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>&quot;logLevel&quot;</span><span class=p>,</span><span class=w> </span><span class=s>&quot;verbose&quot;</span><span class=p>)</span><span class=w></span></code></td> <td><code class=highlight><span class=s2>&quot;verbose&quot;</span><span class=w></span></code></td> </tr> </tbody> </table> </details> <h2 id=note>Note<a class=headerlink href=#note title="Permanent link">&para;</a></h2> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <ul> <li><code>value</code> can only be used with objects. For other types, a <a href=../../../home/exceptions/#jsonexceptiontype_error306><code>basic_json::type_error</code></a> is thrown.</li> </ul> </div> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../checked_access/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Checked access: at" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> Checked access: at </div> </div> </a> <a href=../../iterators/ class="md-footer__link md-footer__link--next" aria-label="Next: Iterators" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Iterators </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013 - 2022 Niels Lohmann </div> </div> <div class=md-social> <a href=https://github.com/nlohmann target=_blank rel=noopener title=github.com class=md-social__link> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 496 512"><!-- Font Awesome Free 6.2.1 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Line truncated
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<span class=w> </span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Mary Smith&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;age&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;hobbies&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>&quot;hiking&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;reading&quot;</span><span class=p>]</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Assume the value is parsed to a <code>json</code> variable <code>j</code>.</p> <table> <thead> <tr> <th>expression</th> <th>value</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>j</span><span class=w></span></code></td> <td><code class=highlight><span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Mary Smith&quot;</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;age&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;hobbies&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>&quot;hiking&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;reading&quot;</span><span class=p>]}</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>&quot;name&quot;</span><span class=p>]</span><span class=w></span></code></td> <td><code class=highlight><span class=s2>&quot;Mary Smith&quot;</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>&quot;age&quot;</span><span class=p>]</span><span class=w></span></code></td> <td><code class=highlight><span class=mi>42</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>&quot;hobbies&quot;</span><span class=p>]</span><span class=w></span></code></td> <td><code class=highlight><span class=p>[</span><span class=s2>&quot;hiking&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;reading&quot;</span><span class=p>]</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>&quot;hobbies&quot;</span><span class=p>][</span><span class=mi>0</span><span class=p>]</span><span class=w></span></code></td> <td><code class=highlight><span class=s2>&quot;hiking&quot;</span><span class=w></span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>&quot;hobbies&quot;</span><span class=p>][</span><span class=mi>1</span><span class=p>]</span><span class=w></span></code></td> <td><code class=highlight><span class=s2>&quot;reading&quot;</span><span class=w></span></code></td> </tr> </tbody> </table> </details> <p>The return value is a reference, so it can modify the original value. In case the passed object key is non-existing, a <code class=highlight><span class=kc>null</span><span class=w></span></code> value is inserted which can be immediately be overwritten.</p> <details class=example> <summary>Write access</summary> <div class=highlight><pre><span></span><code><span class=n>j</span><span class=p>[</span><span class=s>&quot;name&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>&quot;John Smith&quot;</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;maidenName&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>&quot;Jones&quot;</span><span class=p>;</span><span class=w></span>
</code></pre></div> <p>This code produces the following JSON value:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;John Smith&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;maidenName&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Jones&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;age&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;hobbies&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>&quot;hiking&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;reading&quot;</span><span class=p>]</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <p>When accessing an invalid index (i.e., an index greater than or equal to the array size), the JSON array is resized such that the passed index is the new maximal index. Intermediate values are filled with <code class=highlight><span class=kc>null</span><span class=w></span></code>.</p> <details class=example> <summary>Filling up arrays with <code class=highlight><span class=kc>null</span><span class=w></span></code> values</summary> <div class=highlight><pre><span></span><code><span class=n>j</span><span class=p>[</span><span class=s>&quot;hobbies&quot;</span><span class=p>][</span><span class=mi>0</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>&quot;running&quot;</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;hobbies&quot;</span><span class=p>][</span><span class=mi>3</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>&quot;cooking&quot;</span><span class=p>;</span><span class=w></span>
</code></pre></div> <p>This code produces the following JSON value:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;John Smith&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;maidenName&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Jones&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;age&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;hobbies&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>&quot;running&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;reading&quot;</span><span class=p>,</span><span class=w> </span><span class=kc>null</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;cooking&quot;</span><span class=p>]</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <div class="admonition info"> <p class=admonition-title>Design rationale</p> <p>The library behaves differently to <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=w></span></code> and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span><span class=w></span></code>:</p> <ul> <li><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>::</span><span class=k>operator</span><span class=p>[]</span><span class=w></span></code> never inserts a new element.</li> <li><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span><span class=o>::</span><span class=k>operator</span><span class=p>[]</span><span class=w></span></code> is not available for const values.</li> </ul> <p>The type <code class=highlight><span class=n>json</span><span class=w></span></code> wraps all JSON value types. It would be impossible to remove <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> for const objects. At the same time, inserting elements for non-const objects is really convenient as it avoids awkward <code>insert</code> calls. To this end, we decided to have an inserting non-const behavior for both arrays and objects.</p> </div> <div class="admonition info"> <p class=admonition-title>Info</p> <p>The access is unchecked. In case the passed object key does not exist or the passed array index is invalid, no exception is thrown.</p> </div> <div class="admonition danger"> <p class=admonition-title>Danger</p> <ul> <li>It is <strong>undefined behavior</strong> to access a const object with a non-existing key.</li> <li>It is <strong>undefined behavior</strong> to access a const array with an invalid index.</li> <li>In debug mode, an <strong>assertion</strong> will fire in both cases. You can disable assertions by defining the preprocessor symbol <code class=highlight><span class=n>NDEBUG</span><span class=w></span></code> or redefine the macro <a href=../../macros/#json_assertx><code>JSON_ASSERT(x)</code></a>. See the documentation on <a href=../../assertions/ >runtime assertions</a> for more information.</li> </ul> </div> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <p><code>operator[]</code> can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a <a href=../../../home/exceptions/#jsonexceptiontype_error305><code>basic_json::type_error</code></a> is thrown.</p> </div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span><span class=w></span></code> value is returned</td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> <tr> <td>access to invalid array index</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span><span class=w></span></code> value is returned; any index between previous maximal index and passed index are filled with <code class=highlight><span class=kc>null</span><span class=w></span></code></td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> </tbody> </table> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Element Access" rel=prev> <div class="md-footer__button md-icon"> <svg xmlLine truncated
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<span class=k>enum</span><span class=w> </span><span class=nc>TaskState</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>TS_STOPPED</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=n>TS_RUNNING</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=n>TS_COMPLETED</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=n>TS_INVALID</span><span class=o>=</span><span class=mi>-1</span><span class=p>,</span><span class=w></span>
<span class=p>};</span><span class=w></span>
<span class=c1>// map TaskState values to JSON as strings</span>
<span class=n>NLOHMANN_JSON_SERIALIZE_ENUM</span><span class=p>(</span><span class=w> </span><span class=n>TaskState</span><span class=p>,</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=n>TS_INVALID</span><span class=p>,</span><span class=w> </span><span class=k>nullptr</span><span class=p>},</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=n>TS_STOPPED</span><span class=p>,</span><span class=w> </span><span class=s>&quot;stopped&quot;</span><span class=p>},</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=n>TS_RUNNING</span><span class=p>,</span><span class=w> </span><span class=s>&quot;running&quot;</span><span class=p>},</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=n>TS_COMPLETED</span><span class=p>,</span><span class=w> </span><span class=s>&quot;completed&quot;</span><span class=p>},</span><span class=w></span>
<span class=p>})</span><span class=w></span>
</code></pre></div> <p>The <a href=../../api/macros/nlohmann_json_serialize_enum/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM()</code> macro</a> declares a set of <code>to_json()</code> / <code>from_json()</code> functions for type <code>TaskState</code> while avoiding repetition and boilerplate serialization code.</p> <h2 id=usage>Usage<a class=headerlink href=#usage title="Permanent link">&para;</a></h2> <div class=highlight><pre><span></span><code><span class=c1>// enum to JSON as string</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>TS_STOPPED</span><span class=p>;</span><span class=w></span>
<span class=n>assert</span><span class=p>(</span><span class=n>j</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=s>&quot;stopped&quot;</span><span class=p>);</span><span class=w></span>
<span class=c1>// json string to enum</span>
<span class=n>json</span><span class=w> </span><span class=n>j3</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>&quot;running&quot;</span><span class=p>;</span><span class=w></span>
<span class=n>assert</span><span class=p>(</span><span class=n>j3</span><span class=p>.</span><span class=n>get</span><span class=o>&lt;</span><span class=n>TaskState</span><span class=o>&gt;</span><span class=p>()</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>TS_RUNNING</span><span class=p>);</span><span class=w></span>
<span class=c1>// undefined json value to enum (where the first map entry above is the default)</span>
<span class=n>json</span><span class=w> </span><span class=n>jPi</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mf>3.14</span><span class=p>;</span><span class=w></span>
<span class=n>assert</span><span class=p>(</span><span class=n>jPi</span><span class=p>.</span><span class=n>get</span><span class=o>&lt;</span><span class=n>TaskState</span><span class=o>&gt;</span><span class=p>()</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>TS_INVALID</span><span class=w> </span><span class=p>);</span><span class=w></span>
</code></pre></div> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <p>Just as in <a href=../arbitrary_types/ >Arbitrary Type Conversions</a> above,</p> <ul> <li><a href=../../api/macros/nlohmann_json_serialize_enum/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM()</code></a> MUST be declared in your enum type's namespace (which can be the global namespace), or the library will not be able to locate it, and it will default to integer serialization.</li> <li>It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.</li> </ul> <p>Other Important points:</p> <ul> <li>When using <code>get&lt;ENUM_TYPE&gt;()</code>, undefined JSON values will default to the first pair specified in your map. Select this default pair carefully.</li> <li>If an enum or JSON value is specified more than once in your map, the first matching occurrence from the top of the map will be returned when converting to or from JSON.</li> <li>To disable the default serialization of enumerators as integers and force a compiler error instead, see <a href=../../api/macros/json_disable_enum_serialization/ ><code>JSON_DISABLE_ENUM_SERIALIZATION</code></a>.</li> </ul> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../assertions/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Runtime Assertions" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> Runtime Assertions </div> </div> </a> <a href=../macros/ class="md-footer__link md-footer__link--next" aria-label="Next: Supported Macros" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Supported Macros </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013 - 2022 Niels Lohmann </div> </div> <div class=md-social> <a href=https://github.com/nlohmann target=_blank rel=noopener title=github.com class=md-social__link> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 496 512"><!-- Font Awesome Free 6.2.1 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc.--><path d="M165.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6zm-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3zm44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9zM244.8 8C106.1 8 0 113.3 0 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C428.2 457.8 496 362.9 496 252 496 113.3 383.5 8 244.8 8zM97.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1zm-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7zm32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1zm-11.4-14.7c-1.6 1-1.6 3.6 0 5.9 1.6 2.3 4.3 3.3 5.6 2.3 1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2z"/></svg> </a> <a href=https://twitter.com/nlohmann target=_blank rel=noopener title=twitter.com class=mLine truncated
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<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;one&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>1</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;two&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>2</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;three&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>3</span><span class=p>;</span><span class=w></span>
<span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>begin</span><span class=p>();</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>!=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>end</span><span class=p>();</span><span class=w> </span><span class=o>++</span><span class=n>it</span><span class=p>)</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=o>*</span><span class=n>it</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=mi>1</span><span class=w></span>
<span class=mi>3</span><span class=w></span>
<span class=mi>2</span><span class=w></span>
</code></pre></div> <p>The reason for the order is the lexicographic ordering of the object keys "one", "three", "two".</p> </details> <h3 id=access-object-key-during-iteration>Access object key during iteration<a class=headerlink href=#access-object-key-during-iteration title="Permanent link">&para;</a></h3> <p>The JSON iterators have two member functions, <code>key()</code> and <code>value()</code> to access the object key and stored value, respectively. When calling <code>key()</code> on a non-object iterator, an <a href=../../home/exceptions/#jsonexceptioninvalid_iterator207>invalid_iterator.207</a> exception is thrown.</p> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=c1>// create JSON object {&quot;one&quot;: 1, &quot;two&quot;: 2, &quot;three&quot;: 3}</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;one&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>1</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;two&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>2</span><span class=p>;</span><span class=w></span>
<span class=n>j</span><span class=p>[</span><span class=s>&quot;three&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>3</span><span class=p>;</span><span class=w></span>
<span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>begin</span><span class=p>();</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>!=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>end</span><span class=p>();</span><span class=w> </span><span class=o>++</span><span class=n>it</span><span class=p>)</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>it</span><span class=p>.</span><span class=n>key</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot; : &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>it</span><span class=p>.</span><span class=n>value</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=err>o</span><span class=kc>ne</span><span class=w> </span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=w></span>
<span class=kc>t</span><span class=err>hree</span><span class=w> </span><span class=p>:</span><span class=w> </span><span class=mi>3</span><span class=w></span>
<span class=kc>t</span><span class=err>wo</span><span class=w> </span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=w></span>
</code></pre></div> </details> <h3 id=range-based-for-loops>Range-based for loops<a class=headerlink href=#range-based-for-loops title="Permanent link">&para;</a></h3> <p>C++11 allows using range-based for loops to iterate over a container.</p> <div class=highlight><pre><span></span><code><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>j_object</span><span class=p>)</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// &quot;it&quot; is of type json::reference and has no key() member</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;value: &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>For this reason, the <code>items()</code> function allows accessing <code>iterator::key()</code> and <code>iterator::value()</code> during range-based for loops. In these loops, a reference to the JSON values is returned, so there is no access to the underlying iterator.</p> <div class=highlight><pre><span></span><code><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>el</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>j_object</span><span class=p>.</span><span class=n>items</span><span class=p>())</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;key: &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>el</span><span class=p>.</span><span class=n>key</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;, value:&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>el</span><span class=p>.</span><span class=n>value</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>The items() function also allows using structured bindings (C++17):</p> <div class=highlight><pre><span></span><code><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=p>[</span><span class=n>key</span><span class=p>,</span><span class=w> </span><span class=n>val</span><span class=p>]</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>j_object</span><span class=p>.</span><span class=n>items</span><span class=p>())</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;key: &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>key</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;, value:&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>val</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <div class="admonition note"> <p class=admonition-title>Note</p> <p>When iterating over an array, <code>key()</code> will return the index of the element as string. For primitive types (e.g., numbers), <code>key()</code> returns an empty string.</p> </div> <div class="admonition warning"> <p class=admonition-title>Warning</p> <p>Using <code>items()</code> on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See <a href=https://github.com/nlohmann/json/issues/2040>https://github.com/nlohmann/json/issues/2040</a> for more information.</p> </div> <h3 id=reverse-iteration-order>Reverse iteration order<a class=headerlink href=#reverse-iteration-order title="Permanent link">&para;</a></h3> <p><code>rbegin()</code> and <code>rend()</code> return iterators in the reverse sequence.</p> <p><img alt="Illustration from cppreference.com" src=../../images/range-rbegin-rend.svg></p> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>,</span><span class=w> </span><span class=mi>4</span><span class=p>};</span><span class=w></span>
<span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>rbegin</span><span class=p>();</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>!=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>rend</span><span class=p>();</span><span class=w> </span><span class=o>++</span><span class=n>it</span><span class=p>)</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=o>*</span><span class=n>it</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=mi>4</span><span class=w></span>
<span class=mi>3</span><span class=w></span>
<span class=mi>2</span><span class=w></span>
<span class=mi>1</span><span class=w></span>
</code></pre></div> </details> <h3 id=iterating-strings-and-binary-values>Iterating strings and binary values<a class=headerlink href=#iterating-strings-and-binary-values title="Permanent link">&para;</a></h3> <p>Note that "value" means a JSON value in this setting, not values stored in the underlying containers. That is, <code>*begin()</code> returns the complete string or binary array and is also safe the underlying string or binary array is empty.</p> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>&quot;Hello, world&quot;</span><span class=p>;</span><span class=w></span>
<span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>begin</span><span class=p>();</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>!=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>end</span><span class=p>();</span><span class=w> </span><span class=o>++</span><span class=n>it</span><span class=p>)</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=o>*</span><span class=n>it</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=s2>&quot;Hello, world&quot;</span><span class=w></span>
</code></pre></div> </details> <h2 id=iterator-invalidation>Iterator invalidation<a class=headerlink href=#iterator-invalidation title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th>Operations</th> <th>invalidated iterators</th> </tr> </thead> <tbody> <tr> <td><code>clear</code></td> <td>all</td> </tr> </tbody> </table> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../element_access/default_value/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Access with default value: value" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> Access with default value: value </div> </div> </a> <a href=../json_pointer/ class="md-footer__link md-footer__link--next" aria-label="Next: JSON Pointer" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> JSON Pointer </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href=https://json.nlohmann.me/features/json_patch/ rel=canonical><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.4.2, mkdocs-material-8.5.10"><title>JSON Patch and Diff - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.975780f9.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.2505c338.min.css><link rel=preconnect href=https://fonts.gstatic.com crossorigin><link rel=stylesheet href="https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,700,700i%7CJetBrains+Mono:400,400i,700,700i&display=fallback"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=k>namespace</span><span class=w> </span><span class=nn>nlohmann</span><span class=o>::</span><span class=nn>literals</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// the original document</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>doc</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s></span>
<span class=s> {</span>
<span class=s> &quot;baz&quot;: &quot;qux&quot;,</span>
<span class=s> &quot;foo&quot;: &quot;bar&quot;</span>
<span class=s> }</span>
<span class=s> </span><span class=dl>)</span><span class=s>&quot;</span><span class=n>_json</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// the patch</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>patch</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s></span>
<span class=s> [</span>
<span class=s> { &quot;op&quot;: &quot;replace&quot;, &quot;path&quot;: &quot;/baz&quot;, &quot;value&quot;: &quot;boo&quot; },</span>
<span class=s> { &quot;op&quot;: &quot;add&quot;, &quot;path&quot;: &quot;/hello&quot;, &quot;value&quot;: [&quot;world&quot;] },</span>
<span class=s> { &quot;op&quot;: &quot;remove&quot;, &quot;path&quot;: &quot;/foo&quot;}</span>
<span class=s> ]</span>
<span class=s> </span><span class=dl>)</span><span class=s>&quot;</span><span class=n>_json</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// apply the patch</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>patched_doc</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>patch</span><span class=p>(</span><span class=n>patch</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// output original and patched document</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>doc</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;</span><span class=se>\n\n</span><span class=s>&quot;</span><span class=w></span>
<span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>patched_doc</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;baz&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;qux&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;foo&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;bar&quot;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;baz&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;boo&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;hello&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=w></span>
<span class=w> </span><span class=s2>&quot;world&quot;</span><span class=w></span>
<span class=w> </span><span class=p>]</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <h2 id=diff>Diff<a class=headerlink href=#diff title="Permanent link">&para;</a></h2> <p>The library can also calculate a JSON patch (i.e., a <strong>diff</strong>) given two JSON values.</p> <div class="admonition success"> <p class=admonition-title>Invariant</p> <p>For two JSON values <em>source</em> and <em>target</em>, the following code yields always true:</p> <div class=highlight><pre><span></span><code>source.patch(diff(source, target)) == target;
</code></pre></div> </div> <details class=example> <summary>Example</summary> <p>The following code shows how a JSON patch is created as a diff for two JSON values.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=k>namespace</span><span class=w> </span><span class=nn>nlohmann</span><span class=o>::</span><span class=nn>literals</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// the source document</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>source</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s></span>
<span class=s> {</span>
<span class=s> &quot;baz&quot;: &quot;qux&quot;,</span>
<span class=s> &quot;foo&quot;: &quot;bar&quot;</span>
<span class=s> }</span>
<span class=s> </span><span class=dl>)</span><span class=s>&quot;</span><span class=n>_json</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// the target document</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>target</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s></span>
<span class=s> {</span>
<span class=s> &quot;baz&quot;: &quot;boo&quot;,</span>
<span class=s> &quot;hello&quot;: [</span>
<span class=s> &quot;world&quot;</span>
<span class=s> ]</span>
<span class=s> }</span>
<span class=s> </span><span class=dl>)</span><span class=s>&quot;</span><span class=n>_json</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// create the patch</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>patch</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>diff</span><span class=p>(</span><span class=n>source</span><span class=p>,</span><span class=w> </span><span class=n>target</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// roundtrip</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>patched_source</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>source</span><span class=p>.</span><span class=n>patch</span><span class=p>(</span><span class=n>patch</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// output patch and roundtrip result</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>patch</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;</span><span class=se>\n\n</span><span class=s>&quot;</span><span class=w></span>
<span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>patched_source</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>[</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;op&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;replace&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;path&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;/baz&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;value&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;boo&quot;</span><span class=w></span>
<span class=w> </span><span class=p>},</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;op&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;remove&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;path&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;/foo&quot;</span><span class=w></span>
<span class=w> </span><span class=p>},</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;op&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;add&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;path&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;/hello&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;value&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=w></span>
<span class=w> </span><span class=s2>&quot;world&quot;</span><span class=w></span>
<span class=w> </span><span class=p>]</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>]</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;baz&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;boo&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;hello&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=w></span>
<span class=w> </span><span class=s2>&quot;world&quot;</span><span class=w></span>
<span class=w> </span><span class=p>]</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../json_pointer/ class="md-footer__link md-footer__link--prev" aria-label="Previous: JSON Pointer" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> JSON Pointer </div> </div> </a> <a href=../merge_patch/ class="md-footer__link md-footer__link--next" aria-label="Next: JSON Merge Patch" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> JSON Merge Patch </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href=https://json.nlohmann.me/features/json_pointer/ rel=canonical><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.4.2, mkdocs-material-8.5.10"><title>JSON Pointer - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.975780f9.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.2505c338.min.css><link rel=preconnect href=https://fonts.gstatic.com crossorigin><link rel=stylesheet href="https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,700,700i%7CJetBrains+Mono:400,400i,700,700i&display=fallback"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54Z"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3V6m0 5h18v2H3v-2m0 5h18v2H3v-2Z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> JSON Pointer </span> </div> </div> </div> <form class=md-header__option 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<span class=w> </span><span class=nt>&quot;array&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>&quot;A&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;B&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;C&quot;</span><span class=p>],</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;nested&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;one&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;two&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=kc>true</span><span class=p>,</span><span class=w> </span><span class=kc>false</span><span class=p>]</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Then every value inside the JSON document can be identified as follows:</p> <table> <thead> <tr> <th>JSON Pointer</th> <th>JSON value</th> </tr> </thead> <tbody> <tr> <td>``</td> <td><code class=highlight><span class=p>{</span><span class=nt>&quot;array&quot;</span><span class=p>:[</span><span class=s2>&quot;A&quot;</span><span class=p>,</span><span class=s2>&quot;B&quot;</span><span class=p>,</span><span class=s2>&quot;C&quot;</span><span class=p>],</span><span class=nt>&quot;nested&quot;</span><span class=p>:{</span><span class=nt>&quot;one&quot;</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>&quot;two&quot;</span><span class=p>:</span><span class=mi>2</span><span class=p>,</span><span class=nt>&quot;three&quot;</span><span class=p>:[</span><span class=kc>true</span><span class=p>,</span><span class=kc>false</span><span class=p>]}}</span><span class=w></span></code></td> </tr> <tr> <td><code>/array</code></td> <td><code class=highlight><span class=p>[</span><span class=s2>&quot;A&quot;</span><span class=p>,</span><span class=s2>&quot;B&quot;</span><span class=p>,</span><span class=s2>&quot;C&quot;</span><span class=p>]</span><span class=w></span></code></td> </tr> <tr> <td><code>/array/0</code></td> <td><code class=highlight><span class=err>A</span><span class=w></span></code></td> </tr> <tr> <td><code>/array/1</code></td> <td><code class=highlight><span class=err>B</span><span class=w></span></code></td> </tr> <tr> <td><code>/array/2</code></td> <td><code class=highlight><span class=err>C</span><span class=w></span></code></td> </tr> <tr> <td><code>/nested</code></td> <td><code class=highlight><span class=p>{</span><span class=nt>&quot;one&quot;</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>&quot;two&quot;</span><span class=p>:</span><span class=mi>2</span><span class=p>,</span><span class=nt>&quot;three&quot;</span><span class=p>:[</span><span class=kc>true</span><span class=p>,</span><span class=kc>false</span><span class=p>]}</span><span class=w></span></code></td> </tr> <tr> <td><code>/nested/one</code></td> <td><code class=highlight><span class=mi>1</span><span class=w></span></code></td> </tr> <tr> <td><code>/nested/two</code></td> <td><code class=highlight><span class=mi>2</span><span class=w></span></code></td> </tr> <tr> <td><code>/nested/three</code></td> <td><code class=highlight><span class=p>[</span><span class=kc>true</span><span class=p>,</span><span class=kc>false</span><span class=p>]</span><span class=w></span></code></td> </tr> <tr> <td><code>/nested/three/0</code></td> <td><code class=highlight><span class=kc>true</span><span class=w></span></code></td> </tr> <tr> <td><code>/nested/three/1</code></td> <td><code class=highlight><span class=kc>false</span><span class=w></span></code></td> </tr> </tbody> </table> <p>Note <code>/</code> does not identify the root (i.e., the whole document), but an object entry with empty key <code>""</code>. See <a href=https://tools.ietf.org/html/rfc6901>RFC 6901</a> for more information.</p> <h2 id=json-pointer-creation>JSON Pointer creation<a class=headerlink href=#json-pointer-creation title="Permanent link">&para;</a></h2> <p>JSON Pointers can be created from a string:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=o>::</span><span class=n>json_pointer</span><span class=w> </span><span class=n>p</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>&quot;/nested/one&quot;</span><span class=p>;</span><span class=w></span>
</code></pre></div> <p>Furthermore, a user-defined string literal can be used to achieve the same result:</p> <div class=highlight><pre><span></span><code><span class=k>auto</span><span class=w> </span><span class=n>p</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>&quot;/nested/one&quot;</span><span class=n>_json_pointer</span><span class=p>;</span><span class=w></span>
</code></pre></div> <p>The escaping rules of <a href=https://tools.ietf.org/html/rfc6901>RFC 6901</a> are implemented. See the <a href=../../api/json_pointer/json_pointer/ >constructor documentation</a> for more information.</p> <h2 id=value-access>Value access<a class=headerlink href=#value-access title="Permanent link">&para;</a></h2> <p>JSON Pointers can be used in the <a href=../../api/basic_json/at/ ><code>at</code></a>, <a href=../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a>, and <a href=../../api/basic_json/value/ ><code>value</code></a> functions just like object keys or array indices.</p> <div class=highlight><pre><span></span><code><span class=c1>// the JSON value from above</span>
<span class=k>auto</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{</span>
<span class=s> &quot;array&quot;: [&quot;A&quot;, &quot;B&quot;, &quot;C&quot;],</span>
<span class=s> &quot;nested&quot;: {</span>
<span class=s> &quot;one&quot;: 1,</span>
<span class=s> &quot;two&quot;: 2,</span>
<span class=s> &quot;three&quot;: [true, false]</span>
<span class=s> }</span>
<span class=s>}</span><span class=dl>)</span><span class=s>&quot;</span><span class=p>);</span><span class=w></span>
<span class=c1>// access values</span>
<span class=k>auto</span><span class=w> </span><span class=n>val</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>&quot;/&quot;</span><span class=n>_json_pointer</span><span class=p>];</span><span class=w> </span><span class=c1>// {&quot;array&quot;:[&quot;A&quot;,&quot;B&quot;,&quot;C&quot;],...}</span>
<span class=k>auto</span><span class=w> </span><span class=n>val1</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>&quot;/nested/one&quot;</span><span class=n>_json_pointer</span><span class=p>];</span><span class=w> </span><span class=c1>// 1</span>
<span class=k>auto</span><span class=w> </span><span class=n>val2</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>[</span><span class=n>json</span><span class=o>::</span><span class=n>json_pointer</span><span class=p>(</span><span class=s>&quot;/nested/three/1&quot;</span><span class=p>)];</span><span class=w> </span><span class=c1>// false</span>
<span class=k>auto</span><span class=w> </span><span class=n>val3</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>[</span><span class=n>json</span><span class=o>::</span><span class=n>json_pointer</span><span class=p>(</span><span class=s>&quot;/nested/four&quot;</span><span class=p>,</span><span class=w> </span><span class=mi>0</span><span class=p>)];</span><span class=w> </span><span class=c1>// 0</span>
</code></pre></div> <h2 id=flatten-unflatten>Flatten / unflatten<a class=headerlink href=#flatten-unflatten title="Permanent link">&para;</a></h2> <p>The library implements a function <a href=../../api/basic_json/flatten/ ><code>flatten</code></a> to convert any JSON document into a JSON object where each key is a JSON Pointer and each value is a primitive JSON value (i.e., a string, boolean, number, or null).</p> <div class=highlight><pre><span></span><code><span class=c1>// the JSON value from above</span>
<span class=k>auto</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{</span>
<span class=s> &quot;array&quot;: [&quot;A&quot;, &quot;B&quot;, &quot;C&quot;],</span>
<span class=s> &quot;nested&quot;: {</span>
<span class=s> &quot;one&quot;: 1,</span>
<span class=s> &quot;two&quot;: 2,</span>
<span class=s> &quot;three&quot;: [true, false]</span>
<span class=s> }</span>
<span class=s>}</span><span class=dl>)</span><span class=s>&quot;</span><span class=p>);</span><span class=w></span>
<span class=c1>// create flattened value</span>
<span class=k>auto</span><span class=w> </span><span class=n>j_flat</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>flatten</span><span class=p>();</span><span class=w></span>
</code></pre></div> <p>The resulting value <code>j_flat</code> is:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;/array/0&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;A&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;/array/1&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;B&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;/array/2&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;C&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;/nested/one&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;/nested/two&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;/nested/three/0&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;/nested/three/1&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>false</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>The reverse function, <a href=../../api/basic_json/unflatten/ ><code>unflatten</code></a> recreates the original value.</p> <div class=highlight><pre><span></span><code><span class=k>auto</span><span class=w> </span><span class=n>j_original</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j_flat</span><span class=p>.</span><span class=n>unflatten</span><span class=p>();</span><span class=w></span>
</code></pre></div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li>Class <a href=../../api/json_pointer/ ><code>json_pointer</code></a></li> <li>Function <a href=../../api/basic_json/flatten/ ><code>flatten</code></a></li> <li>Function <a href=../../api/basic_json/unflatten/ ><code>unflatten</code></a></li> <li><a href=../json_patch/ >JSON Patch</a></li> </ul> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../iterators/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Iterators" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> Iterators </div> </div> </a> <a href=../json_patch/ class="md-footer__link md-footer__link--next" aria-label="Next: JSON Patch and Diff" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> JSON Patch and Diff </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=c1> // for std::setw</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=k>namespace</span><span class=w> </span><span class=nn>nlohmann</span><span class=o>::</span><span class=nn>literals</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// the original document</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>document</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{</span>
<span class=s> &quot;title&quot;: &quot;Goodbye!&quot;,</span>
<span class=s> &quot;author&quot;: {</span>
<span class=s> &quot;givenName&quot;: &quot;John&quot;,</span>
<span class=s> &quot;familyName&quot;: &quot;Doe&quot;</span>
<span class=s> },</span>
<span class=s> &quot;tags&quot;: [</span>
<span class=s> &quot;example&quot;,</span>
<span class=s> &quot;sample&quot;</span>
<span class=s> ],</span>
<span class=s> &quot;content&quot;: &quot;This will be unchanged&quot;</span>
<span class=s> }</span><span class=dl>)</span><span class=s>&quot;</span><span class=n>_json</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// the patch</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>patch</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{</span>
<span class=s> &quot;title&quot;: &quot;Hello!&quot;,</span>
<span class=s> &quot;phoneNumber&quot;: &quot;+01-123-456-7890&quot;,</span>
<span class=s> &quot;author&quot;: {</span>
<span class=s> &quot;familyName&quot;: null</span>
<span class=s> },</span>
<span class=s> &quot;tags&quot;: [</span>
<span class=s> &quot;example&quot;</span>
<span class=s> ]</span>
<span class=s> }</span><span class=dl>)</span><span class=s>&quot;</span><span class=n>_json</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// apply the patch</span>
<span class=w> </span><span class=n>document</span><span class=p>.</span><span class=n>merge_patch</span><span class=p>(</span><span class=n>patch</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=c1>// output original and patched document</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>document</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;author&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;givenName&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;John&quot;</span><span class=w></span>
<span class=w> </span><span class=p>},</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;content&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;This will be unchanged&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;phoneNumber&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;+01-123-456-7890&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;tags&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=w></span>
<span class=w> </span><span class=s2>&quot;example&quot;</span><span class=w></span>
<span class=w> </span><span class=p>],</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;title&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Hello!&quot;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../json_patch/ class="md-footer__link md-footer__link--prev" aria-label="Previous: JSON Patch and Diff" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> JSON Patch and Diff </div> </div> </a> <a href=../namespace/ class="md-footer__link md-footer__link--next" aria-label="Next: nlohmann Namespace" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> nlohmann Namespace </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> <h2 id=purpose>Purpose<a class=headerlink href=#purpose title="Permanent link">&para;</a></h2> <p>Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different definitions of <a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>. This is illustrated in the diagram below.</p> <p><img src="data:image/svg+xml;base64,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 truncated
<span class=cp>#define NLOHMANN_JSON_NAMESPACE_END }</span>
</code></pre></div> <div class="admonition danger"> <p class=admonition-title>Use at your own risk</p> <p>Overriding the namespace and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!</p> </div> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Introduced inline namespace (<code>json_v3_11_0[_abi-tag]*</code>) in version 3.11.0.</li> <li>Changed structure of inline namespace in version 3.11.2.</li> </ul> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../merge_patch/ class="md-footer__link md-footer__link--prev" aria-label="Previous: JSON Merge Patch" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> JSON Merge Patch </div> </div> </a> <a href=../object_order/ class="md-footer__link md-footer__link--next" aria-label="Next: Object Order" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Object Order </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013 - 2022 Niels Lohmann </div> </div> <div class=md-social> <a href=https://github.com/nlohmann target=_blank rel=noopener title=github.com class=md-social__link> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 496 512"><!-- Font Awesome Free 6.2.1 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc.--><path d="M165.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6zm-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3zm44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9zM244.8 8C106.1 8 0 113.3 0 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C428.2 457.8 496 362.9 496 252 496 113.3 383.5 8 244.8 8zM97.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1zm-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7zm32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1zm-11.4-14.7c-1.6 1-1.6 3.6 0 5.9 1.6 2.3 4.3 3.3 5.6 2.3 1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2z"/></svg> </a> <a href=https://twitter.com/nlohmann target=_blank rel=noopener title=twitter.com class=md-social__link> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 6.2.1 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc.--><path d="M459.37 151.716c.325 4.548.325 9.097.325 13.645 0 138.72-105.583 298.558-298.558 298.558-59.452 0-114.68-17.219-161.137-47.106 8.447.974 16.568 1.299 25.34 1.299 49.055 0 94.213-16.568 130.274-44.832-46.132-.975-84.792-31.188-98.112-72.772 6.498.974 12.995 1.624 19.818 1.624 9.421 0 18.843-1.3 27.614-3.573-48.081-9.747-84.143-51.98-84.143-102.985v-1.299c13.969 7.797 30.214 12.67 47.Line truncated
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href=https://json.nlohmann.me/features/object_order/ rel=canonical><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.4.2, mkdocs-material-8.5.10"><title>Object Order - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.975780f9.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.2505c338.min.css><link rel=preconnect href=https://fonts.gstatic.com crossorigin><link rel=stylesheet href="https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,700,700i%7CJetBrains+Mono:400,400i,700,700i&display=fallback"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54Z"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3V6m0 5h18v2H3v-2m0 5h18v2H3v-2Z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Object Order </span> </div> </div> </div> <form class=md-header__option 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&quot;json.hpp&quot;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>&quot;one&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>1</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>&quot;two&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>2</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>&quot;three&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>3</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;one&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>3</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;two&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <h2 id=alternative-behavior-preserve-insertion-order>Alternative behavior: preserve insertion order<a class=headerlink href=#alternative-behavior-preserve-insertion-order title="Permanent link">&para;</a></h2> <p>If you do want to preserve the <strong>insertion order</strong>, you can try the type <a href=https://github.com/nlohmann/json/issues/2179><code>nlohmann::ordered_json</code></a>.</p> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>ordered_json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>ordered_json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>ordered_json</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>&quot;one&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>1</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>&quot;two&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>2</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>&quot;three&quot;</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>3</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;one&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;two&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>3</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <p>Alternatively, you can use a more sophisticated ordered map like <a href=https://github.com/Tessil/ordered-map><code>tsl::ordered_map</code></a> (<a href=https://github.com/nlohmann/json/issues/546#issuecomment-304447518>integration</a>) or <a href=https://github.com/nlohmann/fifo_map><code>nlohmann::fifo_map</code></a> (<a href=https://github.com/nlohmann/json/issues/485#issuecomment-333652309>integration</a>).</p> <h3 id=notes-on-parsing>Notes on parsing<a class=headerlink href=#notes-on-parsing title="Permanent link">&para;</a></h3> <p>Note that you also need to call the right <a href=../../api/basic_json/parse/ ><code>parse</code></a> function when reading from a file. Assume file <code>input.json</code> contains the JSON object above:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;one&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;two&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>3</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <div class="admonition success"> <p class=admonition-title>Right way</p> <p>The following code correctly calls the <code>parse</code> function from <code>nlohmann::ordered_json</code>:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>i</span><span class=p>(</span><span class=s>&quot;input.json&quot;</span><span class=p>);</span><span class=w></span>
<span class=k>auto</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>ordered_json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>i</span><span class=p>);</span><span class=w></span>
<span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
</code></pre></div> <p>The output will be:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;one&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;two&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>3</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </div> <details class=failure> <summary>Wrong way</summary> <p>The following code incorrectly calls the <code>parse</code> function from <code>nlohmann::json</code> which does not preserve the insertion order, but sorts object keys. Assigning the result to <code>nlohmann::ordered_json</code> compiles, but does not restore the order from the input file.</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>i</span><span class=p>(</span><span class=s>&quot;input.json&quot;</span><span class=p>);</span><span class=w></span>
<span class=n>nlohmann</span><span class=o>::</span><span class=n>ordered_json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>i</span><span class=p>);</span><span class=w></span>
<span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>2</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
</code></pre></div> <p>The output will be:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;one&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>3</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;two&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../namespace/ class="md-footer__link md-footer__link--prev" aria-label="Previous: nlohmann Namespace" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> nlohmann Namespace </div> </div> </a> <a href=../parsing/ class="md-footer__link md-footer__link--next" aria-label="Next: Parsing" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Parsing </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Alexa&quot;</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;wins&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>&quot;two pair&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;4♠&quot;</span><span class=p>],</span><span class=w> </span><span class=p>[</span><span class=s2>&quot;two pair&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;9♠&quot;</span><span class=p>]]}</span><span class=w></span>
<span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;May&quot;</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;wins&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[]}</span><span class=w></span>
<span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Deloise&quot;</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;wins&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>&quot;three of a kind&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;5♣&quot;</span><span class=p>]]}</span><span class=w></span>
</code></pre></div> </div> <p>JSON Lines input with more than one value is treated as invalid JSON by the <a href=../../../api/basic_json/parse/ ><code>parse</code></a> or <a href=../../../api/basic_json/accept/ ><code>accept</code></a> functions. To process it line by line, functions like <a href=https://en.cppreference.com/w/cpp/string/basic_string/getline><code>std::getline</code></a> can be used:</p> <div class="admonition example"> <p class=admonition-title>Example: Parse JSON Text input line by line</p> <p>The example below demonstrates how JSON Lines can be processed.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;sstream&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// JSON Lines (see https://jsonlines.org)</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>stringstream</span><span class=w> </span><span class=n>input</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>input</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{&quot;name&quot;: &quot;Gilbert&quot;, &quot;wins&quot;: [[&quot;straight&quot;, &quot;7♣&quot;], [&quot;one pair&quot;, &quot;10♥&quot;]]}</span>
<span class=s>{&quot;name&quot;: &quot;Alexa&quot;, &quot;wins&quot;: [[&quot;two pair&quot;, &quot;4♠&quot;], [&quot;two pair&quot;, &quot;9♠&quot;]]}</span>
<span class=s>{&quot;name&quot;: &quot;May&quot;, &quot;wins&quot;: []}</span>
<span class=s>{&quot;name&quot;: &quot;Deloise&quot;, &quot;wins&quot;: [[&quot;three of a kind&quot;, &quot;5♣&quot;]]}</span>
<span class=dl>)</span><span class=s>&quot;</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>line</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=k>while</span><span class=w> </span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=n>getline</span><span class=p>(</span><span class=n>input</span><span class=p>,</span><span class=w> </span><span class=n>line</span><span class=p>))</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>line</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=s2>&quot;Gilbert&quot;</span><span class=p>,</span><span class=nt>&quot;wins&quot;</span><span class=p>:[[</span><span class=s2>&quot;straight&quot;</span><span class=p>,</span><span class=s2>&quot;7♣&quot;</span><span class=p>],[</span><span class=s2>&quot;one pair&quot;</span><span class=p>,</span><span class=s2>&quot;10♥&quot;</span><span class=p>]]}</span><span class=w></span>
<span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=s2>&quot;Alexa&quot;</span><span class=p>,</span><span class=nt>&quot;wins&quot;</span><span class=p>:[[</span><span class=s2>&quot;two pair&quot;</span><span class=p>,</span><span class=s2>&quot;4♠&quot;</span><span class=p>],[</span><span class=s2>&quot;two pair&quot;</span><span class=p>,</span><span class=s2>&quot;9♠&quot;</span><span class=p>]]}</span><span class=w></span>
<span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=s2>&quot;May&quot;</span><span class=p>,</span><span class=nt>&quot;wins&quot;</span><span class=p>:[]}</span><span class=w></span>
<span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=s2>&quot;Deloise&quot;</span><span class=p>,</span><span class=nt>&quot;wins&quot;</span><span class=p>:[[</span><span class=s2>&quot;three of a kind&quot;</span><span class=p>,</span><span class=s2>&quot;5♣&quot;</span><span class=p>]]}</span><span class=w></span>
</code></pre></div> </div> <div class="admonition warning"> <p class=admonition-title>Note</p> <p>Using <a href=../../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> like</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w></span>
<span class=k>while</span><span class=w> </span><span class=p>(</span><span class=n>input</span><span class=w> </span><span class=o>&gt;&gt;</span><span class=w> </span><span class=n>j</span><span class=p>)</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>with a JSON Lines input does not work, because the parser will try to parse one value after the last one.</p> </div> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Parsing" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> Parsing </div> </div> </a> <a href=../parse_exceptions/ class="md-footer__link md-footer__link--next" aria-label="Next: Parsing and Exceptions" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Parsing and Exceptions </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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autocomplete=off spellcheck=false data-md-componLine truncated
<span class=k>try</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>my_input</span><span class=p>);</span><span class=w></span>
<span class=p>}</span><span class=w></span>
<span class=k>catch</span><span class=w> </span><span class=p>(</span><span class=n>json</span><span class=o>::</span><span class=n>parse_error</span><span class=o>&amp;</span><span class=w> </span><span class=n>ex</span><span class=p>)</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;parse error at byte &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>ex</span><span class=p>.</span><span class=n>byte</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>In case exceptions are undesired or not supported by the environment, there are different ways to proceed:</p> <h2 id=switch-off-exceptions>Switch off exceptions<a class=headerlink href=#switch-off-exceptions title="Permanent link">&para;</a></h2> <p>The <code>parse()</code> function accepts a <code class=highlight><span class=kt>bool</span><span class=w></span></code> parameter <code>allow_exceptions</code> which controls whether an exception is thrown when a parse error occurs (<code class=highlight><span class=nb>true</span><span class=w></span></code>, default) or whether a discarded value should be returned (<code class=highlight><span class=nb>false</span><span class=w></span></code>).</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>my_input</span><span class=p>,</span><span class=w> </span><span class=k>nullptr</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>);</span><span class=w></span>
<span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>j</span><span class=p>.</span><span class=n>is_discarded</span><span class=p>())</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;parse error&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Note there is no diagnostic information available in this scenario.</p> <h2 id=use-accept-function>Use accept() function<a class=headerlink href=#use-accept-function title="Permanent link">&para;</a></h2> <p>Alternatively, function <code>accept()</code> can be used which does not return a <code>json</code> value, but a <code class=highlight><span class=kt>bool</span><span class=w></span></code> indicating whether the input is valid JSON.</p> <div class=highlight><pre><span></span><code><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=o>!</span><span class=n>json</span><span class=o>::</span><span class=n>accept</span><span class=p>(</span><span class=n>my_input</span><span class=p>))</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;parse error&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Again, there is no diagnostic information available.</p> <h2 id=user-defined-sax-interface>User-defined SAX interface<a class=headerlink href=#user-defined-sax-interface title="Permanent link">&para;</a></h2> <p>Finally, you can implement the <a href=../sax_interface/ >SAX interface</a> and decide what should happen in case of a parse error.</p> <p>This function has the following interface:</p> <div class=highlight><pre><span></span><code><span class=kt>bool</span><span class=w> </span><span class=nf>parse_error</span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=kt>size_t</span><span class=w> </span><span class=n>position</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>&amp;</span><span class=w> </span><span class=n>last_token</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>exception</span><span class=o>&amp;</span><span class=w> </span><span class=n>ex</span><span class=p>);</span><span class=w></span>
</code></pre></div> <p>The return value indicates whether the parsing should continue, so the function should usually return <code class=highlight><span class=nb>false</span><span class=w></span></code>.</p> <details class=example> <summary>Example</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&quot;json.hpp&quot;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=k>class</span><span class=w> </span><span class=nc>sax_no_exception</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=k>public</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>detail</span><span class=o>::</span><span class=n>json_sax_dom_parser</span><span class=o>&lt;</span><span class=n>json</span><span class=o>&gt;</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>public</span><span class=o>:</span><span class=w></span>
<span class=w> </span><span class=n>sax_no_exception</span><span class=p>(</span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>detail</span><span class=o>::</span><span class=n>json_sax_dom_parser</span><span class=o>&lt;</span><span class=n>json</span><span class=o>&gt;</span><span class=p>(</span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{}</span><span class=w></span>
<span class=w> </span><span class=kt>bool</span><span class=w> </span><span class=n>parse_error</span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=kt>size_t</span><span class=w> </span><span class=n>position</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>&amp;</span><span class=w> </span><span class=n>last_token</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>exception</span><span class=o>&amp;</span><span class=w> </span><span class=n>ex</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;parse error at input byte &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>position</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;</span><span class=se>\n</span><span class=s>&quot;</span><span class=w></span>
<span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>ex</span><span class=p>.</span><span class=n>what</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;</span><span class=se>\n</span><span class=s>&quot;</span><span class=w></span>
<span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;last read: </span><span class=se>\&quot;</span><span class=s>&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>last_token</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;</span><span class=se>\&quot;</span><span class=s>&quot;</span><span class=w></span>
<span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=nb>false</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>};</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>myinput</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>&quot;[1,2,3,]&quot;</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>result</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>sax_no_exception</span><span class=w> </span><span class=n>sax</span><span class=p>(</span><span class=n>result</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=kt>bool</span><span class=w> </span><span class=n>parse_result</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>sax_parse</span><span class=p>(</span><span class=n>myinput</span><span class=p>,</span><span class=w> </span><span class=o>&amp;</span><span class=n>sax</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=o>!</span><span class=n>parse_result</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;parsing unsuccessful!&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;parsed value: &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>result</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>parse error at input byte 8
[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected &#39;]&#39;; expected &#39;[&#39;, &#39;{&#39;, or a literal
last read: &quot;3,]&quot;
parsing unsuccessful!
parsed value: [1,2,3]
</code></pre></div> </details> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../json_lines/ class="md-footer__link md-footer__link--prev" aria-label="Previous: JSON Lines" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> JSON Lines </div> </div> </a> <a href=../parser_callbacks/ class="md-footer__link md-footer__link--next" aria-label="Next: Parser Callbacks" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Parser Callbacks </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<span class=k>using</span><span class=w> </span><span class=n>parser_callback_t</span><span class=w> </span><span class=o>=</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>function</span><span class=o>&lt;</span><span class=kt>bool</span><span class=p>(</span><span class=kt>int</span><span class=w> </span><span class=n>depth</span><span class=p>,</span><span class=w> </span><span class=n>parse_event_t</span><span class=w> </span><span class=n>event</span><span class=p>,</span><span class=w> </span><span class=n>BasicJsonType</span><span class=o>&amp;</span><span class=w> </span><span class=n>parsed</span><span class=p>)</span><span class=o>&gt;</span><span class=p>;</span><span class=w></span>
</code></pre></div> <h2 id=callback-event-types>Callback event types<a class=headerlink href=#callback-event-types title="Permanent link">&para;</a></h2> <p>We distinguish six scenarios (determined by the event type) in which the callback function can be called. The following table describes the values of the parameters <code>depth</code>, <code>event</code>, and <code>parsed</code>.</p> <table> <thead> <tr> <th>parameter <code>event</code></th> <th>description</th> <th>parameter <code>depth</code></th> <th>parameter <code>parsed</code></th> </tr> </thead> <tbody> <tr> <td><code>parse_event_t::object_start</code></td> <td>the parser read <code>{</code> and started to process a JSON object</td> <td>depth of the parent of the JSON object</td> <td>a JSON value with type discarded</td> </tr> <tr> <td><code>parse_event_t::key</code></td> <td>the parser read a key of a value in an object</td> <td>depth of the currently parsed JSON object</td> <td>a JSON string containing the key</td> </tr> <tr> <td><code>parse_event_t::object_end</code></td> <td>the parser read <code>}</code> and finished processing a JSON object</td> <td>depth of the parent of the JSON object</td> <td>the parsed JSON object</td> </tr> <tr> <td><code>parse_event_t::array_start</code></td> <td>the parser read <code>[</code> and started to process a JSON array</td> <td>depth of the parent of the JSON array</td> <td>a JSON value with type discarded</td> </tr> <tr> <td><code>parse_event_t::array_end</code></td> <td>the parser read <code>]</code> and finished processing a JSON array</td> <td>depth of the parent of the JSON array</td> <td>the parsed JSON array</td> </tr> <tr> <td><code>parse_event_t::value</code></td> <td>the parser finished reading a JSON value</td> <td>depth of the value</td> <td>the parsed JSON value</td> </tr> </tbody> </table> <details class=example> <summary>Example</summary> <p>When parsing the following JSON text,</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Berlin&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;location&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=w></span>
<span class=w> </span><span class=mf>52.519444</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mf>13.406667</span><span class=w></span>
<span class=w> </span><span class=p>]</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>these calls are made to the callback function:</p> <table> <thead> <tr> <th>event</th> <th>depth</th> <th>parsed</th> </tr> </thead> <tbody> <tr> <td><code>object_start</code></td> <td>0</td> <td><em>discarded</em></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>&quot;name&quot;</span><span class=w></span></code></td> </tr> <tr> <td><code>value</code></td> <td>1</td> <td><code class=highlight><span class=s2>&quot;Berlin&quot;</span><span class=w></span></code></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>&quot;location&quot;</span><span class=w></span></code></td> </tr> <tr> <td><code>array_start</code></td> <td>1</td> <td><em>discarded</em></td> </tr> <tr> <td><code>value</code></td> <td>2</td> <td><code class=highlight><span class=mf>52.519444</span><span class=w></span></code></td> </tr> <tr> <td><code>value</code></td> <td>2</td> <td><code class=highlight><span class=mf>13.406667</span><span class=w></span></code></td> </tr> <tr> <td><code>array_end</code></td> <td>1</td> <td><code class=highlight><span class=p>[</span><span class=mf>52.519444</span><span class=p>,</span><span class=mf>13.406667</span><span class=p>]</span><span class=w></span></code></td> </tr> <tr> <td><code>object_end</code></td> <td>0</td> <td><code class=highlight><span class=p>{</span><span class=nt>&quot;location&quot;</span><span class=p>:[</span><span class=mf>52.519444</span><span class=p>,</span><span class=mf>13.406667</span><span class=p>],</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=s2>&quot;Berlin&quot;</span><span class=p>}</span><span class=w></span></code></td> </tr> </tbody> </table> </details> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">&para;</a></h2> <p>Discarding a value (i.e., returning <code class=highlight><span class=nb>false</span><span class=w></span></code>) has different effects depending on the context in which the function was called:</p> <ul> <li>Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read.</li> <li>In case a value outside a structured type is skipped, it is replaced with <code class=highlight><span class=kc>null</span><span class=w></span></code>. This case happens if the top-level element is skipped.</li> </ul> <details class=example> <summary>Example</summary> <p>The example below demonstrates the <code>parse()</code> function with and without callback function.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span><span class=cp></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// a JSON text</span>
<span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>text</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s></span>
<span class=s> {</span>
<span class=s> &quot;Image&quot;: {</span>
<span class=s> &quot;Width&quot;: 800,</span>
<span class=s> &quot;Height&quot;: 600,</span>
<span class=s> &quot;Title&quot;: &quot;View from 15th Floor&quot;,</span>
<span class=s> &quot;Thumbnail&quot;: {</span>
<span class=s> &quot;Url&quot;: &quot;http://www.example.com/image/481989943&quot;,</span>
<span class=s> &quot;Height&quot;: 125,</span>
<span class=s> &quot;Width&quot;: 100</span>
<span class=s> },</span>
<span class=s> &quot;Animated&quot; : false,</span>
<span class=s> &quot;IDs&quot;: [116, 943, 234, 38793]</span>
<span class=s> }</span>
<span class=s> }</span>
<span class=s> </span><span class=dl>)</span><span class=s>&quot;</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// parse and serialize JSON</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_complete</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>text</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j_complete</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;</span><span class=se>\n\n</span><span class=s>&quot;</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=c1>// define parser callback</span>
<span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parser_callback_t</span><span class=w> </span><span class=n>cb</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>[](</span><span class=kt>int</span><span class=w> </span><span class=n>depth</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse_event_t</span><span class=w> </span><span class=n>event</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>&amp;</span><span class=w> </span><span class=n>parsed</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// skip object elements with key &quot;Thumbnail&quot;</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>event</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse_event_t</span><span class=o>::</span><span class=n>key</span><span class=w> </span><span class=n>and</span><span class=w> </span><span class=n>parsed</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>json</span><span class=p>(</span><span class=s>&quot;Thumbnail&quot;</span><span class=p>))</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=nb>false</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=k>else</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=nb>true</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=p>};</span><span class=w></span>
<span class=w> </span><span class=c1>// parse (with callback) and serialize JSON</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_filtered</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>text</span><span class=p>,</span><span class=w> </span><span class=n>cb</span><span class=p>);</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j_filtered</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Image&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Animated&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>false</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Height&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>600</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;IDs&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=w></span>
<span class=w> </span><span class=mi>116</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mi>943</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mi>234</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mi>38793</span><span class=w></span>
<span class=w> </span><span class=p>],</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Thumbnail&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Height&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>125</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Url&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;http://www.example.com/image/481989943&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>100</span><span class=w></span>
<span class=w> </span><span class=p>},</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Title&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;View from 15th Floor&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>800</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>}</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Image&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Animated&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>false</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Height&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>600</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;IDs&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=w></span>
<span class=w> </span><span class=mi>116</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mi>943</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mi>234</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=mi>38793</span><span class=w></span>
<span class=w> </span><span class=p>],</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Title&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;View from 15th Floor&quot;</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>800</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> </details> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../parse_exceptions/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Parsing and Exceptions" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> Parsing and Exceptions </div> </div> </a> <a href=../sax_interface/ class="md-footer__link md-footer__link--next" aria-label="Next: SAX Interface" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> SAX Interface </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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autocomplete=off spellcheck=false data-md-component=search-query required> <laLine truncated
<span class=kt>bool</span><span class=w> </span><span class=nf>null</span><span class=p>();</span><span class=w></span>
<span class=c1>// called when a boolean is parsed; value is passed</span>
<span class=kt>bool</span><span class=w> </span><span class=nf>boolean</span><span class=p>(</span><span class=kt>bool</span><span class=w> </span><span class=n>val</span><span class=p>);</span><span class=w></span>
<span class=c1>// called when a signed or unsigned integer number is parsed; value is passed</span>
<span class=kt>bool</span><span class=w> </span><span class=nf>number_integer</span><span class=p>(</span><span class=n>number_integer_t</span><span class=w> </span><span class=n>val</span><span class=p>);</span><span class=w></span>
<span class=kt>bool</span><span class=w> </span><span class=nf>number_unsigned</span><span class=p>(</span><span class=n>number_unsigned_t</span><span class=w> </span><span class=n>val</span><span class=p>);</span><span class=w></span>
<span class=c1>// called when a floating-point number is parsed; value and original string is passed</span>
<span class=kt>bool</span><span class=w> </span><span class=nf>number_float</span><span class=p>(</span><span class=n>number_float_t</span><span class=w> </span><span class=n>val</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>string_t</span><span class=o>&amp;</span><span class=w> </span><span class=n>s</span><span class=p>);</span><span class=w></span>
<span class=c1>// called when a string is parsed; value is passed and can be safely moved away</span>
<span class=kt>bool</span><span class=w> </span><span class=nf>string</span><span class=p>(</span><span class=n>string_t</span><span class=o>&amp;</span><span class=w> </span><span class=n>val</span><span class=p>);</span><span class=w></span>
<span class=c1>// called when a binary value is parsed; value is passed and can be safely moved away</span>
<span class=kt>bool</span><span class=w> </span><span class=nf>binary</span><span class=p>(</span><span class=n>binary</span><span class=o>&amp;</span><span class=w> </span><span class=n>val</span><span class=p>);</span><span class=w></span>
<span class=c1>// called when an object or array begins or ends, resp. The number of elements is passed (or -1 if not known)</span>
<span class=kt>bool</span><span class=w> </span><span class=nf>start_object</span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=kt>size_t</span><span class=w> </span><span class=n>elements</span><span class=p>);</span><span class=w></span>
<span class=kt>bool</span><span class=w> </span><span class=nf>end_object</span><span class=p>();</span><span class=w></span>
<span class=kt>bool</span><span class=w> </span><span class=nf>start_array</span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=kt>size_t</span><span class=w> </span><span class=n>elements</span><span class=p>);</span><span class=w></span>
<span class=kt>bool</span><span class=w> </span><span class=nf>end_array</span><span class=p>();</span><span class=w></span>
<span class=c1>// called when an object key is parsed; value is passed and can be safely moved away</span>
<span class=kt>bool</span><span class=w> </span><span class=nf>key</span><span class=p>(</span><span class=n>string_t</span><span class=o>&amp;</span><span class=w> </span><span class=n>val</span><span class=p>);</span><span class=w></span>
<span class=c1>// called when a parse error occurs; byte position, the last token, and an exception is passed</span>
<span class=kt>bool</span><span class=w> </span><span class=nf>parse_error</span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=kt>size_t</span><span class=w> </span><span class=n>position</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>&amp;</span><span class=w> </span><span class=n>last_token</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>exception</span><span class=o>&amp;</span><span class=w> </span><span class=n>ex</span><span class=p>);</span><span class=w></span>
</code></pre></div> <p>The return value of each function determines whether parsing should proceed.</p> <p>To implement your own SAX handler, proceed as follows:</p> <ol> <li>Implement the SAX interface in a class. You can use class <code>nlohmann::json_sax&lt;json&gt;</code> as base class, but you can also use any class where the functions described above are implemented and public.</li> <li>Create an object of your SAX interface class, e.g. <code>my_sax</code>.</li> <li>Call <code class=highlight><span class=kt>bool</span><span class=w> </span><span class=nf>json::sax_parse</span><span class=p>(</span><span class=n>input</span><span class=p>,</span><span class=w> </span><span class=o>&amp;</span><span class=n>my_sax</span><span class=p>);</span><span class=w></span></code> where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your SAX interface.</li> </ol> <p>Note the <code>sax_parse</code> function only returns a <code class=highlight><span class=kt>bool</span><span class=w></span></code> indicating the result of the last executed SAX event. It does not return <code>json</code> value - it is up to you to decide what to do with the SAX events. Furthermore, no exceptions are thrown in case of a parse error - it is up to you what to do with the exception object passed to your <code>parse_error</code> implementation. Internally, the SAX interface is used for the DOM parser (class <code>json_sax_dom_parser</code>) as well as the acceptor (<code>json_sax_acceptor</code>), see file <code>json_sax.hpp</code>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../../api/json_sax/ >json_sax</a> - documentation of the SAX interface</li> <li><a href=../../../api/basic_json/sax_parse/ >sax_parse</a> - SAX parser</li> </ul> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../parser_callbacks/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Parser Callbacks" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> Parser Callbacks </div> </div> </a> <a href=../../assertions/ class="md-footer__link md-footer__link--next" aria-label="Next: Runtime Assertions" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Runtime Assertions </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013 - 2022 Niels Lohmann </div> </div> <div class=md-social> <a href=https://github.com/nlohmann target=_blank rel=noopener title=github.com class=md-social__link> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 496 512"><!-- Font Awesome Free 6.2.1 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc.--><path d="M165.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6zm-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3zm44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9zM244.8 8C106.1 8 0 113.3 0 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 6Line truncated
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href=https://json.nlohmann.me/features/types/ rel=canonical><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.4.2, mkdocs-material-8.5.10"><title>Types - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.975780f9.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.2505c338.min.css><link rel=preconnect href=https://fonts.gstatic.com crossorigin><link rel=stylesheet href="https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,700,700i%7CJetBrains+Mono:400,400i,700,700i&display=fallback"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54Z"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3V6m0 5h18v2H3v-2m0 5h18v2H3v-2Z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Types </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option 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<span class=w> </span><span class=k>template</span><span class=o>&lt;</span><span class=k>typename</span><span class=w> </span><span class=nc>U</span><span class=p>,</span><span class=w> </span><span class=k>typename</span><span class=w> </span><span class=nc>V</span><span class=p>,</span><span class=w> </span><span class=k>typename</span><span class=p>...</span><span class=w> </span><span class=n>Args</span><span class=o>&gt;</span><span class=w> </span><span class=k>class</span><span class=w> </span><span class=nc>ObjectType</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>map</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=k>template</span><span class=o>&lt;</span><span class=k>typename</span><span class=w> </span><span class=nc>U</span><span class=p>,</span><span class=w> </span><span class=k>typename</span><span class=p>...</span><span class=w> </span><span class=n>Args</span><span class=o>&gt;</span><span class=w> </span><span class=k>class</span><span class=w> </span><span class=nc>ArrayType</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=k>class</span><span class=w> </span><span class=nc>StringType</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=k>class</span><span class=w> </span><span class=nc>BooleanType</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=kt>bool</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=k>class</span><span class=w> </span><span class=nc>NumberIntegerType</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=kt>int64_t</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=k>class</span><span class=w> </span><span class=nc>NumberUnsignedType</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=k>class</span><span class=w> </span><span class=nc>NumberFloatType</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=kt>double</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=k>template</span><span class=o>&lt;</span><span class=k>typename</span><span class=w> </span><span class=nc>U</span><span class=o>&gt;</span><span class=w> </span><span class=k>class</span><span class=w> </span><span class=nc>AllocatorType</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>allocator</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=k>template</span><span class=o>&lt;</span><span class=k>typename</span><span class=w> </span><span class=nc>T</span><span class=p>,</span><span class=w> </span><span class=k>typename</span><span class=w> </span><span class=nc>SFINAE</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=kt>void</span><span class=o>&gt;</span><span class=w> </span><span class=k>class</span><span class=w> </span><span class=nc>JSONSerializer</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>adl_serializer</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=k>class</span><span class=w> </span><span class=nc>BinaryType</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w></span>
<span class=o>&gt;</span><span class=w></span>
<span class=k>class</span><span class=w> </span><span class=nc>basic_json</span><span class=p>;</span><span class=w></span>
</code></pre></div> <p>Type <code>json</code> is an alias for <code>basic_json&lt;&gt;</code> and uses the default types.</p> <p>From the template arguments, the following types are derived:</p> <div class=highlight><pre><span></span><code><span class=k>using</span><span class=w> </span><span class=n>object_comparator_t</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>less</span><span class=o>&lt;&gt;</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=n>object_t</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>ObjectType</span><span class=o>&lt;</span><span class=n>StringType</span><span class=p>,</span><span class=w> </span><span class=n>basic_json</span><span class=p>,</span><span class=w> </span><span class=n>object_comparator_t</span><span class=p>,</span><span class=w></span>
<span class=w> </span><span class=n>AllocatorType</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=n>pair</span><span class=o>&lt;</span><span class=k>const</span><span class=w> </span><span class=n>StringType</span><span class=p>,</span><span class=w> </span><span class=n>basic_json</span><span class=o>&gt;&gt;&gt;</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=n>array_t</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>ArrayType</span><span class=o>&lt;</span><span class=n>basic_json</span><span class=p>,</span><span class=w> </span><span class=n>AllocatorType</span><span class=o>&lt;</span><span class=n>basic_json</span><span class=o>&gt;&gt;</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=n>string_t</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>StringType</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=n>boolean_t</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>BooleanType</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=n>number_integer_t</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>NumberIntegerType</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=n>number_unsigned_t</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>NumberUnsignedType</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=n>number_float_t</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>NumberFloatType</span><span class=p>;</span><span class=w></span>
<span class=k>using</span><span class=w> </span><span class=n>binary_t</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>byte_container_with_subtype</span><span class=o>&lt;</span><span class=n>BinaryType</span><span class=o>&gt;</span><span class=p>;</span><span class=w></span>
</code></pre></div> <h2 id=objects>Objects<a class=headerlink href=#objects title="Permanent link">&para;</a></h2> <p><a href=https://tools.ietf.org/html/rfc8259>RFC 8259</a> describes JSON objects as follows:</p> <blockquote> <p>An object is an unordered collection of zero or more name/value pairs, where a name is a string and a value is a string, number, boolean, null, object, or array.</p> </blockquote> <h3 id=default-type>Default type<a class=headerlink href=#default-type title="Permanent link">&para;</a></h3> <p>With the default values for <em>ObjectType</em> (<code>std::map</code>), <em>StringType</em> (<code>std::string</code>), and <em>AllocatorType</em> (<code>std::allocator</code>), the default value for <code>object_t</code> is:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>map</span><span class=o>&lt;</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=p>,</span><span class=w> </span><span class=c1>// key_type</span>
<span class=w> </span><span class=n>basic_json</span><span class=p>,</span><span class=w> </span><span class=c1>// value_type</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>less</span><span class=o>&lt;&gt;</span><span class=p>,</span><span class=w> </span><span class=c1>// key_compare</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>allocator</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=n>pair</span><span class=o>&lt;</span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=p>,</span><span class=w> </span><span class=n>basic_json</span><span class=o>&gt;&gt;</span><span class=w> </span><span class=c1>// allocator_type</span>
<span class=o>&gt;</span><span class=w></span>
</code></pre></div> <h3 id=behavior>Behavior<a class=headerlink href=#behavior title="Permanent link">&para;</a></h3> <p>The choice of <code>object_t</code> influences the behavior of the JSON class. With the default type, objects have the following behavior:</p> <ul> <li>When all names are unique, objects will be interoperable in the sense that all software implementations receiving that object will agree on the name-value mappings.</li> <li>When the names within an object are not unique, it is unspecified which one of the values for a given key will be chosen. For instance, <code class=highlight><span class=p>{</span><span class=nt>&quot;key&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;key&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span><span class=w></span></code> could be equal to either <code class=highlight><span class=p>{</span><span class=nt>&quot;key&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span><span class=w></span></code> or <code class=highlight><span class=p>{</span><span class=nt>&quot;key&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>}</span><span class=w></span></code>.</li> <li>Internally, name/value pairs are stored in lexicographical order of the names. Objects will also be serialized (see <code>dump</code>) in this order. For instance, both <code class=highlight><span class=p>{</span><span class=nt>&quot;b&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;a&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>}</span><span class=w></span></code> and <code class=highlight><span class=p>{</span><span class=nt>&quot;a&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;b&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span><span class=w></span></code> will be stored and serialized as <code class=highlight><span class=p>{</span><span class=nt>&quot;a&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;b&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span><span class=w></span></code>.</li> <li>When comparing objects, the order of the name/value pairs is irrelevant. This makes objects interoperable in the sense that they will not be affected by these differences. For instance, <code class=highlight><span class=p>{</span><span class=nt>&quot;b&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;a&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>}</span><span class=w></span></code> and <code class=highlight><span class=p>{</span><span class=nt>&quot;a&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;b&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span><span class=w></span></code> will be treated as equal.</li> </ul> <h3 id=key-order>Key order<a class=headerlink href=#key-order title="Permanent link">&para;</a></h3> <p>The order name/value pairs are added to the object is <em>not</em> preserved by the library. Therefore, iterating an object may return name/value pairs in a different order than they were originally stored. In fact, keys will be traversed in alphabetical order as <code>std::map</code> with <code>std::less</code> is used by default. Please note this behavior conforms to <a href=https://tools.ietf.org/html/rfc8259>RFC 8259</a>, because any order implements the specified "unordered" nature of JSON objects.</p> <h3 id=limits>Limits<a class=headerlink href=#limits title="Permanent link">&para;</a></h3> <p><a href=https://tools.ietf.org/html/rfc8259>RFC 8259</a> specifies:</p> <blockquote> <p>An implementation may set limits on the maximum depth of nesting.</p> </blockquote> <p>In this class, the object's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the <code>max_size</code> function of a JSON object.</p> <h3 id=storage_1>Storage<a class=headerlink href=#storage_1 title="Permanent link">&para;</a></h3> <p>Objects are stored as pointers in a <code>basic_json</code> type. That is, for any access to object values, a pointer of type <code>object_t*</code> must be dereferenced.</p> <h2 id=arrays>Arrays<a class=headerlinLine truncated
<span class=w> </span><span class=n>basic_json</span><span class=p>,</span><span class=w> </span><span class=c1>// value_type</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>allocator</span><span class=o>&lt;</span><span class=n>basic_json</span><span class=o>&gt;</span><span class=w> </span><span class=c1>// allocator_type</span>
<span class=o>&gt;</span><span class=w></span>
</code></pre></div> <h3 id=limits_1>Limits<a class=headerlink href=#limits_1 title="Permanent link">&para;</a></h3> <p><a href=https://tools.ietf.org/html/rfc8259>RFC 8259</a> specifies:</p> <blockquote> <p>An implementation may set limits on the maximum depth of nesting.</p> </blockquote> <p>In this class, the array's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the <code>max_size</code> function of a JSON array.</p> <h3 id=storage_2>Storage<a class=headerlink href=#storage_2 title="Permanent link">&para;</a></h3> <p>Arrays are stored as pointers in a <code>basic_json</code> type. That is, for any access to array values, a pointer of type <code>array_t*</code> must be dereferenced.</p> <h2 id=strings>Strings<a class=headerlink href=#strings title="Permanent link">&para;</a></h2> <p><a href=https://tools.ietf.org/html/rfc8259>RFC 8259</a> describes JSON strings as follows:</p> <blockquote> <p>A string is a sequence of zero or more Unicode characters.</p> </blockquote> <p>Unicode values are split by the JSON class into byte-sized characters during deserialization.</p> <h3 id=default-type_2>Default type<a class=headerlink href=#default-type_2 title="Permanent link">&para;</a></h3> <p>With the default values for <em>StringType</em> (<code>std::string</code>), the default value for <code>string_t</code> is <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w></span></code>.</p> <h3 id=encoding>Encoding<a class=headerlink href=#encoding title="Permanent link">&para;</a></h3> <p>Strings are stored in UTF-8 encoding. Therefore, functions like <code>std::string::size()</code> or <code>std::string::length()</code> return the number of <strong>bytes</strong> in the string rather than the number of characters or glyphs.</p> <h3 id=string-comparison>String comparison<a class=headerlink href=#string-comparison title="Permanent link">&para;</a></h3> <p><a href=https://tools.ietf.org/html/rfc8259>RFC 8259</a> states:</p> <blockquote> <p>Software implementations are typically required to test names of object members for equality. Implementations that transform the textual representation into sequences of Unicode code units and then perform the comparison numerically, code unit by code unit, are interoperable in the sense that implementations will agree in all cases on equality or inequality of two strings. For example, implementations that compare strings with escaped characters unconverted may incorrectly find that <code>"a\\b"</code> and <code>"a\u005Cb"</code> are not equal.</p> </blockquote> <p>This implementation is interoperable as it does compare strings code unit by code unit.</p> <h3 id=storage_3>Storage<a class=headerlink href=#storage_3 title="Permanent link">&para;</a></h3> <p>String values are stored as pointers in a <code>basic_json</code> type. That is, for any access to string values, a pointer of type <code>string_t*</code> must be dereferenced.</p> <h2 id=booleans>Booleans<a class=headerlink href=#booleans title="Permanent link">&para;</a></h2> <p><a href=https://tools.ietf.org/html/rfc8259>RFC 8259</a> implicitly describes a boolean as a type which differentiates the two literals <code>true</code> and <code>false</code>.</p> <h3 id=default-type_3>Default type<a class=headerlink href=#default-type_3 title="Permanent link">&para;</a></h3> <p>With the default values for <em>BooleanType</em> (<code class=highlight><span class=kt>bool</span><span class=w></span></code>), the default value for <code>boolean_t</code> is <code class=highlight><span class=kt>bool</span><span class=w></span></code>.</p> <h3 id=storage_4>Storage<a class=headerlink href=#storage_4 title="Permanent link">&para;</a></h3> <p>Boolean values are stored directly inside a <code>basic_json</code> type.</p> <h2 id=numbers>Numbers<a class=headerlink href=#numbers title="Permanent link">&para;</a></h2> <p>See the <a href=number_handling/ >number handling</a> article for a detailed discussion on how numbers are handled by this library.</p> <p><a href=https://tools.ietf.org/html/rfc8259>RFC 8259</a> describes numbers as follows:</p> <blockquote> <p>The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.</p> </blockquote> <p>This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is known whether the number is a signed integer, an unsigned integer or a floating-point number. Therefore, three different types, <code>numbLine truncated
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autocomplete=off spellcheck=false data-md-component=search-query requireLine truncated
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>f</span><span class=p>;</span><span class=w></span>
<span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span><span class=p>;</span><span class=w></span>
</code></pre></div> yields <code class=highlight><span class=mf>0.30000001192092896</span><span class=w></span></code>.</li> </ul> <p>All examples here can be reproduced by passing the original double value to</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>printf</span><span class=p>(</span><span class=s>&quot;%.*g</span><span class=se>\n</span><span class=s>&quot;</span><span class=p>,</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>numeric_limits</span><span class=o>&lt;</span><span class=kt>double</span><span class=o>&gt;::</span><span class=n>max_digits10</span><span class=p>,</span><span class=w> </span><span class=n>double_value</span><span class=p>);</span><span class=w></span>
</code></pre></div> </div> <h4 id=nan-handling>NaN handling<a class=headerlink href=#nan-handling title="Permanent link">&para;</a></h4> <p>NaN (not-a-number) cannot be expressed with the number syntax described above and are in fact explicitly excluded:</p> <div class="admonition quote"> <p class=admonition-title><a href=https://tools.ietf.org/html/rfc8259#section-6>RFC 8259</a>, Section 6</p> <p>Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.</p> </div> <p>That is, there is no way to <em>parse</em> a NaN value. However, NaN values can be stored in a JSON value by assignment.</p> <p>This library serializes NaN values as <code class=highlight><span class=kc>null</span><span class=w></span></code>. This corresponds to the behavior of JavaScript's <a href=https://www.w3schools.com/js/js_json_stringify.asp><code>JSON.stringify</code></a> function.</p> <div class="admonition example"> <p class=admonition-title>Example</p> <p>The following example shows how a NaN value is stored in a <code>json</code> value.</p> <div class=highlight><pre><span></span><code><span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=kt>double</span><span class=w> </span><span class=n>val</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>numeric_limits</span><span class=o>&lt;</span><span class=kt>double</span><span class=o>&gt;::</span><span class=n>quiet_NaN</span><span class=p>();</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;val=&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>val</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>val</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;j=&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>val</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w></span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;val=&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>val</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>output:</p> <div class=highlight><pre><span></span><code>val=nan
j=null
val=nan
</code></pre></div> </div> <h3 id=number-comparison>Number comparison<a class=headerlink href=#number-comparison title="Permanent link">&para;</a></h3> <p>Floating-point inside JSON values numbers are compared with <code class=highlight><span class=n>json</span><span class=o>::</span><span class=n>number_float_t</span><span class=o>::</span><span class=n>operator</span><span class=o>==</span><span class=w></span></code> which is <code class=highlight><span class=kt>double</span><span class=o>::</span><span class=n>operator</span><span class=o>==</span><span class=w></span></code> by default.</p> <div class="admonition example"> <p class=admonition-title>Alternative comparison functions</p> <p>To compare floating-point while respecting an epsilon, an alternative <a href=https://github.com/mariokonrad/marnav/blob/master/include/marnav/math/floatingpoint.hpp#L34-#L39>comparison function</a> could be used, for instance</p> <p><div class=highlight><pre><span></span><code><span class=k>template</span><span class=o>&lt;</span><span class=k>typename</span><span class=w> </span><span class=nc>T</span><span class=p>,</span><span class=w> </span><span class=k>typename</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=k>typename</span><span class=w> </span><span class=nc>std</span><span class=o>::</span><span class=n>enable_if</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=n>is_floating_point</span><span class=o>&lt;</span><span class=n>T</span><span class=o>&gt;::</span><span class=n>value</span><span class=p>,</span><span class=w> </span><span class=n>T</span><span class=o>&gt;::</span><span class=n>type</span><span class=o>&gt;</span><span class=w></span>
<span class=kr>inline</span><span class=w> </span><span class=kt>bool</span><span class=w> </span><span class=n>is_same</span><span class=p>(</span><span class=n>T</span><span class=w> </span><span class=n>a</span><span class=p>,</span><span class=w> </span><span class=n>T</span><span class=w> </span><span class=n>b</span><span class=p>,</span><span class=w> </span><span class=n>T</span><span class=w> </span><span class=n>epsilon</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>numeric_limits</span><span class=o>&lt;</span><span class=n>T</span><span class=o>&gt;::</span><span class=n>epsilon</span><span class=p>())</span><span class=w> </span><span class=k>noexcept</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>abs</span><span class=p>(</span><span class=n>a</span><span class=w> </span><span class=o>-</span><span class=w> </span><span class=n>b</span><span class=p>)</span><span class=w> </span><span class=o>&lt;=</span><span class=w> </span><span class=n>epsilon</span><span class=p>;</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> Or you can self-define an operator equal function like this:</p> <div class=highlight><pre><span></span><code><span class=kt>bool</span><span class=w> </span><span class=nf>my_equal</span><span class=p>(</span><span class=n>const_reference</span><span class=w> </span><span class=n>lhs</span><span class=p>,</span><span class=w> </span><span class=n>const_reference</span><span class=w> </span><span class=n>rhs</span><span class=p>)</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>lhs_type</span><span class=w> </span><span class=n>lhs</span><span class=p>.</span><span class=n>type</span><span class=p>();</span><span class=w></span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>rhs_type</span><span class=w> </span><span class=n>rhs</span><span class=p>.</span><span class=n>type</span><span class=p>();</span><span class=w></span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>lhs_type</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>rhs_type</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=k>switch</span><span class=p>(</span><span class=n>lhs_type</span><span class=p>)</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// self_defined case</span>
<span class=w> </span><span class=k>case</span><span class=w> </span><span class=no>value_t</span><span class=o>::</span><span class=no>number_float</span><span class=p>:</span><span class=w></span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>abs</span><span class=p>(</span><span class=n>lhs</span><span class=w> </span><span class=o>-</span><span class=w> </span><span class=n>rhs</span><span class=p>)</span><span class=w> </span><span class=o>&lt;=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>numeric_limits</span><span class=o>&lt;</span><span class=kt>float</span><span class=o>&gt;::</span><span class=n>epsilon</span><span class=p>();</span><span class=w></span>
<span class=w> </span><span class=c1>// other cases remain the same with the original</span>
<span class=w> </span><span class=p>...</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=p>...</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>(see <a href=https://github.com/nlohmann/json/issues/703>#703</a> for more information.)</p> </div> <div class="admonition note"> <p class=admonition-title>Note</p> <p>NaN values never compare equal to themselves or to other NaN values. See <a href=https://github.com/nlohmann/json/issues/514>#514</a>.</p> </div> <h3 id=number-conversion>Number conversion<a class=headerlink href=#number-conversion title="Permanent link">&para;</a></h3> <p>Just like the C++ language itself, the <code>get</code> family of functions allows conversions between unsigned and signed integers, and between integers and floating-point values to integers. This behavior may be surprising.</p> <div class="admonition warning"> <p class=admonition-title>Unconditional number conversions</p> <div class=highlight><pre><span></span><code><span class=kt>double</span><span class=w> </span><span class=n>d</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mf>42.3</span><span class=p>;</span><span class=w> </span><span class=c1>// non-integer double value 42.3</span>
<span class=n>json</span><span class=w> </span><span class=n>jd</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>d</span><span class=p>;</span><span class=w> </span><span class=c1>// stores double value 42.3</span>
<span class=hll><span class=n>std</span><span class=o>::</span><span class=kt>int64_t</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>jd</span><span class=p>.</span><span class=n>get</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>int64_t</span><span class=o>&gt;</span><span class=p>();</span><span class=w> </span><span class=c1>// now i==42; no warning or error is produced</span>
</span></code></pre></div> <p>Note the last line with throw a <a href=../../../home/exceptions/#jsonexceptiontype_error302><code>json.exception.type_error.302</code></a> exception if <code>jd</code> is not a numerical type, for instance a string.</p> </div> <p>The rationale is twofold:</p> <ol> <li>JSON does not define a number type or precision (see <a href=above>#json-specification</a>).</li> <li>C++ also allows to silently convert between number types.</li> </ol> <div class="admonition success"> <p class=admonition-title>Conditional number conversion</p> <p>The code above can be solved by explicitly checking the nature of the value with members such as <a href=../../../api/basic_json/is_number_integer/ ><code>is_number_integer()</code></a> or <a href=../../../api/basic_json/is_number_unsigned/ ><code>is_number_unsigned()</code></a>:</p> <div class=highlight><pre><span></span><code><span class=c1>// check if jd is really integer-valued</span>
<span class=hll><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>jd</span><span class=p>.</span><span class=n>is_number_integer</span><span class=p>())</span><span class=w></span>
</span><span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// if so, do the conversion and use i</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=kt>int64_t</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>jd</span><span class=p>.</span><span class=n>get</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>int64_t</span><span class=o>&gt;</span><span class=p>();</span><span class=w></span>
<span class=w> </span><span class=c1>// ...</span>
<span class=p>}</span><span class=w></span>
<span class=k>else</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// otherwise, take appropriate action</span>
<span class=w> </span><span class=c1>// ...</span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Note this approach also has the advantage that it can react on non-numerical JSON value types such as strings.</p> <p>(Example taken from <a href=https://github.com/nlohmann/json/issues/777#issuecomment-459968458>#777</a>.)</p> </div> <h3 id=determine-number-types>Determine number types<a class=headerlink href=#determine-number-types title="Permanent link">&para;</a></h3> <p>As the example in <a href=#number-conversion>Number conversion</a> shows, there are different functions to determine the type of the stored number:</p> <ul> <li><a href=../../../api/basic_json/is_number/ ><code>is_number()</code></a> returns <code class=highlight><span class=nb>true</span><span class=w></span></code> for any number type</li> <li><a href=../../../api/basic_json/is_number_integer/ ><code>is_number_integer()</code></a> returns <code class=highlight><span class=nb>true</span><span class=w></span></code> for signed and unsigned integers</li> <li><a href=../../../api/basic_json/is_number_unsigned/ ><code>is_number_unsigned()</code></a> returns <code class=highlight><span class=nb>true</span><span class=w></span></code> for unsigned integers only</li> <li><a href=../../../api/basic_json/is_number_float/ ><code>is_number_float()</code></a> returns <code class=highlight><span class=nb>true</span><span class=w></span></code> for floating-point numbers</li> <li><a href=../../../api/basic_json/type_name/ ><code>type_name()</code></a> returns <code class=highlight><span class=s>&quot;number&quot;</span><span class=w></span></code> for any number type</li> <li><a href=../../../api/basic_json/type/ ><code>type()</code></a> returns a different enumerator of <a href=../../../api/basic_json/value_t/ ><code>value_t</code></a> for all number types</li> </ul> <table> <thead> <tr> <th>function</th> <th>unsigned integer</th> <th>signed integer</th> <th>floating-point</th> <th>string</th> </tr> </thead> <tbody> <tr> <td><a href=../../../api/basic_json/is_number/ ><code>is_number()</code></a></td> <td><code class=highlight><span class=nb>true</span><span class=w></span></code></td> <td><code class=highlight><span class=nb>true</span><span class=w></span></code></td> <td><code class=highlight><span class=nb>true</span><span class=w></span></code></td> <td><code class=highlight><span class=nb>false</span><span class=w></span></code></td> </tr> <tr> <td><a href=../../../api/basic_json/is_number_integer/ ><code>is_number_integer()</code></a></td> <td><code class=highlight><span class=nb>true</span><span class=w></span></code></td> <td><code class=highlight><span class=nb>true</span><span class=w></span></code></td> <td><code class=highlight><span class=nb>false</span><span class=w></span></code></td> <td><code class=highlight><span class=nb>false</span><span class=w></span></code></td> </tr> <tr> <td><a href=../../../api/basic_json/is_number_unsigned/ ><code>is_number_unsigned()</code></a></td> <td><code class=highlight><span class=nb>true</span><span class=w></span></code></td> <td><code class=highlight><span class=nb>false</span><span class=w></span></code></td> <td><code class=highlight><span class=nb>false</span><span class=w></span></code></td> <td><code class=highlight><span class=nb>false</span><span class=w></span></code></td> </tr> <tr> <td><a href=../../../api/basic_json/is_number_float/ ><code>is_number_float()</code></a></td> <td><code class=highlight><span class=nb>false</span><span class=w></span></code></td> <td><code class=highlight><span class=nb>false</span><span class=w></span></code></td> <td><code class=highlight><span class=nb>true</span><span class=w></span></code></td> <td><code class=highlight><span class=nb>false</span><span class=w></span></code></td> </tr> <tr> <td><a href=../../../api/basic_json/type_name/ ><code>type_name()</code></a></td> <td><code class=highlight><span class=s>&quot;number&quot;</span><span class=w></span></code></td> <td><code class=highlight><span class=s>&quot;number&quot;</span><span class=w></span></code></td> <td><code class=highlight><span class=s>&quot;number&quot;</span><span class=w></span></code></td> <td><code class=highlight><span class=s>&quot;string&quot;</span><span class=w></span></code></td> </tr> <tr> <td><a href=../../../api/basic_json/type/ ><code>type()</code></a></td> <td><code>number_unsigned</code></td> <td><code>number_integer</code></td> <td><code>number_float</code></td> <td><code>string</code></td> </tr> </tbody> </table> <h3 id=template-number-types>Template number types<a class=headerlink href=#template-number-types title="Permanent link">&para;</a></h3> <p>The number types can be changed with template parameters.</p> <table> <thead> <tr> <th>position</th> <th>number type</th> <th>default type</th> <th>possible values</th> </tr> </thead> <tbody> <tr> <td>5</td> <td>signed integers</td> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>int64_t</span><span class=w></span></code></td> <td><code class=highlight><Line truncated
<span class=hll><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=kt>int64_t</span><span class=p>,</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</span><span class=p>,</span><span class=w> </span><span class=kt>long</span><span class=w> </span><span class=kt>double</span><span class=o>&gt;</span><span class=p>;</span><span class=w></span>
</span></code></pre></div> <p>Note values should then be parsed with <code>json_ld::parse</code> rather than <code>json::parse</code> as the latter would parse floating-point values to <code class=highlight><span class=kt>double</span><span class=w></span></code> before then converting them to <code class=highlight><span class=kt>long</span><span class=w> </span><span class=kt>double</span><span class=w></span></code>.</p> </div> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">March 8, 2023</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> </div> <a href=# class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z"/></svg> Back to top </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Types" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> Types </div> </div> </a> <a href=../../../integration/ class="md-footer__link md-footer__link--next" aria-label="Next: Header only" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> Header only </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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