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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/element_access/default_value/" rel="canonical"><link href=../checked_access/ rel=prev><link href=../../iterators/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Access with default value: value - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><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=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 3.54"/></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 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></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> Access with default value: value </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo 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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/element_access/default_value/" rel="canonical"><link href=../checked_access/ rel=prev><link href=../../iterators/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Access with default value: value - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#access-with-default-value-value class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header 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 3.54"/></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 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></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> Access with default value: value </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo 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6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 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<span class=w> </span><span class=nt>"logOutput"</span><span class=p>:</span><span class=w> </span><span class=s2>"result.log"</span><span class=p>,</span>
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<span class=w> </span><span class=nt>"append"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span>
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<span class=p>}</span>
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</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></code></td> <td><code class=highlight><span class=p>{</span><span class=nt>"logOutput"</span><span class=p>:</span><span class=w> </span><span class=s2>"result.log"</span><span class=p>,</span><span class=w> </span><span class=nt>"append"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>}</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>"logOutput"</span><span class=p>,</span><span class=w> </span><span class=s>"logfile.log"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"result.log"</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>"append"</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>)</span></code></td> <td><code class=highlight><span class=kc>true</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>"append"</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>)</span></code></td> <td><code class=highlight><span class=kc>true</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>"logLevel"</span><span class=p>,</span><span class=w> </span><span class=s>"verbose"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"verbose"</span></code></td> </tr> </tbody> </table> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <ul> <li>With string keys, <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> <li>With <abbr title="JavaScript Object Notation">JSON</abbr> Pointers, <code>value</code> can be used with both objects and arrays. For other types (null, boolean, number, string), a <a href=../../../home/exceptions/#jsonexceptiontype_error306><code>basic_json::type_error</code></a> is thrown.</li> </ul> </div> <div class="admonition warning"> <p class=admonition-title>Return type</p> <p>The value function is a template, and the return type of the function is determined by the type of the provided default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit unsigned integer. We get exactly that value when using <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a>. However, when we call <code>value</code> and provide <code class=highlight><span class=mi>0</span></code> as default value, then <code class=highlight><span class=mi>-1</span></code> is returned. This occurs, because <code class=highlight><span class=mi>0</span></code> has type <code class=highlight><span class=kt>int</span></code> which overflows when handling the value <code class=highlight><span class=mi>18446744073709551615</span></code>.</p> <p>To address this issue, either provide a correctly typed default value or use the template parameter to specify the desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default value is not used as the return value.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
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||||
</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></code></td> <td><code class=highlight><span class=p>{</span><span class=nt>"logOutput"</span><span class=p>:</span><span class=w> </span><span class=s2>"result.log"</span><span class=p>,</span><span class=w> </span><span class=nt>"append"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>}</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>"logOutput"</span><span class=p>,</span><span class=w> </span><span class=s>"logfile.log"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"result.log"</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>"append"</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>)</span></code></td> <td><code class=highlight><span class=kc>true</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>"append"</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>)</span></code></td> <td><code class=highlight><span class=kc>true</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>"logLevel"</span><span class=p>,</span><span class=w> </span><span class=s>"verbose"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"verbose"</span></code></td> </tr> </tbody> </table> </details> <h2 id=nested-values>Nested values<a class=headerlink href=#nested-values title="Permanent link">¶</a></h2> <p>To read a value deep inside a document, pass a <a href=../../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> instead of a key. The default value is returned if the value at the pointer does not exist, including the case that an intermediate key is missing. There is no need to check each level with <a href=../../../api/basic_json/contains/ ><code>contains</code></a> first.</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=p>{{</span><span class=s>"server"</span><span class=p>,</span><span class=w> </span><span class=p>{{</span><span class=s>"port"</span><span class=p>,</span><span class=w> </span><span class=mi>8080</span><span class=p>}}},</span><span class=w> </span><span class=p>{</span><span class=s>"list"</span><span class=p>,</span><span class=w> </span><span class=p>{</span><span class=mi>10</span><span class=p>,</span><span class=w> </span><span class=mi>20</span><span class=p>}}};</span>
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<span class=kt>int</span><span class=w> </span><span class=n>port</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=s>"/server/port"</span><span class=n>_json_pointer</span><span class=p>,</span><span class=w> </span><span class=mi>80</span><span class=p>);</span><span class=w> </span><span class=c1>// 8080</span>
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||||
<span class=kt>int</span><span class=w> </span><span class=n>timeout</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=s>"/server/limits/timeout"</span><span class=n>_json_pointer</span><span class=p>,</span><span class=w> </span><span class=mi>30</span><span class=p>);</span><span class=w> </span><span class=c1>// 30 (missing intermediate key)</span>
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||||
<span class=kt>int</span><span class=w> </span><span class=n>second</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=s>"/list/1"</span><span class=n>_json_pointer</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>// 20 (numeric tokens index arrays)</span>
|
||||
<span class=kt>int</span><span class=w> </span><span class=n>third</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=s>"/list/5"</span><span class=n>_json_pointer</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 (index out of range)</span>
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||||
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<span class=kt>bool</span><span class=w> </span><span class=n>has_port</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>contains</span><span class=p>(</span><span class=s>"/server/port"</span><span class=n>_json_pointer</span><span class=p>);</span><span class=w> </span><span class=c1>// true</span>
|
||||
<span class=kt>bool</span><span class=w> </span><span class=n>has_host</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>contains</span><span class=p>(</span><span class=s>"/server/host/name"</span><span class=n>_json_pointer</span><span class=p>);</span><span class=w> </span><span class=c1>// false</span>
|
||||
</code></pre></div> <p>If the path is only available as a dotted string such as <code class=highlight><span class=s>"server.port"</span></code>, do not build the pointer by concatenating <code class=highlight><span class=s>"/"</span></code> and the parts: keys containing <code>/</code> or <code>~</code> would be misinterpreted. Append each part as a reference token with <a href=../../../api/json_pointer/operator_slasheq/ ><code>operator/=</code></a> instead. It escapes the token for you.</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=nf>to_pointer</span><span class=p>(</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>&</span><span class=w> </span><span class=n>dotted</span><span class=p>)</span>
|
||||
<span class=p>{</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>json_pointer</span><span class=w> </span><span class=n>ptr</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>istringstream</span><span class=w> </span><span class=n>in</span><span class=p>(</span><span class=n>dotted</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>token</span><span class=p>;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>getline</span><span class=p>(</span><span class=n>in</span><span class=p>,</span><span class=w> </span><span class=n>token</span><span class=p>,</span><span class=w> </span><span class=sc>'.'</span><span class=p>);)</span>
|
||||
<span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=n>ptr</span><span class=w> </span><span class=o>/=</span><span class=w> </span><span class=n>token</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>ptr</span><span class=p>;</span>
|
||||
<span class=p>}</span>
|
||||
|
||||
<span class=kt>int</span><span class=w> </span><span class=n>port</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>to_pointer</span><span class=p>(</span><span class=s>"server.port"</span><span class=p>),</span><span class=w> </span><span class=mi>80</span><span class=p>);</span><span class=w> </span><span class=c1>// 8080</span>
|
||||
<span class=kt>int</span><span class=w> </span><span class=n>first</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>to_pointer</span><span class=p>(</span><span class=s>"list.0"</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>// 10</span>
|
||||
</code></pre></div> <p>The key <code class=highlight><span class=s>"a/b"</span></code> yields the pointer <code class=highlight><span class=s>"/a~1b"</span></code>; the dot-splitting itself is up to the caller, so keys containing <code>.</code> need a different separator.</p> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <ul> <li>With string keys, <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> <li>With <abbr title="JavaScript Object Notation">JSON</abbr> Pointers, <code>value</code> can be used with both objects and arrays. For other types (null, boolean, number, string), a <a href=../../../home/exceptions/#jsonexceptiontype_error306><code>basic_json::type_error</code></a> is thrown.</li> </ul> </div> <div class="admonition warning"> <p class=admonition-title><code>null</code> and mistyped members are not missing</p> <p><code>value</code> returns the default value only if the key is <strong>absent</strong>. If the member exists, it is converted to the type of the default value, even if it is <code class=highlight><span class=kc>null</span></code>. For <code class=highlight><span class=p>{</span><span class=nt>"k"</span><span class=p>:</span><span class=w> </span><span class=kc>null</span><span class=p>}</span></code>, the call <code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"k"</span><span class=p>,</span><span class=w> </span><span class=mi>0</span><span class=p>)</span></code> throws a <a href=../../../home/exceptions/#jsonexceptiontype_error302><code>basic_json::type_error</code></a>, and so does a member of another type such as a string where a number is expected. The same holds for <abbr title="JavaScript Object Notation">JSON</abbr> Pointers.</p> <p>To treat <code class=highlight><span class=kc>null</span></code> like a missing value, check for it explicitly:</p> <div class=highlight><pre><span></span><code><span class=kt>int</span><span class=w> </span><span class=n>n</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>(</span><span class=n>j</span><span class=p>.</span><span class=n>contains</span><span class=p>(</span><span class=s>"k"</span><span class=p>)</span><span class=w> </span><span class=o>&&</span><span class=w> </span><span class=o>!</span><span class=n>j</span><span class=p>[</span><span class=s>"k"</span><span class=p>].</span><span class=n>is_null</span><span class=p>())</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>"k"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=p>()</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=mi>0</span><span class=p>;</span>
|
||||
</code></pre></div> <p>With C++17, <a href=../../../api/basic_json/get/ ><code>get<std::optional<T>>()</code></a> maps <code class=highlight><span class=kc>null</span></code> to an empty optional. As <a href=../../../api/basic_json/at/ ><code>at</code></a> throws <a href=../../../home/exceptions/#jsonexceptionout_of_range403><code>out_of_range</code></a> for an absent key, use <a href=../../../api/basic_json/find/ ><code>find</code></a> to cover both cases:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>optional</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=w> </span><span class=n>n</span><span class=p>;</span><span class=w> </span><span class=c1>// empty if "k" is absent or null</span>
|
||||
<span class=k>if</span><span class=w> </span><span class=p>(</span><span class=k>const</span><span class=w> </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>find</span><span class=p>(</span><span class=s>"k"</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=p>{</span>
|
||||
<span class=w> </span><span class=n>n</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>it</span><span class=o>-></span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>optional</span><span class=o><</span><span class=kt>int</span><span class=o>>></span><span class=p>();</span><span class=w> </span><span class=c1>// still throws for a non-number such as "text"</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </div> <div class="admonition warning"> <p class=admonition-title>Return type</p> <p>The value function is a template, and the return type of the function is determined by the type of the provided default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit unsigned integer. We get exactly that value when using <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a>. However, when we call <code>value</code> and provide <code class=highlight><span class=mi>0</span></code> as default value, then <code class=highlight><span class=mi>-1</span></code> is returned. This occurs, because <code class=highlight><span class=mi>0</span></code> has type <code class=highlight><span class=kt>int</span></code> which overflows when handling the value <code class=highlight><span class=mi>18446744073709551615</span></code>.</p> <p>To address this issue, either provide a correctly typed default value or use the template parameter to specify the desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default value is not used as the return value.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></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>
|
||||
@@ -20,4 +48,4 @@
|
||||
<span class=err>de</span><span class=kc>fault</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=err>(i</span><span class=kc>nt</span><span class=err>)</span><span class=p>:</span><span class=w> </span><span class=mi>-1</span>
|
||||
<span class=err>de</span><span class=kc>fault</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=err>(ui</span><span class=kc>nt</span><span class=mi>64</span><span class=err>_</span><span class=kc>t</span><span class=err>)</span><span class=p>:</span><span class=w> </span><span class=mi>18446744073709551615</span>
|
||||
<span class=err>explici</span><span class=kc>t</span><span class=w> </span><span class=err>re</span><span class=kc>turn</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=kc>t</span><span class=err>ype</span><span class=p>:</span><span class=w> </span><span class=mi>18446744073709551615</span>
|
||||
</code></pre></div> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../../api/basic_json/value/ ><code>value</code></a> for access with default value</li> <li>documentation on <a href=../checked_access/ >checked access</a></li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 2, 2026 09:32:15 UTC">October 2, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var 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> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button 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 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <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 © 2013-2026 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 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../../api/basic_json/value/ ><code>value</code></a> for access with default value</li> <li>documentation on <a href=../checked_access/ >checked access</a></li> <li>documentation on <a href=../../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a></li> <li><a href=../../../api/basic_json/contains/ ><code>contains</code></a> to check whether a key or <abbr title="JavaScript Object Notation">JSON</abbr> Pointer exists</li> <li><a href=../../../api/json_pointer/operator_slasheq/ ><code>json_pointer::operator/=</code></a> to build a pointer token by token</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 11, 2026 06:22:10 UTC">October 11, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var 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> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button 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 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <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 © 2013-2026 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 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -27,6 +27,42 @@ Assume the value is parsed to a `json` variable `j`.
|
||||
| `j.value("append", false)` | `true` |
|
||||
| `j.value("logLevel", "verbose")` | `"verbose"` |
|
||||
|
||||
## Nested values
|
||||
|
||||
To read a value deep inside a document, pass a [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) instead of a key. The default value is returned if the value at the pointer does not exist, including the case that an intermediate key is missing. There is no need to check each level with [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) first.
|
||||
|
||||
```
|
||||
json j = {{"server", {{"port", 8080}}}, {"list", {10, 20}}};
|
||||
|
||||
int port = j.value("/server/port"_json_pointer, 80); // 8080
|
||||
int timeout = j.value("/server/limits/timeout"_json_pointer, 30); // 30 (missing intermediate key)
|
||||
int second = j.value("/list/1"_json_pointer, 0); // 20 (numeric tokens index arrays)
|
||||
int third = j.value("/list/5"_json_pointer, 0); // 0 (index out of range)
|
||||
|
||||
bool has_port = j.contains("/server/port"_json_pointer); // true
|
||||
bool has_host = j.contains("/server/host/name"_json_pointer); // false
|
||||
```
|
||||
|
||||
If the path is only available as a dotted string such as `"server.port"`, do not build the pointer by concatenating `"/"` and the parts: keys containing `/` or `~` would be misinterpreted. Append each part as a reference token with [`operator/=`](https://json.nlohmann.me/api/json_pointer/operator_slasheq/index.md) instead. It escapes the token for you.
|
||||
|
||||
```
|
||||
json::json_pointer to_pointer(const std::string& dotted)
|
||||
{
|
||||
json::json_pointer ptr;
|
||||
std::istringstream in(dotted);
|
||||
for (std::string token; std::getline(in, token, '.');)
|
||||
{
|
||||
ptr /= token;
|
||||
}
|
||||
return ptr;
|
||||
}
|
||||
|
||||
int port = j.value(to_pointer("server.port"), 80); // 8080
|
||||
int first = j.value(to_pointer("list.0"), 0); // 10
|
||||
```
|
||||
|
||||
The key `"a/b"` yields the pointer `"/a~1b"`; the dot-splitting itself is up to the caller, so keys containing `.` need a different separator.
|
||||
|
||||
## Notes
|
||||
|
||||
Exceptions
|
||||
@@ -34,6 +70,26 @@ Exceptions
|
||||
- With string keys, `value` can only be used with objects. For other types, a [`basic_json::type_error`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error306) is thrown.
|
||||
- With JSON Pointers, `value` can be used with both objects and arrays. For other types (null, boolean, number, string), a [`basic_json::type_error`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error306) is thrown.
|
||||
|
||||
`null` and mistyped members are not missing
|
||||
|
||||
`value` returns the default value only if the key is **absent**. If the member exists, it is converted to the type of the default value, even if it is `null`. For `{"k": null}`, the call `j.value("k", 0)` throws a [`basic_json::type_error`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error302), and so does a member of another type such as a string where a number is expected. The same holds for JSON Pointers.
|
||||
|
||||
To treat `null` like a missing value, check for it explicitly:
|
||||
|
||||
```
|
||||
int n = (j.contains("k") && !j["k"].is_null()) ? j["k"].get<int>() : 0;
|
||||
```
|
||||
|
||||
With C++17, [`get<std::optional<T>>()`](https://json.nlohmann.me/api/basic_json/get/index.md) maps `null` to an empty optional. As [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) throws [`out_of_range`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range403) for an absent key, use [`find`](https://json.nlohmann.me/api/basic_json/find/index.md) to cover both cases:
|
||||
|
||||
```
|
||||
std::optional<int> n; // empty if "k" is absent or null
|
||||
if (const auto it = j.find("k"); it != j.end())
|
||||
{
|
||||
n = it->get<std::optional<int>>(); // still throws for a non-number such as "text"
|
||||
}
|
||||
```
|
||||
|
||||
Return type
|
||||
|
||||
The value function is a template, and the return type of the function is determined by the type of the provided default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit unsigned integer. We get exactly that value when using [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md). However, when we call `value` and provide `0` as default value, then `-1` is returned. This occurs, because `0` has type `int` which overflows when handling the value `18446744073709551615`.
|
||||
@@ -70,3 +126,6 @@ explicit return value type: 18446744073709551615
|
||||
|
||||
- [`value`](https://json.nlohmann.me/api/basic_json/value/index.md) for access with default value
|
||||
- documentation on [checked access](https://json.nlohmann.me/features/element_access/checked_access/index.md)
|
||||
- documentation on [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md)
|
||||
- [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) to check whether a key or JSON Pointer exists
|
||||
- [`json_pointer::operator/=`](https://json.nlohmann.me/api/json_pointer/operator_slasheq/index.md) to build a pointer token by token
|
||||
Reference in new issue
Block a user