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@@ -26,7 +26,8 @@ To store objects in C++, a type is defined by the template parameters described
`StringType`
: the type of the keys or names (e.g., `std::string`). The comparison function `std::less<StringType>` is used to
order elements inside the container.
order elements inside the container. `object_t::key_type` must be implicitly convertible to `string_t` (required by the
binary formats).
`AllocatorType`
: the allocator to use for objects (e.g., `std::allocator`)
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@@ -2,7 +2,7 @@
<span class=w> </span><span class=n>basic_json</span><span class=p>,</span>
<span class=w> </span><span class=n>default_object_comparator_t</span><span class=p>,</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>
</code></pre></div> <p>The type used to store <abbr title="JavaScript Object Notation">JSON</abbr> objects.</p> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> describes <abbr title="JavaScript Object Notation">JSON</abbr> 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> <p>To store objects in C++, a type is defined by the template parameters described below.</p> <h2 id=template-parameters>Template parameters<a class=headerlink href=#template-parameters title="Permanent link">&para;</a></h2> <dl> <dt><code>ObjectType</code></dt> <dd>the container to store objects. Its template parameters must have the same order and meaning as those of <code>std::map</code>; in particular, the third parameter is a comparator. <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>unordered_map</span></code>, whose third parameter is a hash function, therefore needs an adapter -- see <a href=../../../features/types/template_parameters/#objecttype>Template Parameter Requirements</a> for the full list of requirements, an adapter example, and the containers that are known to work.</dd> <dt><code>StringType</code></dt> <dd>the type of the keys or names (e.g., <code>std::string</code>). The comparison function <code>std::less&lt;StringType&gt;</code> is used to order elements inside the container.</dd> <dt><code>AllocatorType</code></dt> <dd>the allocator to use for objects (e.g., <code>std::allocator</code>)</dd> </dl> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <h3 id=default-type>Default type<a class=headerlink href=#default-type title="Permanent link">&para;</a></h3> <p>With the default values for <code>ObjectType</code> (<code>std::map</code>), <code>StringType</code> (<code>std::string</code>), and <code>AllocatorType</code> (<code>std::allocator</code>), the default value for <code>object_t</code> is:</p> <div class=highlight><pre><span></span><code><span class=c1>// until C++14</span>
</code></pre></div> <p>The type used to store <abbr title="JavaScript Object Notation">JSON</abbr> objects.</p> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> describes <abbr title="JavaScript Object Notation">JSON</abbr> 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> <p>To store objects in C++, a type is defined by the template parameters described below.</p> <h2 id=template-parameters>Template parameters<a class=headerlink href=#template-parameters title="Permanent link">&para;</a></h2> <dl> <dt><code>ObjectType</code></dt> <dd>the container to store objects. Its template parameters must have the same order and meaning as those of <code>std::map</code>; in particular, the third parameter is a comparator. <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>unordered_map</span></code>, whose third parameter is a hash function, therefore needs an adapter -- see <a href=../../../features/types/template_parameters/#objecttype>Template Parameter Requirements</a> for the full list of requirements, an adapter example, and the containers that are known to work.</dd> <dt><code>StringType</code></dt> <dd>the type of the keys or names (e.g., <code>std::string</code>). The comparison function <code>std::less&lt;StringType&gt;</code> is used to order elements inside the container. <code>object_t::key_type</code> must be implicitly convertible to <code>string_t</code> (required by the binary formats).</dd> <dt><code>AllocatorType</code></dt> <dd>the allocator to use for objects (e.g., <code>std::allocator</code>)</dd> </dl> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <h3 id=default-type>Default type<a class=headerlink href=#default-type title="Permanent link">&para;</a></h3> <p>With the default values for <code>ObjectType</code> (<code>std::map</code>), <code>StringType</code> (<code>std::string</code>), and <code>AllocatorType</code> (<code>std::allocator</code>), the default value for <code>object_t</code> is:</p> <div class=highlight><pre><span></span><code><span class=c1>// until C++14</span>
<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=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>
@@ -28,4 +28,4 @@
<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>boolalpha</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>is_same</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=n>map</span><span class=o>&lt;</span><span class=n>json</span><span class=o>::</span><span class=n>string_t</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>&gt;</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>object_t</span><span class=o>&gt;::</span><span class=n>value</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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=kc>true</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../array_t/ >array_t</a> the type used to store <abbr title="JavaScript Object Notation">JSON</abbr> arrays</li> <li><a href=../string_t/ >string_t</a> the type used to store <abbr title="JavaScript Object Notation">JSON</abbr> strings</li> <li><a href=../object_comparator_t/ >object_comparator_t</a> the comparator used to order object keys</li> <li><a href=../../../features/object_order/ >Object Order</a> - the article on object key ordering</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Added in version 1.0.0.</li> <li>Allowed object types whose <code>erase(iterator)</code> returns <code class=highlight><span class=kt>void</span></code> in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>.</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 &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../array_t/ >array_t</a> the type used to store <abbr title="JavaScript Object Notation">JSON</abbr> arrays</li> <li><a href=../string_t/ >string_t</a> the type used to store <abbr title="JavaScript Object Notation">JSON</abbr> strings</li> <li><a href=../object_comparator_t/ >object_comparator_t</a> the comparator used to order object keys</li> <li><a href=../../../features/object_order/ >Object Order</a> - the article on object key ordering</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Added in version 1.0.0.</li> <li>Allowed object types whose <code>erase(iterator)</code> returns <code class=highlight><span class=kt>void</span></code> in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>.</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 9, 2026 11:22:07 UTC">October 9, 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 &copy; 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@@ -19,7 +19,7 @@ To store objects in C++, a type is defined by the template parameters described
`ObjectType` : the container to store objects. Its template parameters must have the same order and meaning as those of `std::map`; in particular, the third parameter is a comparator. `std::unordered_map`, whose third parameter is a hash function, therefore needs an adapter -- see [Template Parameter Requirements](https://json.nlohmann.me/features/types/template_parameters/#objecttype) for the full list of requirements, an adapter example, and the containers that are known to work.
`StringType` : the type of the keys or names (e.g., `std::string`). The comparison function `std::less<StringType>` is used to order elements inside the container.
`StringType` : the type of the keys or names (e.g., `std::string`). The comparison function `std::less<StringType>` is used to order elements inside the container. `object_t::key_type` must be implicitly convertible to `string_t` (required by the binary formats).
`AllocatorType` : the allocator to use for objects (e.g., `std::allocator`)
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{"config":{"lang":["en"],"separator":"[\\s\\-\\.]","pipeline":["stopWordFilter"],"fields":{"title":{"boost":1000.0},"text":{"boost":1.0},"tags":{"boost":1000000.0}}},"docs":[{"location":"","title":"JSON for Modern C++","text":"<p>JSON for Modern C++ is a header-only C++11 library that turns JSON into a first-class C++ data type, using the operator magic of modern C++ so that creating, reading, and modifying JSON values feels as natural as it does in languages like Python. The whole library is available as a single header, <code>json.hpp</code>, with no dependencies, no subproject, and no complex build system to set up; a companion header, <code>json_fwd.hpp</code>, provides forward declarations to keep compile times down. See header-only integration for details. It is heavily unit-tested with 100% code coverage, checked with Valgrind and the Clang Sanitizers for memory leaks, and continuously fuzz-tested by Google OSS-Fuzz.</p>"},{"location":"#quick-start","title":"Quick start","text":"<p>Add the single header to your project and use the library like this:</p> <pre><code>#include &lt;iostream&gt;\n#include &lt;nlohmann/json.hpp&gt;\n\nusing json = nlohmann::json;\n\nint main()\n{\n // parse a JSON string\n json j = json::parse(R\"({\"happy\": true, \"pi\": 3.141})\");\n\n // access and modify values\n j[\"name\"] = \"Niels\";\n j[\"list\"] = {1, 0, 2};\n\n // serialize with an indent of 4 spaces\n std::cout &lt;&lt; j.dump(4) &lt;&lt; '\\n';\n}\n</code></pre> <p>Get the library by copying the single header <code>json.hpp</code> from the releases page into a directory <code>nlohmann</code> on your include path, or by installing it with a package manager:</p> <pre><code>brew install nlohmann-json # Homebrew\nvcpkg install nlohmann-json # vcpkg\n</code></pre> <pre><code>find_package(nlohmann_json 3.12.0 REQUIRED)\ntarget_link_libraries(myproject PRIVATE nlohmann_json::nlohmann_json)\n</code></pre> <p>See Integration for CMake in detail, all supported package managers (Conan, Meson, Bazel, Conda, and more), and pkg-config.</p>"},{"location":"#explore-the-documentation","title":"Explore the documentation","text":"<ul> <li> <p> Features</p> <p>Creating, parsing, accessing, and serializing JSON values, JSON Pointer/Patch, binary formats, and more.</p> <p> Features</p> </li> <li> <p> Integration</p> <p>Add the library to your project via a single header, CMake, a package manager, or pkg-config.</p> <p> Integration</p> </li> <li> <p> API documentation</p> <p>The complete reference for <code>basic_json</code> and its member functions, types, and related classes.</p> <p> API documentation</p> </li> <li> <p> FAQ</p> <p>Answers to common questions and known surprises when using the library.</p> <p> FAQ</p> </li> <li> <p> Releases</p> <p>What changed in each release, with links to the relevant documentation.</p> <p> Releases</p> </li> <li> <p> Community</p> <p>The ecosystem, contribution guidelines, governance, and quality assurance around the project.</p> <p> Community</p> </li> </ul> <p>Unreleased changes</p> <p>This documentation is built from the <code>develop</code> branch and may describe changes that are not part of a release yet. Their version numbers are followed by an unreleased badge; see Releases for what shipped in each version.</p> <p>The library is licensed under the MIT License. The source code, issue tracker, and discussions are on GitHub.</p>"},{"location":"api/json/","title":"nlohmann::json","text":"<pre><code>using json = basic_json&lt;&gt;;\n</code></pre> <p>This type is the default specialization of the basic_json class which uses the standard template types.</p>"},{"location":"api/json/#examples","title":"Examples","text":"Example <p>The example below demonstrates how to use the type <code>nlohmann::json</code>.</p> <pre><code>#include &lt;iostream&gt;\n#include &lt;iomanip&gt;\n#include &lt;nlohmann/json.hpp&gt;\n\nusing json = nlohmann::json;\n\nint main()\n{\n // create a JSON object\n json j =\n {\n {\"pi\", 3.141},\n {\"happy\", true},\n {\"name\", \"Niels\"},\n {\"nothing\", nullptr},\n {\n \"answer\", {\n {\"everything\", 42}\n }\n },\n {\"list\", {1, 0, 2}},\n {\n \"object\", {\n {\"currency\", \"USD\"},\n {\"value\", 42.99}\n }\n }\n };\n\n // add new values\n j[\"new\"][\"key\"][\"value\"] = {\"another\", \"list\"};\n\n // count elements\n auto s = j.size();\n j[\"size\"] = s;\n\n // pretty print with indent of 4 spaces\n std::cout &lt;&lt; std::setw(4) &lt;&lt; j &lt;&lt; '\\n';\n}\n</code></pre> <p>Output:</p> <pre><code>{\n \"answer\": {\n \"everything\": 42\n },\n \"happy\": true,\n \"list\": [\n 1,\n 0,\n 2\n ],\n \"name\": \"Niels\",\n \"new\": {\n \"key\": {\n \"value\": [\n \"another\",\n Line truncated
{"config":{"lang":["en"],"separator":"[\\s\\-\\.]","pipeline":["stopWordFilter"],"fields":{"title":{"boost":1000.0},"text":{"boost":1.0},"tags":{"boost":1000000.0}}},"docs":[{"location":"","title":"JSON for Modern C++","text":"<p>JSON for Modern C++ is a header-only C++11 library that turns JSON into a first-class C++ data type, using the operator magic of modern C++ so that creating, reading, and modifying JSON values feels as natural as it does in languages like Python. The whole library is available as a single header, <code>json.hpp</code>, with no dependencies, no subproject, and no complex build system to set up; a companion header, <code>json_fwd.hpp</code>, provides forward declarations to keep compile times down. See header-only integration for details. It is heavily unit-tested with 100% code coverage, checked with Valgrind and the Clang Sanitizers for memory leaks, and continuously fuzz-tested by Google OSS-Fuzz.</p>"},{"location":"#quick-start","title":"Quick start","text":"<p>Add the single header to your project and use the library like this:</p> <pre><code>#include &lt;iostream&gt;\n#include &lt;nlohmann/json.hpp&gt;\n\nusing json = nlohmann::json;\n\nint main()\n{\n // parse a JSON string\n json j = json::parse(R\"({\"happy\": true, \"pi\": 3.141})\");\n\n // access and modify values\n j[\"name\"] = \"Niels\";\n j[\"list\"] = {1, 0, 2};\n\n // serialize with an indent of 4 spaces\n std::cout &lt;&lt; j.dump(4) &lt;&lt; '\\n';\n}\n</code></pre> <p>Get the library by copying the single header <code>json.hpp</code> from the releases page into a directory <code>nlohmann</code> on your include path, or by installing it with a package manager:</p> <pre><code>brew install nlohmann-json # Homebrew\nvcpkg install nlohmann-json # vcpkg\n</code></pre> <pre><code>find_package(nlohmann_json 3.12.0 REQUIRED)\ntarget_link_libraries(myproject PRIVATE nlohmann_json::nlohmann_json)\n</code></pre> <p>See Integration for CMake in detail, all supported package managers (Conan, Meson, Bazel, Conda, and more), and pkg-config.</p>"},{"location":"#explore-the-documentation","title":"Explore the documentation","text":"<ul> <li> <p> Features</p> <p>Creating, parsing, accessing, and serializing JSON values, JSON Pointer/Patch, binary formats, and more.</p> <p> Features</p> </li> <li> <p> Integration</p> <p>Add the library to your project via a single header, CMake, a package manager, or pkg-config.</p> <p> Integration</p> </li> <li> <p> API documentation</p> <p>The complete reference for <code>basic_json</code> and its member functions, types, and related classes.</p> <p> API documentation</p> </li> <li> <p> FAQ</p> <p>Answers to common questions and known surprises when using the library.</p> <p> FAQ</p> </li> <li> <p> Releases</p> <p>What changed in each release, with links to the relevant documentation.</p> <p> Releases</p> </li> <li> <p> Community</p> <p>The ecosystem, contribution guidelines, governance, and quality assurance around the project.</p> <p> Community</p> </li> </ul> <p>Unreleased changes</p> <p>This documentation is built from the <code>develop</code> branch and may describe changes that are not part of a release yet. Their version numbers are followed by an unreleased badge; see Releases for what shipped in each version.</p> <p>The library is licensed under the MIT License. The source code, issue tracker, and discussions are on GitHub.</p>"},{"location":"api/json/","title":"nlohmann::json","text":"<pre><code>using json = basic_json&lt;&gt;;\n</code></pre> <p>This type is the default specialization of the basic_json class which uses the standard template types.</p>"},{"location":"api/json/#examples","title":"Examples","text":"Example <p>The example below demonstrates how to use the type <code>nlohmann::json</code>.</p> <pre><code>#include &lt;iostream&gt;\n#include &lt;iomanip&gt;\n#include &lt;nlohmann/json.hpp&gt;\n\nusing json = nlohmann::json;\n\nint main()\n{\n // create a JSON object\n json j =\n {\n {\"pi\", 3.141},\n {\"happy\", true},\n {\"name\", \"Niels\"},\n {\"nothing\", nullptr},\n {\n \"answer\", {\n {\"everything\", 42}\n }\n },\n {\"list\", {1, 0, 2}},\n {\n \"object\", {\n {\"currency\", \"USD\"},\n {\"value\", 42.99}\n }\n }\n };\n\n // add new values\n j[\"new\"][\"key\"][\"value\"] = {\"another\", \"list\"};\n\n // count elements\n auto s = j.size();\n j[\"size\"] = s;\n\n // pretty print with indent of 4 spaces\n std::cout &lt;&lt; std::setw(4) &lt;&lt; j &lt;&lt; '\\n';\n}\n</code></pre> <p>Output:</p> <pre><code>{\n \"answer\": {\n \"everything\": 42\n },\n \"happy\": true,\n \"list\": [\n 1,\n 0,\n 2\n ],\n \"name\": \"Niels\",\n \"new\": {\n \"key\": {\n \"value\": [\n \"another\",\n Line truncated