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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=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=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=kt>int</span><span class=w> </span><span class=n>age</span><span class=p>;</span>
<span class=w> </span><span class=p>};</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=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=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=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=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=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=n>j</span><span class=p>[</span><span class=s>&quot;name&quot;</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>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=n>j</span><span class=p>[</span><span class=s>&quot;address&quot;</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>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=n>j</span><span class=p>[</span><span class=s>&quot;age&quot;</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=kt>int</span><span class=o>&gt;</span><span class=p>()</span>
<span class=p>};</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=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=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=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=k>template</span><span class=w> </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=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>
</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=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=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=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=p>}</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=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=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=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=p>}</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>template 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">#1108</a> for errors that may occur otherwise.</li> <li>When using <code>template 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.html><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 <abbr title="Standard Template Library">STL</abbr> 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 six macros to make your life easier as long as you (1) want to use a <abbr title="JavaScript Object Notation">JSON</abbr> 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.html><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 <abbr title="JavaScript Object Notation">JSON</abbr> object.</li> <li><a href=../api/macros/nlohmann_define_type_non_intrusive.html><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 <abbr title="JavaScript Object Notation">JSON</abbr> object, but fills in values from an object which is default-constructed by the type.</li> <li><a href=../api/macros/nlohmann_define_type_non_intrusive.html><code>NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(name, member1, member2, ...)</code></a> is to be defined inside the namespace of the class/struct to create code for. It does not define a <code>from_json()</code> function which is needed in case the type does not have a default constructor.</li> <li><a href=../api/macros/nlohmann_define_type_intrusive.html><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 <abbr title="JavaScript Object Notation">JSON</abbr> object.</li> <li><a href=../api/macros/nlohmann_define_type_intrusive.html><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 <abbr title="JavaScript Object Notation">JSON</abbr> object, but fills in values from an object which is default-constructed by the type.</li> <li><a href=../api/macros/nlohmann_define_type_intrusive.html><code>NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE(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 does not define a <code>from_json()</code> function which is needed in case the type does not have a default constructor.</li> </ul> <p>Furthermore, there exist versions to use in the case of derived classes:</p> <table> <thead> <tr> <th>Need access to private members</th> <th>Need only de-serialization</th> <th>Allow missing values when de-serializing</th> <th>macro</th> </tr> </thead> <tbody> <tr> <td><div stLine truncated
<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=p>}</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=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=k>private</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=w> </span><span class=n>street</span><span class=p>;</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=kt>int</span><span class=w> </span><span class=n>postcode</span><span class=p>;</span>
<span class=w> </span><span class=k>public</span><span class=o>:</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=p>};</span>
<span class=p>}</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 us see how this conversion mechanism works:</p> <p>The library uses <strong><abbr title="JavaScript Object Notation">JSON</abbr> Serializers</strong> to convert types to <abbr title="JavaScript Object Notation">JSON</abbr>. The default serializer for <code>nlohmann::json</code> is <code>nlohmann::adl_serializer</code> (<abbr title="Argument-dependent lookup">ADL</abbr> 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=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=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=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=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=c1>// same thing, but with the &quot;from_json&quot; method</span>
<span class=w> </span><span class=p>}</span>
<span class=p>};</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=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=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=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=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=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=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=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=p>}</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=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=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=k>else</span><span class=w> </span><span class=p>{</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=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>// 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=p>}</span>
<span class=p>};</span>
<span class=n>NLOHMANN_JSON_NAMESPACE_END</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.html><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></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=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=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=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=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=kt>int</span><span class=w> </span><span class=n>i</span><span class=p>;</span>
<span class=p>};</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=k>template</span><span class=w> </span><span class=o>&lt;&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>move_only_type</span><span class=o>&gt;</span><span class=w> </span><span class=p>{</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=k>return</span><span class=w> </span><span class=p>{</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=kt>int</span><span class=o>&gt;</span><span class=p>()};</span>
<span class=w> </span><span class=p>}</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=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=p>}</span>
<span class=w> </span><span class=p>};</span>
<span class=p>}</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 will 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 <abbr title="Argument-dependent lookup">ADL</abbr></li> </ul> <p>Here is an example, without simplifications, that only accepts types with a size &lt;= 32, and uses <abbr title="Argument-dependent lookup">ADL</abbr>.</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=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=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=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=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=p>}</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=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=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=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=p>}</span>
<span class=p>};</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=k>struct</span><span class=w> </span><span class=nc>bad_serializer</span>
<span class=p>{</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=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=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=p>}</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=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=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=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=p>};</span>
</code></pre></div> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<!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 href=../../home/sponsors/ rel=prev><link href=../binary_formats/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Arbitrary Type Conversions - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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<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=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=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=kt>int</span><span class=w> </span><span class=n>age</span><span class=p>;</span>
<span class=w> </span><span class=p>};</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=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=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=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=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=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=n>j</span><span class=p>[</span><span class=s>&quot;name&quot;</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>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=n>j</span><span class=p>[</span><span class=s>&quot;address&quot;</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>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=n>j</span><span class=p>[</span><span class=s>&quot;age&quot;</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=kt>int</span><span class=o>&gt;</span><span class=p>()</span>
<span class=p>};</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=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=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=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=k>template</span><span class=w> </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=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>
</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=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=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=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=p>}</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=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=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=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=p>}</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>template 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">#1108</a> for errors that may occur otherwise.</li> <li>When using <code>template 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 <abbr title="Standard Template Library">STL</abbr> 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 six macros to make your life easier as long as you (1) want to use a <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> object, but fills in values from an object which is default-constructed by the type.</li> <li><a href=../../api/macros/nlohmann_define_type_non_intrusive/ ><code>NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(name, member1, member2, ...)</code></a> is to be defined inside the namespace of the class/struct to create code for. It does not define a <code>from_json()</code> function which is needed in case the type does not have a default constructor.</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 <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> object, but fills in values from an object which is default-constructed by the type.</li> <li><a href=../../api/macros/nlohmann_define_type_intrusive/ ><code>NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE(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 does not define a <code>from_json()</code> function which is needed in case the type does not have a default constructor.</li> </ul> <p>Furthermore, there exist versions to use in the case of derived classes:</p> <table> <thead> <tr> <th>Need access to private members</th> <th>Need only de-serialization</th> <th>Allow missing values when de-serializing</th> <th>macro</th> </tr> </thead> <tbody> <tr> <td><div stLine truncated
<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=p>}</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=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=k>private</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=w> </span><span class=n>street</span><span class=p>;</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=kt>int</span><span class=w> </span><span class=n>postcode</span><span class=p>;</span>
<span class=w> </span><span class=k>public</span><span class=o>:</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=p>};</span>
<span class=p>}</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 us see how this conversion mechanism works:</p> <p>The library uses <strong><abbr title="JavaScript Object Notation">JSON</abbr> Serializers</strong> to convert types to <abbr title="JavaScript Object Notation">JSON</abbr>. The default serializer for <code>nlohmann::json</code> is <code>nlohmann::adl_serializer</code> (<abbr title="Argument-dependent lookup">ADL</abbr> 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=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=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=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=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=c1>// same thing, but with the &quot;from_json&quot; method</span>
<span class=w> </span><span class=p>}</span>
<span class=p>};</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=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=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=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=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=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=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=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=p>}</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=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=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=k>else</span><span class=w> </span><span class=p>{</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=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>// 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=p>}</span>
<span class=p>};</span>
<span class=n>NLOHMANN_JSON_NAMESPACE_END</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></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=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=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=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=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=kt>int</span><span class=w> </span><span class=n>i</span><span class=p>;</span>
<span class=p>};</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=k>template</span><span class=w> </span><span class=o>&lt;&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>move_only_type</span><span class=o>&gt;</span><span class=w> </span><span class=p>{</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=k>return</span><span class=w> </span><span class=p>{</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=kt>int</span><span class=o>&gt;</span><span class=p>()};</span>
<span class=w> </span><span class=p>}</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=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=p>}</span>
<span class=w> </span><span class=p>};</span>
<span class=p>}</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 will 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 <abbr title="Argument-dependent lookup">ADL</abbr></li> </ul> <p>Here is an example, without simplifications, that only accepts types with a size &lt;= 32, and uses <abbr title="Argument-dependent lookup">ADL</abbr>.</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=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=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=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=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=p>}</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=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=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=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=p>}</span>
<span class=p>};</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=k>struct</span><span class=w> </span><span class=nc>bad_serializer</span>
<span class=p>{</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=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=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=p>}</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=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=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=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=p>};</span>
</code></pre></div> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<!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 rel="canonical" href="https://json.nlohmann.me/features/assertions.html"><link rel=prev href=parsing/sax_interface.html><link rel=next href=enum_conversion.html><link rel=icon href=../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Runtime Assertions - JSON for Modern C++</title><link rel=stylesheet href=../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../assets/stylesheets/palette.06af60db.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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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> Runtime Assertions </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 aria-label="Switch to dark mode" type=radio 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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </labLine truncated
<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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</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=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=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=p>}</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 <abbr title="JavaScript Object Notation">JSON</abbr> value from an iterator range (see <a href=../api/basic_json/basic_json.html>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=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=kt>int</span><span class=w> </span><span class=nf>main</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>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=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=p>}</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 <abbr title="JavaScript Object Notation">JSON</abbr> 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=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=kt>int</span><span class=w> </span><span class=nf>main</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>iterator</span><span class=w> </span><span class=n>it</span><span class=p>;</span>
<span class=w> </span><span class=o>++</span><span class=n>it</span><span class=p>;</span>
<span class=p>}</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> <h2 id=changes>Changes<a class=headerlink href=#changes title="Permanent link">&para;</a></h2> <h3 id=reading-from-a-null-file-or-char-pointer>Reading from a null <code>FILE</code> or <code>char</code> pointer<a class=headerlink href=#reading-from-a-null-file-or-char-pointer title="Permanent link">&para;</a></h3> <p>Reading from a null <code class=highlight><span class=kt>FILE</span></code> or <code class=highlight><span class=kt>char</span></code> pointer in C++ is undefined behavior. Until version 3.12.0, this library asserted that the pointer was not <code>nullptr</code> using a runtime assertion. If assertions were disabled, this would result in undefined behavior. Since version 3.12.0, this library checks for <code>nullptr</code> and throws a <a href=../home/exceptions.html#jsonexceptionparse_error101><code>parse_error.101</code></a> to prevent the undefined behavior.</p> <details class=example> <summary>Example 4: Reading from null pointer</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;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=k>try</span><span class=w> </span><span class=p>{</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><span class=k>catch</span><span class=w> </span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=n>exception</span><span class=o>&amp;</span><span class=w> </span><span class=n>e</span><span class=p>)</span><span class=w> </span><span class=p>{</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=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=p>}</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>[json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON
</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=../api/macros/json_assert.html>JSON_ASSERT</a> - control behavior of runtime assertions</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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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+46
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@@ -0,0 +1,46 @@
<!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/assertions/" rel="canonical"><link href=../parsing/sax_interface/ rel=prev><link href=../enum_conversion/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Runtime Assertions - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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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> Runtime Assertions </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 aria-label="Switch to dark mode" type=radio 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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5Line truncated
<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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</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=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=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=p>}</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 <abbr title="JavaScript Object Notation">JSON</abbr> 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=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=kt>int</span><span class=w> </span><span class=nf>main</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>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=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=p>}</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 <abbr title="JavaScript Object Notation">JSON</abbr> 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=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=kt>int</span><span class=w> </span><span class=nf>main</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>iterator</span><span class=w> </span><span class=n>it</span><span class=p>;</span>
<span class=w> </span><span class=o>++</span><span class=n>it</span><span class=p>;</span>
<span class=p>}</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> <h2 id=changes>Changes<a class=headerlink href=#changes title="Permanent link">&para;</a></h2> <h3 id=reading-from-a-null-file-or-char-pointer>Reading from a null <code>FILE</code> or <code>char</code> pointer<a class=headerlink href=#reading-from-a-null-file-or-char-pointer title="Permanent link">&para;</a></h3> <p>Reading from a null <code class=highlight><span class=kt>FILE</span></code> or <code class=highlight><span class=kt>char</span></code> pointer in C++ is undefined behavior. Until version 3.12.0, this library asserted that the pointer was not <code>nullptr</code> using a runtime assertion. If assertions were disabled, this would result in undefined behavior. Since version 3.12.0, this library checks for <code>nullptr</code> and throws a <a href=../../home/exceptions/#jsonexceptionparse_error101><code>parse_error.101</code></a> to prevent the undefined behavior.</p> <details class=example> <summary>Example 4: Reading from null pointer</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;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=k>try</span><span class=w> </span><span class=p>{</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><span class=k>catch</span><span class=w> </span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=n>exception</span><span class=o>&amp;</span><span class=w> </span><span class=n>e</span><span class=p>)</span><span class=w> </span><span class=p>{</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=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=p>}</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>[json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON
</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=../../api/macros/json_assert/ >JSON_ASSERT</a> - control behavior of runtime assertions</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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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-98
View File
@@ -1,98 +0,0 @@
<!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 rel="canonical" href="https://json.nlohmann.me/features/binary_formats/bjdata.html"><link rel=prev href=index.html><link rel=next href=bson.html><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>BJData - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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<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=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=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=p>}</span>
</code></pre></div> <p>Likewise, when a <abbr title="JavaScript Object Notation">JSON</abbr> object in the above form is serialzed using <a href=../../api/basic_json/to_bjdata.html><code>to_bjdata</code></a>, it is automatically converted into a compact <abbr title="Binary JData">BJData</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> array input to a <abbr title="Binary JData">BJData</abbr> 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 <abbr title="Binary JData">BJData</abbr>, 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>UiuImlMLhdDCB</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 <abbr title="Binary JData">BJData</abbr>.</p> </div> <div class="admonition info"> <p class=admonition-title>Binary values</p> <p><abbr title="Binary JData">BJData</abbr> provides a dedicated <code>B</code> marker (defined in the <a href="https://github.com/NeuroJSON/bjdata/blob/master/Binary_JData_Specification.md#optimized-binary-array)"><abbr title="Binary JData">BJData</abbr> specification (Draft 3)</a>) that is used in optimized arrays to designate binary data. This means that, unlike <abbr title="Universal Binary JSON">UBJSON</abbr>, binary data can be both serialized and deserialized.</p> <p>To preserve compatibility with <abbr title="Binary JData">BJData</abbr> Draft 2, the Draft 3 optimized binary array must be explicitly enabled using the <code>version</code> parameter of <a href=../../api/basic_json/to_bjdata.html><code>to_bjdata</code></a>.</p> <p>In Draft2 mode (default), if the <abbr title="JavaScript Object Notation">JSON</abbr> data contains the binary type, the value stored as a list of integers, as suggested by the <abbr title="Binary JData">BJData</abbr> documentation. In particular, this means that the serialization and the deserialization of <abbr title="JavaScript Object Notation">JSON</abbr> containing binary values into <abbr title="Binary JData">BJData</abbr> and back will result in a different <abbr title="JavaScript Object Notation">JSON</abbr> 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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=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=p>{</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=k>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=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=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=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=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=p>}</span>
<span class=p>}</span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=p>{</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=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>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</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=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=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=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=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=p>{</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=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>std</span><span class=o>::</span><span class=n>endl</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=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=p>{</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=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>std</span><span class=o>::</span><span class=n>endl</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=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=p>{</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=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>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=p>{</span><span class=n>i7compactTi6schemaF</span><span class=p>}</span>
<span class=p>[</span><span class=n>i1i2i3i4i5i6i7i8</span><span class=p>]</span>
<span class=p>[</span><span class=err>#</span><span class=n>i8i1i2i3i4i5i6i7i8</span>
<span class=p>[</span><span class=n>$i</span><span class=err>#</span><span class=n>i812345678</span>
</code></pre></div> </details> <h2 id=deserialization>Deserialization<a class=headerlink href=#deserialization title="Permanent link">&para;</a></h2> <p>The library maps <abbr title="Binary JData">BJData</abbr> types to <abbr title="JavaScript Object Notation">JSON</abbr> value types as follows:</p> <table> <thead> <tr> <th><abbr title="Binary JData">BJData</abbr> type</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> 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>byte</td> <td>number_unsigned</td> <td><code>B</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>[$.#[.</code></td> </tr> <tr> <td>object</td> <td>object (optimized values are supported)</td> <td><code>{</code></td> </tr> <tr> <td>binary</td> <td>binary (strongly-typed byte array)</td> <td><code>[$B</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 <abbr title="Binary JData">BJData</abbr> value can be converted to a <abbr title="JavaScript Object Notation">JSON</abbr> 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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=p>};</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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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+98
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@@ -0,0 +1,98 @@
<!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/binary_formats/bjdata/" rel="canonical"><link href=../ rel=prev><link href=../bson/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>BJData - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.06af60db.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=#bjdata 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> BJData </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 aria-label="Switch to dark mode" type=radio name=__palette 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6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div class=md-searLine truncated
<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=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=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=p>}</span>
</code></pre></div> <p>Likewise, when a <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="Binary JData">BJData</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> array input to a <abbr title="Binary JData">BJData</abbr> 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 <abbr title="Binary JData">BJData</abbr>, 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>UiuImlMLhdDCB</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 <abbr title="Binary JData">BJData</abbr>.</p> </div> <div class="admonition info"> <p class=admonition-title>Binary values</p> <p><abbr title="Binary JData">BJData</abbr> provides a dedicated <code>B</code> marker (defined in the <a href="https://github.com/NeuroJSON/bjdata/blob/master/Binary_JData_Specification.md#optimized-binary-array)"><abbr title="Binary JData">BJData</abbr> specification (Draft 3)</a>) that is used in optimized arrays to designate binary data. This means that, unlike <abbr title="Universal Binary JSON">UBJSON</abbr>, binary data can be both serialized and deserialized.</p> <p>To preserve compatibility with <abbr title="Binary JData">BJData</abbr> Draft 2, the Draft 3 optimized binary array must be explicitly enabled using the <code>version</code> parameter of <a href=../../../api/basic_json/to_bjdata/ ><code>to_bjdata</code></a>.</p> <p>In Draft2 mode (default), if the <abbr title="JavaScript Object Notation">JSON</abbr> data contains the binary type, the value stored as a list of integers, as suggested by the <abbr title="Binary JData">BJData</abbr> documentation. In particular, this means that the serialization and the deserialization of <abbr title="JavaScript Object Notation">JSON</abbr> containing binary values into <abbr title="Binary JData">BJData</abbr> and back will result in a different <abbr title="JavaScript Object Notation">JSON</abbr> 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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=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=p>{</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=k>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=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=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=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=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=p>}</span>
<span class=p>}</span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=p>{</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=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>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</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=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=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=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=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=p>{</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=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>std</span><span class=o>::</span><span class=n>endl</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=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=p>{</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=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>std</span><span class=o>::</span><span class=n>endl</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=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=p>{</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=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>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=p>{</span><span class=n>i7compactTi6schemaF</span><span class=p>}</span>
<span class=p>[</span><span class=n>i1i2i3i4i5i6i7i8</span><span class=p>]</span>
<span class=p>[</span><span class=err>#</span><span class=n>i8i1i2i3i4i5i6i7i8</span>
<span class=p>[</span><span class=n>$i</span><span class=err>#</span><span class=n>i812345678</span>
</code></pre></div> </details> <h2 id=deserialization>Deserialization<a class=headerlink href=#deserialization title="Permanent link">&para;</a></h2> <p>The library maps <abbr title="Binary JData">BJData</abbr> types to <abbr title="JavaScript Object Notation">JSON</abbr> value types as follows:</p> <table> <thead> <tr> <th><abbr title="Binary JData">BJData</abbr> type</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> 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>byte</td> <td>number_unsigned</td> <td><code>B</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>[$.#[.</code></td> </tr> <tr> <td>object</td> <td>object (optimized values are supported)</td> <td><code>{</code></td> </tr> <tr> <td>binary</td> <td>binary (strongly-typed byte array)</td> <td><code>[$B</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 <abbr title="Binary JData">BJData</abbr> value can be converted to a <abbr title="JavaScript Object Notation">JSON</abbr> 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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=p>};</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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<!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 rel="canonical" href="https://json.nlohmann.me/features/binary_formats/bson.html"><link rel=prev href=bjdata.html><link rel=next href=cbor.html><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>BSON - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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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> BSON </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 aria-label="Switch to dark mode" type=radio name=__palette 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=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=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=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>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=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 <abbr title="Binary JSON">BSON</abbr> record types to <abbr title="JavaScript Object Notation">JSON</abbr> value types as follows:</p> <table> <thead> <tr> <th><abbr title="Binary JSON">BSON</abbr> type</th> <th><abbr title="Binary JSON">BSON</abbr> marker byte</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> 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>uint64(Timestamp)</td> <td>0x11</td> <td>number_unsigned</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> <div class="admonition note"> <p class=admonition-title>Handling of <abbr title="Binary JSON">BSON</abbr> type 0x11</p> <p><abbr title="Binary JSON">BSON</abbr> type 0x11 is used to represent uint64 numbers. This library treats these values purely as uint64 numbers and does not parse them into date-related formats.</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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=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=p>};</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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<!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/binary_formats/bson/" rel="canonical"><link href=../bjdata/ rel=prev><link href=../cbor/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>BSON - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.06af60db.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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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> BSON </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 aria-label="Switch to dark mode" type=radio name=__palette 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6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div class=md-searcLine truncated
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=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=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=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>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=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 <abbr title="Binary JSON">BSON</abbr> record types to <abbr title="JavaScript Object Notation">JSON</abbr> value types as follows:</p> <table> <thead> <tr> <th><abbr title="Binary JSON">BSON</abbr> type</th> <th><abbr title="Binary JSON">BSON</abbr> marker byte</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> 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>uint64(Timestamp)</td> <td>0x11</td> <td>number_unsigned</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> <div class="admonition note"> <p class=admonition-title>Handling of <abbr title="Binary JSON">BSON</abbr> type 0x11</p> <p><abbr title="Binary JSON">BSON</abbr> type 0x11 is used to represent uint64 numbers. This library treats these values purely as uint64 numbers and does not parse them into date-related formats.</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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=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=p>};</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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=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=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=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>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=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 <abbr title="Concise Binary Object Representation">CBOR</abbr> types to <abbr title="JavaScript Object Notation">JSON</abbr> value types as follows:</p> <table> <thead> <tr> <th><abbr title="Concise Binary Object Representation">CBOR</abbr> type</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> 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><abbr title="Unicode Transformation Format">UTF</abbr>-8 string</td> <td>string</td> <td>0x60..0x77</td> </tr> <tr> <td><abbr title="Unicode Transformation Format">UTF</abbr>-8 string</td> <td>string</td> <td>0x78</td> </tr> <tr> <td><abbr title="Unicode Transformation Format">UTF</abbr>-8 string</td> <td>string</td> <td>0x79</td> </tr> <tr> <td><abbr title="Unicode Transformation Format">UTF</abbr>-8 string</td> <td>string</td> <td>0x7A</td> </tr> <tr> <td><abbr title="Unicode Transformation Format">UTF</abbr>-8 string</td> <td>string</td> <td>0x7B</td> </tr> <tr> <td><abbr title="Unicode Transformation Format">UTF</abbr>-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 <abbr title="Concise Binary Object Representation">CBOR</abbr> types can be converted to a <abbr title="JavaScript Object Notation">JSON</abbr> value. The following <abbr title="Concise Binary Object Representation">CBOR</abbr> 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><abbr title="Concise Binary Object Representation">CBOR</abbr> allows map keys of any type, whereas <abbr title="JavaScript Object Notation">JSON</abbr> only allows strings as keys in object values. Therefore, <abbr title="Concise Binary Object Representation">CBOR</abbr> maps with keys other than <abbr title="Unicode Transformation Format">UTF</abbr>-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><Line truncated
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=mh>0x61</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span>
<span class=w> </span><span class=p>};</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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<!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/binary_formats/cbor/" rel="canonical"><link href=../bson/ rel=prev><link href=../messagepack/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>CBOR - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.06af60db.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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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> CBOR </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 aria-label="Switch to dark mode" type=radio name=__palette 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=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=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=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>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=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 <abbr title="Concise Binary Object Representation">CBOR</abbr> types to <abbr title="JavaScript Object Notation">JSON</abbr> value types as follows:</p> <table> <thead> <tr> <th><abbr title="Concise Binary Object Representation">CBOR</abbr> type</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> 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><abbr title="Unicode Transformation Format">UTF</abbr>-8 string</td> <td>string</td> <td>0x60..0x77</td> </tr> <tr> <td><abbr title="Unicode Transformation Format">UTF</abbr>-8 string</td> <td>string</td> <td>0x78</td> </tr> <tr> <td><abbr title="Unicode Transformation Format">UTF</abbr>-8 string</td> <td>string</td> <td>0x79</td> </tr> <tr> <td><abbr title="Unicode Transformation Format">UTF</abbr>-8 string</td> <td>string</td> <td>0x7A</td> </tr> <tr> <td><abbr title="Unicode Transformation Format">UTF</abbr>-8 string</td> <td>string</td> <td>0x7B</td> </tr> <tr> <td><abbr title="Unicode Transformation Format">UTF</abbr>-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 <abbr title="Concise Binary Object Representation">CBOR</abbr> types can be converted to a <abbr title="JavaScript Object Notation">JSON</abbr> value. The following <abbr title="Concise Binary Object Representation">CBOR</abbr> 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><abbr title="Concise Binary Object Representation">CBOR</abbr> allows map keys of any type, whereas <abbr title="JavaScript Object Notation">JSON</abbr> only allows strings as keys in object values. Therefore, <abbr title="Concise Binary Object Representation">CBOR</abbr> maps with keys other than <abbr title="Unicode Transformation Format">UTF</abbr>-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><Line truncated
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=mh>0x61</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span>
<span class=w> </span><span class=p>};</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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=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=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=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>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=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 <abbr title="JavaScript Object Notation">JSON</abbr> value types as follows:</p> <table> <thead> <tr> <th>MessagePack type</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> 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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=mh>0x61</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span>
<span class=w> </span><span class=p>};</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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<!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/binary_formats/messagepack/" rel="canonical"><link href=../cbor/ rel=prev><link href=../ubjson/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>MessagePack - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.06af60db.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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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> Line truncated
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=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=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=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>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=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 <abbr title="JavaScript Object Notation">JSON</abbr> value types as follows:</p> <table> <thead> <tr> <th>MessagePack type</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> 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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=mh>0x61</span><span class=p>,</span><span class=w> </span><span class=mh>0x00</span>
<span class=w> </span><span class=p>};</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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=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=p>{</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=k>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=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=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=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=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=p>}</span>
<span class=p>}</span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=p>{</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=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>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</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=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=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=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=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=p>{</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=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>std</span><span class=o>::</span><span class=n>endl</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=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=p>{</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=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>std</span><span class=o>::</span><span class=n>endl</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=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=p>{</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=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>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=p>{</span><span class=n>i7compactTi6schemaF</span><span class=p>}</span>
<span class=p>[</span><span class=n>i1i2i3i4i5i6i7i8</span><span class=p>]</span>
<span class=p>[</span><span class=err>#</span><span class=n>i8i1i2i3i4i5i6i7i8</span>
<span class=p>[</span><span class=n>$i</span><span class=err>#</span><span class=n>i812345678</span>
</code></pre></div> </details> <h2 id=deserialization>Deserialization<a class=headerlink href=#deserialization title="Permanent link">&para;</a></h2> <p>The library maps <abbr title="Universal Binary JSON">UBJSON</abbr> types to <abbr title="JavaScript Object Notation">JSON</abbr> value types as follows:</p> <table> <thead> <tr> <th><abbr title="Universal Binary JSON">UBJSON</abbr> type</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="Universal Binary JSON">UBJSON</abbr> value can be converted to a <abbr title="JavaScript Object Notation">JSON</abbr> 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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=p>};</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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<!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/binary_formats/ubjson/" rel="canonical"><link href=../messagepack/ rel=prev><link href=../../binary_values/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>UBJSON - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.06af60db.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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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> UBJSON </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 aria-label="Switch to dark mode" type=radio name=__palette 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=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=p>{</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=k>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=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=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=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=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=p>}</span>
<span class=p>}</span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=p>{</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=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>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</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=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=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=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=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=p>{</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=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>std</span><span class=o>::</span><span class=n>endl</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=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=p>{</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=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>std</span><span class=o>::</span><span class=n>endl</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=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=p>{</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=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>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=p>{</span><span class=n>i7compactTi6schemaF</span><span class=p>}</span>
<span class=p>[</span><span class=n>i1i2i3i4i5i6i7i8</span><span class=p>]</span>
<span class=p>[</span><span class=err>#</span><span class=n>i8i1i2i3i4i5i6i7i8</span>
<span class=p>[</span><span class=n>$i</span><span class=err>#</span><span class=n>i812345678</span>
</code></pre></div> </details> <h2 id=deserialization>Deserialization<a class=headerlink href=#deserialization title="Permanent link">&para;</a></h2> <p>The library maps <abbr title="Universal Binary JSON">UBJSON</abbr> types to <abbr title="JavaScript Object Notation">JSON</abbr> value types as follows:</p> <table> <thead> <tr> <th><abbr title="Universal Binary JSON">UBJSON</abbr> type</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="Universal Binary JSON">UBJSON</abbr> value can be converted to a <abbr title="JavaScript Object Notation">JSON</abbr> 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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=p>};</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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;schema&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div class=md-sLine truncated
class binary_t ["json::binary_t"] {
+void set_subtype(std::uint64_t subtype)
+void clear_subtype()
+std::uint64_t subtype() const
+bool has_subtype() const
}
class vector ["std::vector&lt;uint8_t&gt;"]
vector &lt;|-- binary_t</code></pre> <p>By default, binary values are stored as <code>std::vector&lt;std::uint8_t&gt;</code>. This type can be changed by providing a template parameter to the <code>basic_json</code> type. To store binary subtypes, the storage type is extended and exposed as <code>json::binary_t</code>:</p> <div class=highlight><pre><span></span><code><span class=k>auto</span><span class=w> </span><span class=n>binary</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=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>
</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=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=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=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><abbr title="JavaScript Object Notation">JSON</abbr> 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>
</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 <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> 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=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=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><abbr title="JavaScript Object Notation">JSON</abbr><a class=headerlink href=#json title="Permanent link">&para;</a></h3> <p><abbr title="JavaScript Object Notation">JSON</abbr> 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=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=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>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=nt>&quot;subtype&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> </details> <div class="admonition warning"> <p class=admonition-title>No roundtrip for binary values</p> <p>The <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> is only implemented for debugging purposes.</p> </div> <h3 id=bjdata><abbr title="Binary JData">BJData</abbr><a class=headerlink href=#bjdata title="Permanent link">&para;</a></h3> <p><a href=binary_formats/bjdata.html><abbr title="Binary JData">BJData</abbr></a> neither supports binary values nor subtypes and proposes to serialize binary values as an array of uint8 values. The library implements this translation.</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=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=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 <abbr title="Universal Binary JSON">UBJSON</abbr>:</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>
</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 <abbr title="Universal Binary JSON">UBJSON</abbr> 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=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=p>}</span>
</code></pre></div> </details> <h3 id=bson><abbr title="Binary JSON">BSON</abbr><a class=headerlink href=#bson title="Permanent link">&para;</a></h3> <p><a href=binary_formats/bson.html><abbr title="Binary JSON">BSON</abbr></a> supports binary values and subtypes. If a subtype is given, it is used and added as an 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=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=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=nt>&quot;binary&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</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=nt>&quot;subtype&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> </details> <h3 id=cbor><abbr title="Concise Binary Object Representation">CBOR</abbr><a class=headerlink href=#cbor title="Permanent link">&para;</a></h3> <p><a href=binary_formats/cbor.html><abbr title="Concise Binary Object Representation">CBOR</abbr></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=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=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=nt>&quot;binary&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</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=nt>&quot;subtype&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>null</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</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.html>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 a 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=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=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=nt>&quot;binary&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</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=nt>&quot;subtype&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> </details> <h3 id=ubjson><abbr title="Universal Binary JSON">UBJSON</abbr><a class=headerlink href=#ubjson title="Permanent link">&para;</a></h3> <p><a href=binary_formats/ubjson.html><abbr title="Universal Binary JSON">UBJSON</abbr></a> neither supports binary values nor subtypes and proposes to serialize binary values as an array of uint8 values. The library implements this translation.</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=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=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 <abbr title="Universal Binary JSON">UBJSON</abbr>:</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>
</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 <abbr title="Universal Binary JSON">UBJSON</abbr> 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=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=p>}</span>
</code></pre></div> </details> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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class binary_t ["json::binary_t"] {
+void set_subtype(std::uint64_t subtype)
+void clear_subtype()
+std::uint64_t subtype() const
+bool has_subtype() const
}
class vector ["std::vector&lt;uint8_t&gt;"]
vector &lt;|-- binary_t</code></pre> <p>By default, binary values are stored as <code>std::vector&lt;std::uint8_t&gt;</code>. This type can be changed by providing a template parameter to the <code>basic_json</code> type. To store binary subtypes, the storage type is extended and exposed as <code>json::binary_t</code>:</p> <div class=highlight><pre><span></span><code><span class=k>auto</span><span class=w> </span><span class=n>binary</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=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>
</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=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=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=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><abbr title="JavaScript Object Notation">JSON</abbr> 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>
</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 <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> 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=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=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><abbr title="JavaScript Object Notation">JSON</abbr><a class=headerlink href=#json title="Permanent link">&para;</a></h3> <p><abbr title="JavaScript Object Notation">JSON</abbr> 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=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=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>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=nt>&quot;subtype&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> </details> <div class="admonition warning"> <p class=admonition-title>No roundtrip for binary values</p> <p>The <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> is only implemented for debugging purposes.</p> </div> <h3 id=bjdata><abbr title="Binary JData">BJData</abbr><a class=headerlink href=#bjdata title="Permanent link">&para;</a></h3> <p><a href=../binary_formats/bjdata/ ><abbr title="Binary JData">BJData</abbr></a> neither supports binary values nor subtypes and proposes to serialize binary values as an array of uint8 values. The library implements this translation.</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=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=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 <abbr title="Universal Binary JSON">UBJSON</abbr>:</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>
</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 <abbr title="Universal Binary JSON">UBJSON</abbr> 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=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=p>}</span>
</code></pre></div> </details> <h3 id=bson><abbr title="Binary JSON">BSON</abbr><a class=headerlink href=#bson title="Permanent link">&para;</a></h3> <p><a href=../binary_formats/bson/ ><abbr title="Binary JSON">BSON</abbr></a> supports binary values and subtypes. If a subtype is given, it is used and added as an 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=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=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=nt>&quot;binary&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</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=nt>&quot;subtype&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> </details> <h3 id=cbor><abbr title="Concise Binary Object Representation">CBOR</abbr><a class=headerlink href=#cbor title="Permanent link">&para;</a></h3> <p><a href=../binary_formats/cbor/ ><abbr title="Concise Binary Object Representation">CBOR</abbr></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=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=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=nt>&quot;binary&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</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=nt>&quot;subtype&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>null</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</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 a 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=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=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=nt>&quot;binary&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</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=nt>&quot;subtype&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>42</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> </details> <h3 id=ubjson><abbr title="Universal Binary JSON">UBJSON</abbr><a class=headerlink href=#ubjson title="Permanent link">&para;</a></h3> <p><a href=../binary_formats/ubjson/ ><abbr title="Universal Binary JSON">UBJSON</abbr></a> neither supports binary values nor subtypes and proposes to serialize binary values as an array of uint8 values. The library implements this translation.</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=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=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 <abbr title="Universal Binary JSON">UBJSON</abbr>:</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>
</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 <abbr title="Universal Binary JSON">UBJSON</abbr> 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=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=p>}</span>
</code></pre></div> </details> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<span class=w> </span><span class=c1>// update in 2006: removed Pluto</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=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=p>}</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>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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> // 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=k>try</span>
<span class=w> </span><span class=p>{</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=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>exception</span><span class=o>&amp;</span><span class=w> </span><span class=n>e</span><span class=p>)</span>
<span class=w> </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>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=p>}</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=cm>/* callback */</span><span class=w> </span><span class=k>nullptr</span><span class=p>,</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=cm>/* ignore_comments */</span><span class=w> </span><span class=nb>true</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=sc>&#39;\n&#39;</span><span class=p>;</span>
<span class=p>}</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
{
&quot;planets&quot;: [
&quot;Mercury&quot;,
&quot;Venus&quot;,
&quot;Earth&quot;,
&quot;Mars&quot;,
&quot;Jupiter&quot;,
&quot;Uranus&quot;,
&quot;Neptune&quot;
]
}
</code></pre></div> </div> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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+48
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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/comments/" rel="canonical"><link href=../binary_values/ rel=prev><link href=../trailing_commas/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Comments - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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<span class=w> </span><span class=c1>// update in 2006: removed Pluto</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=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=p>}</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>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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> // 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=k>try</span>
<span class=w> </span><span class=p>{</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=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>exception</span><span class=o>&amp;</span><span class=w> </span><span class=n>e</span><span class=p>)</span>
<span class=w> </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>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=p>}</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=cm>/* callback */</span><span class=w> </span><span class=k>nullptr</span><span class=p>,</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=cm>/* ignore_comments */</span><span class=w> </span><span class=nb>true</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=sc>&#39;\n&#39;</span><span class=p>;</span>
<span class=p>}</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
{
&quot;planets&quot;: [
&quot;Mercury&quot;,
&quot;Venus&quot;,
&quot;Earth&quot;,
&quot;Mars&quot;,
&quot;Jupiter&quot;,
&quot;Uranus&quot;,
&quot;Neptune&quot;
]
}
</code></pre></div> </div> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<!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 rel="canonical" href="https://json.nlohmann.me/features/element_access/checked_access.html"><link rel=prev href=unchecked_access.html><link rel=next href=default_value.html><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Checked access: at - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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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> Checked access: at </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 aria-label="Switch to dark mode" type=radio 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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=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=p>}</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></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></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></code></td> <td><code class=highlight><span class=s2>&quot;Mary Smith&quot;</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></code></td> <td><code class=highlight><span class=mi>42</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></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></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></code></td> <td><code class=highlight><span class=s2>&quot;hiking&quot;</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></code></td> <td><code class=highlight><span class=s2>&quot;reading&quot;</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>
</code></pre></div> <p>This code produces the following <abbr title="JavaScript Object Notation">JSON</abbr> value:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=p>}</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>
</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.html#extended-diagnostic-messages>extended diagnostic messages</a> are enabled by defining <a href=../../api/macros/json_diagnostics.html><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.html><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.html#jsonexceptiontype_error304><code>basic_json::type_error</code></a> is thrown.</li> <li><a href=../../home/exceptions.html#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> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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; 2013-2025 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 7.0.0 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 2025 Fonticons, Inc.--><path fill=currentColor d="M173.9 397.4c0 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 3.6m-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 2.3m44.2-1.7c-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-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 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 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-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-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-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-.3-4.3.7m32.4 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.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/element_access/checked_access/" rel="canonical"><link href=../unchecked_access/ rel=prev><link href=../default_value/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Checked access: at - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.06af60db.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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<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> Checked access: at </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 aria-label="Switch to dark mode" type=radio 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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=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=p>}</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></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></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></code></td> <td><code class=highlight><span class=s2>&quot;Mary Smith&quot;</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></code></td> <td><code class=highlight><span class=mi>42</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></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></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></code></td> <td><code class=highlight><span class=s2>&quot;hiking&quot;</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></code></td> <td><code class=highlight><span class=s2>&quot;reading&quot;</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>
</code></pre></div> <p>This code produces the following <abbr title="JavaScript Object Notation">JSON</abbr> value:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=p>}</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>
</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> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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; 2013-2025 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 7.0.0 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 2025 Fonticons, Inc.--><path fill=currentColor d="M173.9 397.4c0 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 3.6m-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 2.3m44.2-1.7c-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-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 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 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-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-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-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-.3-4.3.7m32.4 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-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 rel="canonical" href="https://json.nlohmann.me/features/element_access/default_value.html"><link rel=prev href=checked_access.html><link rel=next href=../iterators.html><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Access with default value: value - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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<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=nt>&quot;append&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>true</span>
<span class=p>}</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></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></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></code></td> <td><code class=highlight><span class=s2>&quot;result.log&quot;</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></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>&quot;append&quot;</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>&quot;logLevel&quot;</span><span class=p>,</span><span class=w> </span><span class=s>&quot;verbose&quot;</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>&quot;verbose&quot;</span></code></td> </tr> </tbody> </table> </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><code>value</code> can only be used with objects. For other types, a <a href=../../home/exceptions.html#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.html><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. The 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>&lt;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{&quot;uint64&quot;: 18446744073709551615}</span><span class=dl>)</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>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;operator[]: &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=s>&quot;uint64&quot;</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=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;default value (int): &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>value</span><span class=p>(</span><span class=s>&quot;uint64&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=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span>
<span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;default value (uint64_t): &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>value</span><span class=p>(</span><span class=s>&quot;uint64&quot;</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=mi>0</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=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;explict return value type: &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>value</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</span><span class=o>&gt;</span><span class=p>(</span><span class=s>&quot;uint64&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=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span><span class=p>;</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=err>opera</span><span class=kc>t</span><span class=err>or</span><span class=p>[]:</span><span class=w> </span><span class=mi>18446744073709551615</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>(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>explic</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">&para;</a></h2> <ul> <li><a href=../../api/basic_json/value.html><code>value</code></a> for access with default value</li> <li>documentation on <a href=checked_access.html>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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<!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-9.6.16"><title>Access with default value: value - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.06af60db.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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<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=nt>&quot;append&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>true</span>
<span class=p>}</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></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></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></code></td> <td><code class=highlight><span class=s2>&quot;result.log&quot;</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></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>&quot;append&quot;</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>&quot;logLevel&quot;</span><span class=p>,</span><span class=w> </span><span class=s>&quot;verbose&quot;</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>&quot;verbose&quot;</span></code></td> </tr> </tbody> </table> </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><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> <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. The 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>&lt;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{&quot;uint64&quot;: 18446744073709551615}</span><span class=dl>)</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>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;operator[]: &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=s>&quot;uint64&quot;</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=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;default value (int): &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>value</span><span class=p>(</span><span class=s>&quot;uint64&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=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span>
<span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;default value (uint64_t): &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>value</span><span class=p>(</span><span class=s>&quot;uint64&quot;</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=mi>0</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=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;explict return value type: &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>value</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</span><span class=o>&gt;</span><span class=p>(</span><span class=s>&quot;uint64&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=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</span><span class=p>;</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=err>opera</span><span class=kc>t</span><span class=err>or</span><span class=p>[]:</span><span class=w> </span><span class=mi>18446744073709551615</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>(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>explic</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">&para;</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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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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>&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=p>}</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></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></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></code></td> <td><code class=highlight><span class=s2>&quot;Mary Smith&quot;</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></code></td> <td><code class=highlight><span class=mi>42</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></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></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></code></td> <td><code class=highlight><span class=s2>&quot;hiking&quot;</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></code></td> <td><code class=highlight><span class=s2>&quot;reading&quot;</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></code> value is inserted which can 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=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>
</code></pre></div> <p>This code produces the following <abbr title="JavaScript Object Notation">JSON</abbr> value:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=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=p>}</span>
</code></pre></div> </details> <p>When accessing an invalid index (i.e., an index greater than or equal to the array size), the <abbr title="JavaScript Object Notation">JSON</abbr> 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></code>.</p> <details class=example> <summary>Filling up arrays with <code class=highlight><span class=kc>null</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=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>
</code></pre></div> <p>This code produces the following <abbr title="JavaScript Object Notation">JSON</abbr> value:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=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;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=p>}</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></code> and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</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></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></code> is not available for const values.</li> </ul> <p>The type <code class=highlight><span class=n>json</span></code> wraps all <abbr title="JavaScript Object Notation">JSON</abbr> value types. It would be impossible to remove <a href=../../api/basic_json/operator%5B%5D.html><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></code> or redefine the macro <a href=../macros.html#json_assertx><code>JSON_ASSERT(x)</code></a>. See the documentation on <a href=../assertions.html>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.html#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></code> value is returned</td> <td><strong>undefined behavior</strong>; <a href=../assertions.html>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></code> value is returned; any index between previous maximal index and passed index are filled with <code class=highlight><span class=kc>null</span></code></td> <td><strong>undefined behavior</strong>; <a href=../assertions.html>runtime assertion</a> in debug mode</td> </tr> </tbody> </table> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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.getElementByIdLine 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/element_access/unchecked_access/" rel="canonical"><link href=../ rel=prev><link href=../checked_access/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Unchecked access: operator[] - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.06af60db.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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4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5Line 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=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=p>}</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></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></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></code></td> <td><code class=highlight><span class=s2>&quot;Mary Smith&quot;</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></code></td> <td><code class=highlight><span class=mi>42</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></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></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></code></td> <td><code class=highlight><span class=s2>&quot;hiking&quot;</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></code></td> <td><code class=highlight><span class=s2>&quot;reading&quot;</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></code> value is inserted which can 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=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>
</code></pre></div> <p>This code produces the following <abbr title="JavaScript Object Notation">JSON</abbr> value:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=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=p>}</span>
</code></pre></div> </details> <p>When accessing an invalid index (i.e., an index greater than or equal to the array size), the <abbr title="JavaScript Object Notation">JSON</abbr> 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></code>.</p> <details class=example> <summary>Filling up arrays with <code class=highlight><span class=kc>null</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=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>
</code></pre></div> <p>This code produces the following <abbr title="JavaScript Object Notation">JSON</abbr> value:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=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;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=p>}</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></code> and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</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></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></code> is not available for const values.</li> </ul> <p>The type <code class=highlight><span class=n>json</span></code> wraps all <abbr title="JavaScript Object Notation">JSON</abbr> 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></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></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></code> value is returned; any index between previous maximal index and passed index are filled with <code class=highlight><span class=kc>null</span></code></td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> </tbody> </table> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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(lLine 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 rel="canonical" href="https://json.nlohmann.me/features/enum_conversion.html"><link rel=prev href=assertions.html><link rel=next href=macros.html><link rel=icon href=../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Specializing enum conversion - JSON for Modern C++</title><link rel=stylesheet href=../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../assets/stylesheets/palette.06af60db.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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<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=n>TS_STOPPED</span><span class=p>,</span>
<span class=w> </span><span class=n>TS_RUNNING</span><span class=p>,</span>
<span class=w> </span><span class=n>TS_COMPLETED</span><span class=p>,</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=p>};</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=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=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=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=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=p>})</span>
</code></pre></div> <p>The <a href=../api/macros/nlohmann_json_serialize_enum.html><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=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=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=n>assert</span><span class=p>(</span><span class=n>j3</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>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=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=n>assert</span><span class=p>(</span><span class=n>jPi</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>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>
</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.html>Arbitrary Type Conversions</a> above,</p> <ul> <li><a href=../api/macros/nlohmann_json_serialize_enum.html><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>template get&lt;ENUM_TYPE&gt;()</code>, undefined <abbr title="JavaScript Object Notation">JSON</abbr> values will default to the first pair specified in your map. Select this default pair carefully.</li> <li>If an enum or <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr>.</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.html><code>JSON_DISABLE_ENUM_SERIALIZATION</code></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="August 2, 2025 19:06:01 UTC">August 2, 2025</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; 2013-2025 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 7.0.0 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 2025 Fonticons, Inc.--><path fill=currentColor d="M173.9 397.4c0 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 3.6m-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 2.3m44.2-1.7c-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-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 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 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-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-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-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-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.6 1-1.6 3.6 0 5.9s4.3 3.3 5.6 2.3c1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2"/></svg> </a> <a href="https://www.linkedin.com/in/nielslohmann/" target="_blank" rel="noopener" title="www.linkedin.com" class="md-social__link"> <svg xmlns=Line 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=n>TS_STOPPED</span><span class=p>,</span>
<span class=w> </span><span class=n>TS_RUNNING</span><span class=p>,</span>
<span class=w> </span><span class=n>TS_COMPLETED</span><span class=p>,</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=p>};</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=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=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=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=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=p>})</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=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=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=n>assert</span><span class=p>(</span><span class=n>j3</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>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=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=n>assert</span><span class=p>(</span><span class=n>jPi</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>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>
</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>template get&lt;ENUM_TYPE&gt;()</code>, undefined <abbr title="JavaScript Object Notation">JSON</abbr> values will default to the first pair specified in your map. Select this default pair carefully.</li> <li>If an enum or <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr>.</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> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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; 2013-2025 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 7.0.0 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 2025 Fonticons, Inc.--><path fill=currentColor d="M173.9 397.4c0 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 3.6m-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 2.3m44.2-1.7c-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-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 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 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-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-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-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-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.6 1-1.6 3.6 0 5.9s4.3 3.3 5.6 2.3c1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2"/></svg> </a> <a href="https://www.linkedin.com/in/nielslohmann/" target="_blank" rel="noopener" title="www.linkedin.com" class="md-social__link"> <svg xmlns=Line 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=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=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=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=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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=mi>1</span>
<span class=mi>3</span>
<span class=mi>2</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-keys-during-iteration>Access object keys during iteration<a class=headerlink href=#access-object-keys-during-iteration title="Permanent link">&para;</a></h3> <p>The <abbr title="JavaScript Object Notation">JSON</abbr> 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.html#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=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=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=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=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=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>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=p>}</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=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=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>
</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=p>{</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=p>}</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 <abbr title="JavaScript Object Notation">JSON</abbr> 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=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=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=p>}</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=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=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=p>}</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">#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=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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=mi>4</span>
<span class=mi>3</span>
<span class=mi>2</span>
<span class=mi>1</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 <abbr title="JavaScript Object Notation">JSON</abbr> 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=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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=s2>&quot;Hello, world&quot;</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> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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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> Iterators </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 aria-label="Switch to dark mode" type=radio 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<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</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=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=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=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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=mi>1</span>
<span class=mi>3</span>
<span class=mi>2</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-keys-during-iteration>Access object keys during iteration<a class=headerlink href=#access-object-keys-during-iteration title="Permanent link">&para;</a></h3> <p>The <abbr title="JavaScript Object Notation">JSON</abbr> 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=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=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=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=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=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>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=p>}</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=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=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>
</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=p>{</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=p>}</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 <abbr title="JavaScript Object Notation">JSON</abbr> 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=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=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=p>}</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=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=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=p>}</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">#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=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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=mi>4</span>
<span class=mi>3</span>
<span class=mi>2</span>
<span class=mi>1</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 <abbr title="JavaScript Object Notation">JSON</abbr> 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=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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=s2>&quot;Hello, world&quot;</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> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<!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 rel="canonical" href="https://json.nlohmann.me/features/json_patch.html"><link rel=prev href=json_pointer.html><link rel=next href=merge_patch.html><link rel=icon href=../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>JSON Patch and Diff - JSON for Modern C++</title><link rel=stylesheet href=../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../assets/stylesheets/palette.06af60db.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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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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> &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=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;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=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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;foo&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;bar&quot;</span>
<span class=p>}</span>
<span class=p>{</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=nt>&quot;hello&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=s2>&quot;world&quot;</span>
<span class=w> </span><span class=p>]</span>
<span class=p>}</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 <abbr title="JavaScript Object Notation">JSON</abbr> patch (i.e., a <strong>diff</strong>) given two <abbr title="JavaScript Object Notation">JSON</abbr> values.</p> <div class="admonition success"> <p class=admonition-title>Invariant</p> <p>For two <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> patch is created as a diff for two <abbr title="JavaScript Object Notation">JSON</abbr> 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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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> &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=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> &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=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=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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>[</span>
<span class=w> </span><span class=p>{</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=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=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=p>},</span>
<span class=w> </span><span class=p>{</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=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=p>},</span>
<span class=w> </span><span class=p>{</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=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=nt>&quot;value&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=s2>&quot;world&quot;</span>
<span class=w> </span><span class=p>]</span>
<span class=w> </span><span class=p>}</span>
<span class=p>]</span>
<span class=p>{</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=nt>&quot;hello&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=s2>&quot;world&quot;</span>
<span class=w> </span><span class=p>]</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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> &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=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;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=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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;foo&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;bar&quot;</span>
<span class=p>}</span>
<span class=p>{</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=nt>&quot;hello&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=s2>&quot;world&quot;</span>
<span class=w> </span><span class=p>]</span>
<span class=p>}</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 <abbr title="JavaScript Object Notation">JSON</abbr> patch (i.e., a <strong>diff</strong>) given two <abbr title="JavaScript Object Notation">JSON</abbr> values.</p> <div class="admonition success"> <p class=admonition-title>Invariant</p> <p>For two <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> patch is created as a diff for two <abbr title="JavaScript Object Notation">JSON</abbr> 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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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> &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=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> &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=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=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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>[</span>
<span class=w> </span><span class=p>{</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=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=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=p>},</span>
<span class=w> </span><span class=p>{</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=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=p>},</span>
<span class=w> </span><span class=p>{</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=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=nt>&quot;value&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=s2>&quot;world&quot;</span>
<span class=w> </span><span class=p>]</span>
<span class=w> </span><span class=p>}</span>
<span class=p>]</span>
<span class=p>{</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=nt>&quot;hello&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=s2>&quot;world&quot;</span>
<span class=w> </span><span class=p>]</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<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=nt>&quot;nested&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</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=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=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=p>}</span>
<span class=p>}</span>
</code></pre></div> <p>Then every value inside the <abbr title="JavaScript Object Notation">JSON</abbr> document can be identified as follows:</p> <table> <thead> <tr> <th><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> 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></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></code></td> </tr> <tr> <td><code>/array/0</code></td> <td><code class=highlight><span class=err>A</span></code></td> </tr> <tr> <td><code>/array/1</code></td> <td><code class=highlight><span class=err>B</span></code></td> </tr> <tr> <td><code>/array/2</code></td> <td><code class=highlight><span class=err>C</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></code></td> </tr> <tr> <td><code>/nested/one</code></td> <td><code class=highlight><span class=mi>1</span></code></td> </tr> <tr> <td><code>/nested/two</code></td> <td><code class=highlight><span class=mi>2</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></code></td> </tr> <tr> <td><code>/nested/three/0</code></td> <td><code class=highlight><span class=kc>true</span></code></td> </tr> <tr> <td><code>/nested/three/1</code></td> <td><code class=highlight><span class=kc>false</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"><abbr title="Request for Comments">RFC</abbr> 6901</a> for more information.</p> <h2 id=json-pointer-creation><abbr title="JavaScript Object Notation">JSON</abbr> Pointer creation<a class=headerlink href=#json-pointer-creation title="Permanent link">&para;</a></h2> <p><abbr title="JavaScript Object Notation">JSON</abbr> 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=p>(</span><span class=s>&quot;/nested/one&quot;</span><span class=p>);</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>
</code></pre></div> <p>The escaping rules of <a href="https://tools.ietf.org/html/rfc6901"><abbr title="Request for Comments">RFC</abbr> 6901</a> are implemented. See the <a href=../api/json_pointer/json_pointer.html>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><abbr title="JavaScript Object Notation">JSON</abbr> Pointers can be used in the <a href=../api/basic_json/at.html><code>at</code></a>, <a href=../api/basic_json/operator%5B%5D.html><code>operator[]</code></a>, and <a href=../api/basic_json/value.html><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=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.html><code>flatten</code></a> to convert any <abbr title="JavaScript Object Notation">JSON</abbr> document into a <abbr title="JavaScript Object Notation">JSON</abbr> object where each key is a <abbr title="JavaScript Object Notation">JSON</abbr> Pointer and each value is a primitive <abbr title="JavaScript Object Notation">JSON</abbr> 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=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>
</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=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=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=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=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=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=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=nt>&quot;/nested/three/1&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>false</span>
<span class=p>}</span>
</code></pre></div> <p>The reverse function, <a href=../api/basic_json/unflatten.html><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>
</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/index.html><code>json_pointer</code></a></li> <li>Function <a href=../api/basic_json/flatten.html><code>flatten</code></a></li> <li>Function <a href=../api/basic_json/unflatten.html><code>unflatten</code></a></li> <li><a href=json_patch.html><abbr title="JavaScript Object Notation">JSON</abbr> Patch</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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<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=nt>&quot;nested&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</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=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=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=p>}</span>
<span class=p>}</span>
</code></pre></div> <p>Then every value inside the <abbr title="JavaScript Object Notation">JSON</abbr> document can be identified as follows:</p> <table> <thead> <tr> <th><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> 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></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></code></td> </tr> <tr> <td><code>/array/0</code></td> <td><code class=highlight><span class=err>A</span></code></td> </tr> <tr> <td><code>/array/1</code></td> <td><code class=highlight><span class=err>B</span></code></td> </tr> <tr> <td><code>/array/2</code></td> <td><code class=highlight><span class=err>C</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></code></td> </tr> <tr> <td><code>/nested/one</code></td> <td><code class=highlight><span class=mi>1</span></code></td> </tr> <tr> <td><code>/nested/two</code></td> <td><code class=highlight><span class=mi>2</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></code></td> </tr> <tr> <td><code>/nested/three/0</code></td> <td><code class=highlight><span class=kc>true</span></code></td> </tr> <tr> <td><code>/nested/three/1</code></td> <td><code class=highlight><span class=kc>false</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"><abbr title="Request for Comments">RFC</abbr> 6901</a> for more information.</p> <h2 id=json-pointer-creation><abbr title="JavaScript Object Notation">JSON</abbr> Pointer creation<a class=headerlink href=#json-pointer-creation title="Permanent link">&para;</a></h2> <p><abbr title="JavaScript Object Notation">JSON</abbr> 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=p>(</span><span class=s>&quot;/nested/one&quot;</span><span class=p>);</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>
</code></pre></div> <p>The escaping rules of <a href="https://tools.ietf.org/html/rfc6901"><abbr title="Request for Comments">RFC</abbr> 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><abbr title="JavaScript Object Notation">JSON</abbr> 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=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 <abbr title="JavaScript Object Notation">JSON</abbr> document into a <abbr title="JavaScript Object Notation">JSON</abbr> object where each key is a <abbr title="JavaScript Object Notation">JSON</abbr> Pointer and each value is a primitive <abbr title="JavaScript Object Notation">JSON</abbr> 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=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>
</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=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=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=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=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=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=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=nt>&quot;/nested/three/1&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>false</span>
<span class=p>}</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>
</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/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch</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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=p>},</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=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=nt>&quot;tags&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=s2>&quot;example&quot;</span>
<span class=w> </span><span class=p>],</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=p>}</span>
</code></pre></div> </details> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<!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/merge_patch/" rel="canonical"><link href=../json_patch/ rel=prev><link href=../modules/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>JSON Merge Patch - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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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> JSON Merge Patch </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 aria-label="Switch to dark mode" type=radio 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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div Line truncated
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=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=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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=p>},</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=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=nt>&quot;tags&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=s2>&quot;example&quot;</span>
<span class=w> </span><span class=p>],</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=p>}</span>
</code></pre></div> </details> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<!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 rel="canonical" href="https://json.nlohmann.me/features/modules.html"><link rel=prev href=merge_patch.html><link rel=next href=namespace.html><link rel=icon href=../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Modules - JSON for Modern C++</title><link rel=stylesheet href=../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../assets/stylesheets/palette.06af60db.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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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> Modules </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 aria-label="Switch to dark mode" type=radio name=__palette 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<span class=k>import</span><span class=w> </span><span class=n>nlohmann</span><span class=p>.</span><span class=n>json</span><span class=p>;</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=c1>// ...</span>
<span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>f</span><span class=p>(</span><span class=s>&quot;example.json&quot;</span><span class=p>);</span>
<span class=n>json</span><span class=w> </span><span class=n>data</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>
</code></pre></div></p> <h2 id=modules-do-not-export-macros>Modules do not export macros<a class=headerlink href=#modules-do-not-export-macros title="Permanent link">&para;</a></h2> <p>It should be noted that as modules do not export macros, the <code>nlohmann.json</code> module will not export any macros, but rather only the following symbols: - <code>nlohmann::adl_serializer</code> - <code>nlohmann::basic_json</code> - <code>nlohmann::json</code> - <code>nlohmann::json_pointer</code> - <code>nlohmann::ordered_map</code> - <code>nlohmann::ordered_json</code></p> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<span class=k>import</span><span class=w> </span><span class=n>nlohmann</span><span class=p>.</span><span class=n>json</span><span class=p>;</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=c1>// ...</span>
<span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>f</span><span class=p>(</span><span class=s>&quot;example.json&quot;</span><span class=p>);</span>
<span class=n>json</span><span class=w> </span><span class=n>data</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>
</code></pre></div></p> <h2 id=modules-do-not-export-macros>Modules do not export macros<a class=headerlink href=#modules-do-not-export-macros title="Permanent link">&para;</a></h2> <p>It should be noted that as modules do not export macros, the <code>nlohmann.json</code> module will not export any macros, but rather only the following symbols:</p> <ul> <li><code>nlohmann::adl_serializer</code></li> <li><code>nlohmann::basic_json</code></li> <li><code>nlohmann::json</code></li> <li><code>nlohmann::json_pointer</code></li> <li><code>nlohmann::ordered_map</code></li> <li><code>nlohmann::ordered_json</code></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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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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> nlohmann Namespace </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 aria-label="Switch to dark mode" type=radio 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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.html><code>JSON_DIAGNOSTICS</code></a>. This is illustrated in the diagram below.</p> <pre class=mermaid><code>graph
json["&lt;strong&gt;nlohmann_json (v3.10.5)&lt;/strong&gt;&lt;br&gt;JSON_DIAGNOSTICS=0"]
json_diag["&lt;strong&gt;nlohmann_json (v3.10.5)&lt;/strong&gt;&lt;br&gt;JSON_DIAGNOSTICS=1"]
library["&lt;strong&gt;some library&lt;/strong&gt;"]
app["&lt;strong&gt;application&lt;/strong&gt;"]
library --&gt; json
app --&gt; json_diag
app --&gt; library</code></pre> <p>In releases prior to 3.11.0, mixing any version of the <abbr title="JavaScript Object Notation">JSON</abbr> library with different <code>JSON_DIAGNOSTICS</code> settings would result in a crashing application. If <code>some_library</code> never passes instances of <abbr title="JavaScript Object Notation">JSON</abbr> library types to the application, this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.</p> <h2 id=limitations>Limitations<a class=headerlink href=#limitations title="Permanent link">&para;</a></h2> <p>Neither the compiler nor the linker will issue as much as a warning when translation units – intended to be linked together and that include different versions and/or configurations of the <abbr title="JavaScript Object Notation">JSON</abbr> library – exchange and use library types.</p> <p>There is an exception when forward declarations are used (i.e., when including <code>json_fwd.hpp</code>) in which case the linker may complain about undefined references.</p> <h2 id=disabling-the-version-component>Disabling the version component<a class=headerlink href=#disabling-the-version-component title="Permanent link">&para;</a></h2> <p>Different versions are not necessarily ABI-incompatible, but the project does not actively track changes in the ABI and recommends that all parts of a codebase exchanging library types be built with the same version. Users can, <strong>at their own risk</strong>, disable the version component of the linline namespace, allowing different versions – but not configurations – to be used in cases where the linker would otherwise output undefined reference errors.</p> <p>To do so, define <a href=../api/macros/nlohmann_json_namespace_no_version.html><code>NLOHMANN_JSON_NAMESPACE_NO_VERSION</code></a> to <code>1</code>.</p> <p>This applies to version 3.11.2 and above only; versions 3.11.0 and 3.11.1 can apply the technique described in the next section to emulate the effect of the <code>NLOHMANN_JSON_NAMESPACE_NO_VERSION</code> macro.</p> <div class="admonition danger"> <p class=admonition-title>Use at your own risk</p> <p>Disabling the namespace version component and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!</p> </div> <h2 id=disabling-the-inline-namespace-completely>Disabling the inline namespace completely<a class=headerlink href=#disabling-the-inline-namespace-completely title="Permanent link">&para;</a></h2> <p>When interoperability with code using a pre-3.11.0 version of the library is required, users can, <strong>at their own risk</strong> restore the old namespace layout by redefining <a href=../api/macros/nlohmann_json_namespace_begin.html><code>NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END</code></a> as follows:</p> <div class=highlight><pre><span></span><code><span class=cp>#define NLOHMANN_JSON_NAMESPACE_BEGIN namespace nlohmann {</span>
<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> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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; 2013-2025 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 7.0.0 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 2025 Fonticons, Inc.--><path fill=currentColor d="M173.9 397.4c0 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 3.6m-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 2.3m44.2-1.7c-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-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 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 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-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-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-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-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.6 1-1.6 3.6 0 5.9s4.3 3.3 5.6 2.3c1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2"/></svg> </a> <a href="https://www.linkedin.com/in/nielslohmann/" target="_blank" rel="noopener" title="www.linkedin.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 448 512"><!-- Font Awesome Free 7.0.0 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 2025 Fonticons, Inc.--><path fill=currentColor d="M416 32H31.9C14.3 32 0 46.5 0 64.3v383.4C0 465.5 14.3 480 31.9 480H416c17.6 0 32-14.5 32-32.3V64.3c0-17.8-14.4-32.3-32-32.3M135.4 416H69V202.2h66.5V416zM102.2 96a38.5 38.5 0 1 1 0 77 38.5 38.5 0 1 1 0-77m282.1 320h-66.4V312c0-24.8-.5-56.7-34.5-56.7-34.6 0-39.9 27-39.9 54.9V416h-66.4V202.2h63.7v29.2h.9c8.9-16.8 30.6-34.5 62.9-34.5 67.2 0 79.7 44.3 79.7 101.9z"/></svg> </a> <a href="https://www.xing.com/profile/Niels_Lohmann" target="_blank" rel="noopener" title="www.xing.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svgLine truncated
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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> <pre class=mermaid><code>graph
json["&lt;strong&gt;nlohmann_json (v3.10.5)&lt;/strong&gt;&lt;br&gt;JSON_DIAGNOSTICS=0"]
json_diag["&lt;strong&gt;nlohmann_json (v3.10.5)&lt;/strong&gt;&lt;br&gt;JSON_DIAGNOSTICS=1"]
library["&lt;strong&gt;some library&lt;/strong&gt;"]
app["&lt;strong&gt;application&lt;/strong&gt;"]
library --&gt; json
app --&gt; json_diag
app --&gt; library</code></pre> <p>In releases prior to 3.11.0, mixing any version of the <abbr title="JavaScript Object Notation">JSON</abbr> library with different <code>JSON_DIAGNOSTICS</code> settings would result in a crashing application. If <code>some_library</code> never passes instances of <abbr title="JavaScript Object Notation">JSON</abbr> library types to the application, this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.</p> <h2 id=limitations>Limitations<a class=headerlink href=#limitations title="Permanent link">&para;</a></h2> <p>Neither the compiler nor the linker will issue as much as a warning when translation units – intended to be linked together and that include different versions and/or configurations of the <abbr title="JavaScript Object Notation">JSON</abbr> library – exchange and use library types.</p> <p>There is an exception when forward declarations are used (i.e., when including <code>json_fwd.hpp</code>) in which case the linker may complain about undefined references.</p> <h2 id=disabling-the-version-component>Disabling the version component<a class=headerlink href=#disabling-the-version-component title="Permanent link">&para;</a></h2> <p>Different versions are not necessarily ABI-incompatible, but the project does not actively track changes in the ABI and recommends that all parts of a codebase exchanging library types be built with the same version. Users can, <strong>at their own risk</strong>, disable the version component of the linline namespace, allowing different versions – but not configurations – to be used in cases where the linker would otherwise output undefined reference errors.</p> <p>To do so, define <a href=../../api/macros/nlohmann_json_namespace_no_version/ ><code>NLOHMANN_JSON_NAMESPACE_NO_VERSION</code></a> to <code>1</code>.</p> <p>This applies to version 3.11.2 and above only; versions 3.11.0 and 3.11.1 can apply the technique described in the next section to emulate the effect of the <code>NLOHMANN_JSON_NAMESPACE_NO_VERSION</code> macro.</p> <div class="admonition danger"> <p class=admonition-title>Use at your own risk</p> <p>Disabling the namespace version component and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!</p> </div> <h2 id=disabling-the-inline-namespace-completely>Disabling the inline namespace completely<a class=headerlink href=#disabling-the-inline-namespace-completely title="Permanent link">&para;</a></h2> <p>When interoperability with code using a pre-3.11.0 version of the library is required, users can, <strong>at their own risk</strong> restore the old namespace layout by redefining <a href=../../api/macros/nlohmann_json_namespace_begin/ ><code>NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END</code></a> as follows:</p> <div class=highlight><pre><span></span><code><span class=cp>#define NLOHMANN_JSON_NAMESPACE_BEGIN namespace nlohmann {</span>
<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> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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; 2013-2025 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 7.0.0 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 2025 Fonticons, Inc.--><path fill=currentColor d="M173.9 397.4c0 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 3.6m-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 2.3m44.2-1.7c-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-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 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 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-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-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-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-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.6 1-1.6 3.6 0 5.9s4.3 3.3 5.6 2.3c1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2"/></svg> </a> <a href="https://www.linkedin.com/in/nielslohmann/" target="_blank" rel="noopener" title="www.linkedin.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 448 512"><!-- Font Awesome Free 7.0.0 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 2025 Fonticons, Inc.--><path fill=currentColor d="M416 32H31.9C14.3 32 0 46.5 0 64.3v383.4C0 465.5 14.3 480 31.9 480H416c17.6 0 32-14.5 32-32.3V64.3c0-17.8-14.4-32.3-32-32.3M135.4 416H69V202.2h66.5V416zM102.2 96a38.5 38.5 0 1 1 0 77 38.5 38.5 0 1 1 0-77m282.1 320h-66.4V312c0-24.8-.5-56.7-34.5-56.7-34.6 0-39.9 27-39.9 54.9V416h-66.4V202.2h63.7v29.2h.9c8.9-16.8 30.6-34.5 62.9-34.5 67.2 0 79.7 44.3 79.7 101.9z"/></svg> </a> <a href="https://www.xing.com/profile/Niels_Lohmann" target="_blank" rel="noopener" title="www.xing.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svgLine 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 rel="canonical" href="https://json.nlohmann.me/features/object_order.html"><link rel=prev href=namespace.html><link rel=next href=parsing/index.html><link rel=icon href=../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Object Order - JSON for Modern C++</title><link rel=stylesheet href=../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../assets/stylesheets/palette.06af60db.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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<span class=cp>#include</span><span class=w> </span><span class=cpf>&quot;json.hpp&quot;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=nt>&quot;two&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span>
<span class=p>}</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>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>3</span>
<span class=p>}</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.html><code>parse</code></a> function when reading from a file. Assume file <code>input.json</code> contains the <abbr title="JavaScript Object Notation">JSON</abbr> object above:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>3</span>
<span class=p>}</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=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=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>
</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=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=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=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>3</span>
<span class=p>}</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=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=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>
</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=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=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>3</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=p>}</span>
</code></pre></div> </details> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<!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 href=../namespace/ rel=prev><link href=../parsing/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Object Order - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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<span class=cp>#include</span><span class=w> </span><span class=cpf>&quot;json.hpp&quot;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=nt>&quot;two&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>2</span>
<span class=p>}</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>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>3</span>
<span class=p>}</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 <abbr title="JavaScript Object Notation">JSON</abbr> object above:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>3</span>
<span class=p>}</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=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=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>
</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=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=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=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>3</span>
<span class=p>}</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=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=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>
</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=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=nt>&quot;three&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>3</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=p>}</span>
</code></pre></div> </details> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<!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 rel="canonical" href="https://json.nlohmann.me/features/parsing/json_lines.html"><link rel=prev href=index.html><link rel=next href=parse_exceptions.html><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>JSON Lines - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div cLine truncated
<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=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=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>
</code></pre></div> </div> <p><abbr title="JavaScript Object Notation">JSON</abbr> Lines input with more than one value is treated as invalid <abbr title="JavaScript Object Notation">JSON</abbr> by the <a href=../../api/basic_json/parse.html><code>parse</code></a> or <a href=../../api/basic_json/accept.html><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 <abbr title="JavaScript Object Notation">JSON</abbr> Text input line by line</p> <p>The example below demonstrates how <abbr title="JavaScript Object Notation">JSON</abbr> 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>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=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=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>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=p>}</span>
<span class=p>}</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=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=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=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>
</code></pre></div> </div> <div class="admonition warning"> <p class=admonition-title>Note</p> <p>Using <a href=../../api/operator_gtgt.html><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=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=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=p>}</span>
</code></pre></div> <p>with a <abbr title="JavaScript Object Notation">JSON</abbr> Lines input does not work, because the parser will try to parse one value after the last one.</p> </div> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<!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/parsing/json_lines/" rel="canonical"><link href=../ rel=prev><link href=../parse_exceptions/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>JSON Lines - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.06af60db.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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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> JSON Lines </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 aria-label="Switch to dark mode" type=radio 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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </labeLine truncated
<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=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=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>
</code></pre></div> </div> <p><abbr title="JavaScript Object Notation">JSON</abbr> Lines input with more than one value is treated as invalid <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> Text input line by line</p> <p>The example below demonstrates how <abbr title="JavaScript Object Notation">JSON</abbr> 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>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=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=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=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=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>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=p>}</span>
<span class=p>}</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=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=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=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>
</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=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=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=p>}</span>
</code></pre></div> <p>with a <abbr title="JavaScript Object Notation">JSON</abbr> Lines input does not work, because the parser will try to parse one value after the last one.</p> </div> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<!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 rel="canonical" href="https://json.nlohmann.me/features/parsing/parse_exceptions.html"><link rel=prev href=json_lines.html><link rel=next href=parser_callbacks.html><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Parsing and Exceptions - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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<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> Parsing and Exceptions </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 aria-label="Switch to dark mode" 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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 4.23l.27.27h.79l5 5-1.5 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<span class=k>try</span>
<span class=p>{</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=p>}</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=p>{</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=p>}</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></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></code>, default) or whether a discarded value should be returned (<code class=highlight><span class=nb>false</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=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=p>{</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=p>}</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></code> indicating whether the input is valid <abbr title="JavaScript Object Notation">JSON</abbr>.</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=p>{</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=p>}</span>
</code></pre></div> <p>Again, there is no diagnostic information available.</p> <h2 id=user-defined-sax-interface>User-defined <abbr title="Simple API for XML">SAX</abbr> 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.html><abbr title="Simple API for XML">SAX</abbr> 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=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>
</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></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>#include</span><span class=w> </span><span class=cpf>&quot;json.hpp&quot;</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=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=p>{</span>
<span class=w> </span><span class=k>public</span><span class=o>:</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=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=p>{}</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=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><span class=p>{</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=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=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=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=k>return</span><span class=w> </span><span class=nb>false</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
<span class=p>};</span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=n>json</span><span class=w> </span><span class=n>result</span><span class=p>;</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=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=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=p>{</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=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=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=p>}</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> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<!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/parsing/parse_exceptions/" rel="canonical"><link href=../json_lines/ rel=prev><link href=../parser_callbacks/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Parsing and Exceptions - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.06af60db.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> Parsing and Exceptions </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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<span class=k>try</span>
<span class=p>{</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=p>}</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=p>{</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=p>}</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></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></code>, default) or whether a discarded value should be returned (<code class=highlight><span class=nb>false</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=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=p>{</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=p>}</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></code> indicating whether the input is valid <abbr title="JavaScript Object Notation">JSON</abbr>.</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=p>{</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=p>}</span>
</code></pre></div> <p>Again, there is no diagnostic information available.</p> <h2 id=user-defined-sax-interface>User-defined <abbr title="Simple API for XML">SAX</abbr> 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/ ><abbr title="Simple API for XML">SAX</abbr> 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=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>
</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></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>#include</span><span class=w> </span><span class=cpf>&quot;json.hpp&quot;</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=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=p>{</span>
<span class=w> </span><span class=k>public</span><span class=o>:</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=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=p>{}</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=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><span class=p>{</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=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=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=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=k>return</span><span class=w> </span><span class=nb>false</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
<span class=p>};</span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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=n>json</span><span class=w> </span><span class=n>result</span><span class=p>;</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=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=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=p>{</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=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=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=p>}</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> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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-93
View File
@@ -1,93 +0,0 @@
<!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 rel="canonical" href="https://json.nlohmann.me/features/parsing/parser_callbacks.html"><link rel=prev href=parse_exceptions.html><link rel=next href=sax_interface.html><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Parser Callbacks - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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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> Parser Callbacks </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 aria-label="Switch to dark mode" type=radio 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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 Line truncated
<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=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>
</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 <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> object</td> </tr> <tr> <td><code>parse_event_t::array_start</code></td> <td>the parser read <code>[</code> and started to process a <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> array</td> </tr> <tr> <td><code>parse_event_t::value</code></td> <td>the parser finished reading a <abbr title="JavaScript Object Notation">JSON</abbr> value</td> <td>depth of the value</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> value</td> </tr> </tbody> </table> <details class=example> <summary>Example</summary> <p>When parsing the following <abbr title="JavaScript Object Notation">JSON</abbr> text,</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;location&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=mf>52.519444</span><span class=p>,</span>
<span class=w> </span><span class=mf>13.406667</span>
<span class=w> </span><span class=p>]</span>
<span class=p>}</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></code></td> </tr> <tr> <td><code>value</code></td> <td>1</td> <td><code class=highlight><span class=s2>&quot;Berlin&quot;</span></code></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>&quot;location&quot;</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></code></td> </tr> <tr> <td><code>value</code></td> <td>2</td> <td><code class=highlight><span class=mf>13.406667</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></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></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></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></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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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> &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=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=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=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=p>{</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=k>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=p>{</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=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=k>return</span><span class=w> </span><span class=nb>true</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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=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=nt>&quot;IDs&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=mi>116</span><span class=p>,</span>
<span class=w> </span><span class=mi>943</span><span class=p>,</span>
<span class=w> </span><span class=mi>234</span><span class=p>,</span>
<span class=w> </span><span class=mi>38793</span>
<span class=w> </span><span class=p>],</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=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=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=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>100</span>
<span class=w> </span><span class=p>},</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=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
<span class=p>{</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=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=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=nt>&quot;IDs&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=mi>116</span><span class=p>,</span>
<span class=w> </span><span class=mi>943</span><span class=p>,</span>
<span class=w> </span><span class=mi>234</span><span class=p>,</span>
<span class=w> </span><span class=mi>38793</span>
<span class=w> </span><span class=p>],</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=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<!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/parsing/parser_callbacks/" rel="canonical"><link href=../parse_exceptions/ rel=prev><link href=../sax_interface/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Parser Callbacks - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.06af60db.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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<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> Parser Callbacks </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 aria-label="Switch to dark mode" type=radio 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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=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>
</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 <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> object</td> </tr> <tr> <td><code>parse_event_t::array_start</code></td> <td>the parser read <code>[</code> and started to process a <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> array</td> </tr> <tr> <td><code>parse_event_t::value</code></td> <td>the parser finished reading a <abbr title="JavaScript Object Notation">JSON</abbr> value</td> <td>depth of the value</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> value</td> </tr> </tbody> </table> <details class=example> <summary>Example</summary> <p>When parsing the following <abbr title="JavaScript Object Notation">JSON</abbr> text,</p> <div class=highlight><pre><span></span><code><span class=p>{</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=nt>&quot;location&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=mf>52.519444</span><span class=p>,</span>
<span class=w> </span><span class=mf>13.406667</span>
<span class=w> </span><span class=p>]</span>
<span class=p>}</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></code></td> </tr> <tr> <td><code>value</code></td> <td>1</td> <td><code class=highlight><span class=s2>&quot;Berlin&quot;</span></code></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>&quot;location&quot;</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></code></td> </tr> <tr> <td><code>value</code></td> <td>2</td> <td><code class=highlight><span class=mf>13.406667</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></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></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></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></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>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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> &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=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=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=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=p>{</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=k>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=p>{</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=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=k>return</span><span class=w> </span><span class=nb>true</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=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=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=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</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=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=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=nt>&quot;IDs&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=mi>116</span><span class=p>,</span>
<span class=w> </span><span class=mi>943</span><span class=p>,</span>
<span class=w> </span><span class=mi>234</span><span class=p>,</span>
<span class=w> </span><span class=mi>38793</span>
<span class=w> </span><span class=p>],</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=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=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=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>100</span>
<span class=w> </span><span class=p>},</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=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
<span class=p>{</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=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=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=nt>&quot;IDs&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=mi>116</span><span class=p>,</span>
<span class=w> </span><span class=mi>943</span><span class=p>,</span>
<span class=w> </span><span class=mi>234</span><span class=p>,</span>
<span class=w> </span><span class=mi>38793</span>
<span class=w> </span><span class=p>],</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=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> </details> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<!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 rel="canonical" href="https://json.nlohmann.me/features/parsing/sax_interface.html"><link rel=prev href=parser_callbacks.html><link rel=next href=../assertions.html><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>SAX Interface - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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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> SAX Interface </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 aria-label="Switch to dark mode" type=radio 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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></Line truncated
class sax_t ["json::sax_t"] {
&lt;&lt;interface&gt;&gt;
+bool null()*
+bool boolean(bool val)*
+bool number_integer(number_integer_t val)*
+bool number_unsigned(number_unsigned_t val)*
+bool number_float(number_float_t val, const string_t&amp; s)*
+bool string(string_t&amp; val)*
+bool binary(binary_t&amp; val)*
+bool start_object(std::size_t elements)*
+bool end_object()*
+bool start_array(std::size_t elements)*
+bool end_array()*
+bool key(string_t&amp; val)*
+bool parse_error(std::size_t position, const std::string&amp; last_token, const json::exception&amp; ex)*
}</code></pre> <div class=highlight><pre><span></span><code><span class=c1>// called when null is parsed</span>
<span class=kt>bool</span><span class=w> </span><span class=nf>null</span><span class=p>();</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=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=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=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=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=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_t</span><span class=o>&amp;</span><span class=w> </span><span class=n>val</span><span class=p>);</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=kt>bool</span><span class=w> </span><span class=nf>end_object</span><span class=p>();</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=kt>bool</span><span class=w> </span><span class=nf>end_array</span><span class=p>();</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=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>
</code></pre></div> <p>The return value of each function determines whether parsing should proceed.</p> <p>To implement your own <abbr title="Simple API for XML">SAX</abbr> handler, proceed as follows:</p> <ol> <li>Implement the <abbr title="Simple API for XML">SAX</abbr> 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 <abbr title="Simple API for XML">SAX</abbr> 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></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 <abbr title="Simple API for XML">SAX</abbr> interface.</li> </ol> <p>Note the <code>sax_parse</code> function only returns a <code class=highlight><span class=kt>bool</span></code> indicating the result of the last executed <abbr title="Simple API for XML">SAX</abbr> event. It does not return <code>json</code> value - it is up to you to decide what to do with the <abbr title="Simple API for XML">SAX</abbr> 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 <abbr title="Simple API for XML">SAX</abbr> interface is used for the <abbr title="Document Object Model">DOM</abbr> 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/index.html>json_sax</a> - documentation of the <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../api/basic_json/sax_parse.html>sax_parse</a> - <abbr title="Simple API for XML">SAX</abbr> parser</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="August 2, 2025 19:06:01 UTC">August 2, 2025</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; 2013-2025 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 7.0.0 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 2025 Fonticons, Inc.--><path fill=currentColor d="M173.9 397.4c0 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 3.6m-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 2.3m44.2-1.7c-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-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 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 0-2Line truncated
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<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> SAX Interface </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 aria-label="Switch to dark mode" type=radio 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class sax_t ["json::sax_t"] {
&lt;&lt;interface&gt;&gt;
+bool null()*
+bool boolean(bool val)*
+bool number_integer(number_integer_t val)*
+bool number_unsigned(number_unsigned_t val)*
+bool number_float(number_float_t val, const string_t&amp; s)*
+bool string(string_t&amp; val)*
+bool binary(binary_t&amp; val)*
+bool start_object(std::size_t elements)*
+bool end_object()*
+bool start_array(std::size_t elements)*
+bool end_array()*
+bool key(string_t&amp; val)*
+bool parse_error(std::size_t position, const std::string&amp; last_token, const json::exception&amp; ex)*
}</code></pre> <div class=highlight><pre><span></span><code><span class=c1>// called when null is parsed</span>
<span class=kt>bool</span><span class=w> </span><span class=nf>null</span><span class=p>();</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=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=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=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=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=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_t</span><span class=o>&amp;</span><span class=w> </span><span class=n>val</span><span class=p>);</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=kt>bool</span><span class=w> </span><span class=nf>end_object</span><span class=p>();</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=kt>bool</span><span class=w> </span><span class=nf>end_array</span><span class=p>();</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=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>
</code></pre></div> <p>The return value of each function determines whether parsing should proceed.</p> <p>To implement your own <abbr title="Simple API for XML">SAX</abbr> handler, proceed as follows:</p> <ol> <li>Implement the <abbr title="Simple API for XML">SAX</abbr> 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 <abbr title="Simple API for XML">SAX</abbr> 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></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 <abbr title="Simple API for XML">SAX</abbr> interface.</li> </ol> <p>Note the <code>sax_parse</code> function only returns a <code class=highlight><span class=kt>bool</span></code> indicating the result of the last executed <abbr title="Simple API for XML">SAX</abbr> event. It does not return <code>json</code> value - it is up to you to decide what to do with the <abbr title="Simple API for XML">SAX</abbr> 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 <abbr title="Simple API for XML">SAX</abbr> interface is used for the <abbr title="Document Object Model">DOM</abbr> 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 <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../../api/basic_json/sax_parse/ >sax_parse</a> - <abbr title="Simple API for XML">SAX</abbr> parser</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="August 3, 2025 08:02:12 UTC">August 3, 2025</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; 2013-2025 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 7.0.0 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 2025 Fonticons, Inc.--><path fill=currentColor d="M173.9 397.4c0 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 3.6m-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 2.3m44.2-1.7c-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-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 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.Line truncated
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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> Trailing Commas </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 aria-label="Switch to dark mode" type=radio 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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div claLine truncated
<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=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=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=p>,</span>
<span class=w> </span><span class=p>]</span>
<span class=p>}</span>
</code></pre></div> <p>When calling <code>parse</code> without additional argument, a parse error exception is thrown. If <code>ignore_trailing_commas</code> is set to <code>true</code>, the trailing commas 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>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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> &quot;planets&quot;: [</span>
<span class=s> &quot;Mercury&quot;,</span>
<span class=s> &quot;Venus&quot;,</span>
<span class=s> &quot;Earth&quot;,</span>
<span class=s> &quot;Mars&quot;,</span>
<span class=s> &quot;Jupiter&quot;,</span>
<span class=s> &quot;Uranus&quot;,</span>
<span class=s> &quot;Neptune&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=p>;</span>
<span class=w> </span><span class=k>try</span>
<span class=w> </span><span class=p>{</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=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>exception</span><span class=o>&amp;</span><span class=w> </span><span class=n>e</span><span class=p>)</span>
<span class=w> </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>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=p>}</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=cm>/* callback */</span><span class=w> </span><span class=k>nullptr</span><span class=p>,</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=cm>/* ignore_comments */</span><span class=w> </span><span class=nb>false</span><span class=p>,</span>
<span class=w> </span><span class=cm>/* ignore_trailing_commas */</span><span class=w> </span><span class=nb>true</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=sc>&#39;\n&#39;</span><span class=p>;</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>[json.exception.parse_error.101] parse error at line 11, column 9: syntax error while parsing value - unexpected &#39;]&#39;; expected &#39;[&#39;, &#39;{&#39;, or a literal
{
&quot;planets&quot;: [
&quot;Mercury&quot;,
&quot;Venus&quot;,
&quot;Earth&quot;,
&quot;Mars&quot;,
&quot;Jupiter&quot;,
&quot;Uranus&quot;,
&quot;Neptune&quot;
]
}
</code></pre></div> </div> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<!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/trailing_commas/" rel="canonical"><link href=../comments/ rel=prev><link href=../element_access/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Trailing Commas - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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=#trailing-commas 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> Trailing Commas </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 aria-label="Switch to dark mode" type=radio 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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label>Line truncated
<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=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=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=p>,</span>
<span class=w> </span><span class=p>]</span>
<span class=p>}</span>
</code></pre></div> <p>When calling <code>parse</code> without additional argument, a parse error exception is thrown. If <code>ignore_trailing_commas</code> is set to <code>true</code>, the trailing commas 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>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</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=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</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> &quot;planets&quot;: [</span>
<span class=s> &quot;Mercury&quot;,</span>
<span class=s> &quot;Venus&quot;,</span>
<span class=s> &quot;Earth&quot;,</span>
<span class=s> &quot;Mars&quot;,</span>
<span class=s> &quot;Jupiter&quot;,</span>
<span class=s> &quot;Uranus&quot;,</span>
<span class=s> &quot;Neptune&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=p>;</span>
<span class=w> </span><span class=k>try</span>
<span class=w> </span><span class=p>{</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=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>exception</span><span class=o>&amp;</span><span class=w> </span><span class=n>e</span><span class=p>)</span>
<span class=w> </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>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=p>}</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=cm>/* callback */</span><span class=w> </span><span class=k>nullptr</span><span class=p>,</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=cm>/* ignore_comments */</span><span class=w> </span><span class=nb>false</span><span class=p>,</span>
<span class=w> </span><span class=cm>/* ignore_trailing_commas */</span><span class=w> </span><span class=nb>true</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=sc>&#39;\n&#39;</span><span class=p>;</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>[json.exception.parse_error.101] parse error at line 11, column 9: syntax error while parsing value - unexpected &#39;]&#39;; expected &#39;[&#39;, &#39;{&#39;, or a literal
{
&quot;planets&quot;: [
&quot;Mercury&quot;,
&quot;Venus&quot;,
&quot;Earth&quot;,
&quot;Mars&quot;,
&quot;Jupiter&quot;,
&quot;Uranus&quot;,
&quot;Neptune&quot;
]
}
</code></pre></div> </div> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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<!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 rel="canonical" href="https://json.nlohmann.me/features/types/index.html"><link rel=prev href=../macros.html><link rel=next href=number_handling.html><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Types - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.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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class value_t {
&lt;&lt;enumeration&gt;&gt;
@@ -77,4 +77,4 @@ basic_json .. value_t</code></pre> <h2 id=template-arguments>Template arguments<
<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>
</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"><abbr title="Request for Comments">RFC</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> 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"><abbr title="Request for Comments">RFC</abbr> 8259</a> describes <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> 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></code>.</p> <h3 id=encoding>Encoding<a class=headerlink href=#encoding title="Permanent link">&para;</a></h3> <p>Strings are stored in <abbr title="Unicode Transformation Format">UTF</abbr>-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"><abbr title="Request for Comments">RFC</abbr> 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"><abbr title="Request for Comments">RFC</abbr> 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></code>), the default value for <code>boolean_t</code> is <code class=highlight><span class=kt>bool</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.html>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"><abbr title="Request for Comments">RFC</abbr> 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 rLine truncated
</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"><abbr title="Request for Comments">RFC</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> 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"><abbr title="Request for Comments">RFC</abbr> 8259</a> describes <abbr title="JavaScript Object Notation">JSON</abbr> 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 <abbr title="JavaScript Object Notation">JSON</abbr> 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></code>.</p> <h3 id=encoding>Encoding<a class=headerlink href=#encoding title="Permanent link">&para;</a></h3> <p>Strings are stored in <abbr title="Unicode Transformation Format">UTF</abbr>-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"><abbr title="Request for Comments">RFC</abbr> 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"><abbr title="Request for Comments">RFC</abbr> 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></code>), the default value for <code>boolean_t</code> is <code class=highlight><span class=kt>bool</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"><abbr title="Request for Comments">RFC</abbr> 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 reprLine truncated
-57
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@@ -1,57 +0,0 @@
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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5Line 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=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>
</code></pre></div> yields <code class=highlight><span class=mf>0.30000001192092896</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>
</code></pre></div> </div> <h4 id=nan-handling><abbr title="Not a Number">NaN</abbr> handling<a class=headerlink href=#nan-handling title="Permanent link">&para;</a></h4> <p><abbr title="Not a Number">NaN</abbr> (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"><abbr title="Request for Comments">RFC</abbr> 8259</a>, Section 6</p> <p>Numeric values that cannot be represented in the grammar below (such as Infinity and <abbr title="Not a Number">NaN</abbr>) are not permitted.</p> </div> <p>That is, there is no way to <em>parse</em> a <abbr title="Not a Number">NaN</abbr> value. However, assignments can store <abbr title="Not a Number">NaN</abbr> values in a <abbr title="JavaScript Object Notation">JSON</abbr> value.</p> <p>This library serializes <abbr title="Not a Number">NaN</abbr> values as <code class=highlight><span class=kc>null</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 <abbr title="Not a Number">NaN</abbr> 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=p>{</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=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=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=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=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=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=p>}</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 <abbr title="JavaScript Object Notation">JSON</abbr> 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></code> which is <code class=highlight><span class=kt>double</span><span class=o>::</span><span class=n>operator</span><span class=o>==</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=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=p>{</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=p>}</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=p>{</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=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=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=p>{</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=p>{</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=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=c1>// other cases remain the same with the original</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=p>...</span>
<span class=p>}</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><abbr title="Not a Number">NaN</abbr> values never compare equal to themselves or to other <abbr title="Not a Number">NaN</abbr> 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=k>template</span><span class=w> </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.html#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><abbr title="JavaScript Object Notation">JSON</abbr> does not define a number type or precision (see above).</li> <li>C++ also allows silently converting 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.html><code>is_number_integer()</code></a> or <a href=../../api/basic_json/is_number_unsigned.html><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><span class=p>{</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=k>template</span><span class=w> </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>// ...</span>
<span class=p>}</span>
<span class=k>else</span>
<span class=p>{</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>
</code></pre></div> <p>Note this approach also has the advantage that it can react on non-numerical <abbr title="JavaScript Object Notation">JSON</abbr> 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.html><code>is_number()</code></a> returns <code class=highlight><span class=nb>true</span></code> for any number type</li> <li><a href=../../api/basic_json/is_number_integer.html><code>is_number_integer()</code></a> returns <code class=highlight><span class=nb>true</span></code> for signed and unsigned integers</li> <li><a href=../../api/basic_json/is_number_unsigned.html><code>is_number_unsigned()</code></a> returns <code class=highlight><span class=nb>true</span></code> for unsigned integers only</li> <li><a href=../../api/basic_json/is_number_float.html><code>is_number_float()</code></a> returns <code class=highlight><span class=nb>true</span></code> for floating-point numbers</li> <li><a href=../../api/basic_json/type_name.html><code>type_name()</code></a> returns <code class=highlight><span class=s>&quot;number&quot;</span></code> for any number type</li> <li><a href=../../api/basic_json/type.html><code>type()</code></a> returns a different enumerator of <a href=../../api/basic_json/value_t.html><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.html><code>is_number()</code></a></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> </tr> <tr> <td><a href=../../api/basic_json/is_number_integer.html><code>is_number_integer()</code></a></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> </tr> <tr> <td><a href=../../api/basic_json/is_number_unsigned.html><code>is_number_unsigned()</code></a></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> </tr> <tr> <td><a href=../../api/basic_json/is_number_float.html><code>is_number_float()</code></a></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> </tr> <tr> <td><a href=../../api/basic_json/type_name.html><code>type_name()</code></a></td> <td><code class=highlight><span class=s>&quot;number&quot;</span></code></td> <td><code class=highlight><span class=s>&quot;number&quot;</span></code></td> <td><code class=highlight><span class=s>&quot;number&quot;</span></code></td> <td><code class=highlight><span class=s>&quot;string&quot;</span></code></td> </tr> <tr> <td><a href=../../api/basic_json/type.html><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></code></td> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>int32_t</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>int16_t</span></code>, etc.</td> </tr> <tr> <td>6</td> <td>unsigned integers</td> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</span></code></td> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>uint32_t</span></code>, <code class=highliLine 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></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></code> before then converting them to <code class=highlight><span class=kt>long</span><span class=w> </span><span class=kt>double</span></code>.</p> </div> <!-- 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="August 2, 2025 19:06:01 UTC">August 2, 2025</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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<!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/number_handling/" rel="canonical"><link href=../ rel=prev><link href=../../../integration/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.6.16"><title>Number Handling - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.7e37652d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.06af60db.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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<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> Number Handling </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 aria-label="Switch to dark mode" type=radio 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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.Line 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=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>
</code></pre></div> yields <code class=highlight><span class=mf>0.30000001192092896</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>
</code></pre></div> </div> <h4 id=nan-handling><abbr title="Not a Number">NaN</abbr> handling<a class=headerlink href=#nan-handling title="Permanent link">&para;</a></h4> <p><abbr title="Not a Number">NaN</abbr> (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"><abbr title="Request for Comments">RFC</abbr> 8259</a>, Section 6</p> <p>Numeric values that cannot be represented in the grammar below (such as Infinity and <abbr title="Not a Number">NaN</abbr>) are not permitted.</p> </div> <p>That is, there is no way to <em>parse</em> a <abbr title="Not a Number">NaN</abbr> value. However, assignments can store <abbr title="Not a Number">NaN</abbr> values in a <abbr title="JavaScript Object Notation">JSON</abbr> value.</p> <p>This library serializes <abbr title="Not a Number">NaN</abbr> values as <code class=highlight><span class=kc>null</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 <abbr title="Not a Number">NaN</abbr> 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=p>{</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=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=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=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=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=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=p>}</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 <abbr title="JavaScript Object Notation">JSON</abbr> 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></code> which is <code class=highlight><span class=kt>double</span><span class=o>::</span><span class=n>operator</span><span class=o>==</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=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=p>{</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=p>}</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=p>{</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=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=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=p>{</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=p>{</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=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=c1>// other cases remain the same with the original</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=p>...</span>
<span class=p>}</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><abbr title="Not a Number">NaN</abbr> values never compare equal to themselves or to other <abbr title="Not a Number">NaN</abbr> 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=k>template</span><span class=w> </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><abbr title="JavaScript Object Notation">JSON</abbr> does not define a number type or precision (see above).</li> <li>C++ also allows silently converting 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><span class=p>{</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=k>template</span><span class=w> </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>// ...</span>
<span class=p>}</span>
<span class=k>else</span>
<span class=p>{</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>
</code></pre></div> <p>Note this approach also has the advantage that it can react on non-numerical <abbr title="JavaScript Object Notation">JSON</abbr> 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></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></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></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></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></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></code></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>false</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></code></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>false</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></code></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>false</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></code></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>false</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></code></td> <td><code class=highlight><span class=s>&quot;number&quot;</span></code></td> <td><code class=highlight><span class=s>&quot;number&quot;</span></code></td> <td><code class=highlight><span class=s>&quot;string&quot;</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></code></td> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>int32_t</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>int16_t</span></code>, etc.</td> </tr> <tr> <td>6</td> <td>unsigned integers</td> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</span></code></td> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>uint32_t</span></code>, <code class=highliLine 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></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></code> before then converting them to <code class=highlight><span class=kt>long</span><span class=w> </span><span class=kt>double</span></code>.</p> </div> <!-- 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="August 3, 2025 08:02:12 UTC">August 3, 2025</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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