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@@ -7,47 +7,47 @@
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<span class=w> </span><span class=p>};</span>
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<span class=p>}</span><span class=w> </span><span class=c1>// namespace ns</span>
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<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>"Ned Flanders"</span><span class=p>,</span><span class=w> </span><span class=s>"744 Evergreen Terrace"</span><span class=p>,</span><span class=w> </span><span class=mi>60</span><span class=p>};</span>
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<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>"Ned Flanders"</span><span class=p>,</span><span class=w> </span><span class=s>"744 Evergreen Terrace"</span><span class=p>,</span><span class=w> </span><span class=mi>60</span><span class=p>};</span>
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<span class=c1>// convert to JSON: copy each value into the JSON object</span>
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<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span>
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<span class=n>j</span><span class=p>[</span><span class=s>"name"</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>
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<span class=n>j</span><span class=p>[</span><span class=s>"address"</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>
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<span class=n>j</span><span class=p>[</span><span class=s>"age"</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>
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<span class=n>j</span><span class=p>[</span><span class=s>"name"</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>
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<span class=n>j</span><span class=p>[</span><span class=s>"address"</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>
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<span class=n>j</span><span class=p>[</span><span class=s>"age"</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>
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<span class=c1>// ...</span>
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<span class=c1>// convert from JSON: copy each value from the JSON object</span>
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<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>
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<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>(),</span>
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<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"address"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>(),</span>
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<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"age"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=p>()</span>
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<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>(),</span>
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<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"address"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>(),</span>
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<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"age"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=p>()</span>
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<span class=p>};</span>
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</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>
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<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>"Ned Flanders"</span><span class=p>,</span><span class=w> </span><span class=s>"744 Evergreen Terrace"</span><span class=p>,</span><span class=w> </span><span class=mi>60</span><span class=p>};</span>
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<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>"Ned Flanders"</span><span class=p>,</span><span class=w> </span><span class=s>"744 Evergreen Terrace"</span><span class=p>,</span><span class=w> </span><span class=mi>60</span><span class=p>};</span>
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<span class=c1>// conversion: person -> json</span>
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<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>
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<span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</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>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span>
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<span class=c1>// {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}</span>
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<span class=c1>// {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}</span>
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<span class=c1>// conversion: json -> person</span>
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<span class=k>auto</span><span class=w> </span><span class=n>p2</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=n>ns</span><span class=o>::</span><span class=n>person</span><span class=o>></span><span class=p>();</span>
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<span class=c1>// that's it</span>
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<span class=c1>// that's it</span>
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<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>
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</code></pre></div> <h2 id=basic-usage>Basic usage<a class=headerlink href=#basic-usage title="Permanent link">¶</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>
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<span class=k>namespace</span><span class=w> </span><span class=nn>ns</span><span class=w> </span><span class=p>{</span>
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<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>&</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>&</span><span class=w> </span><span class=n>p</span><span class=p>)</span><span class=w> </span><span class=p>{</span>
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<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>"name"</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>"address"</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>"age"</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>
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<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>"name"</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>"address"</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>"age"</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>
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<span class=w> </span><span class=p>}</span>
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<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>&</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>&</span><span class=w> </span><span class=n>p</span><span class=p>)</span><span class=w> </span><span class=p>{</span>
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<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>"name"</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>
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<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>"address"</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>
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<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>"age"</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>
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<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>"name"</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>
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<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>"address"</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>
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<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>"age"</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>
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<span class=w> </span><span class=p>}</span>
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<span class=p>}</span><span class=w> </span><span class=c1>// namespace ns</span>
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</code></pre></div> <p>That's all! When calling the <code>json</code> constructor with your type, your custom <code>to_json</code> method will be automatically called. Likewise, when calling <code>get<your_type>()</code> or <code>get_to(your_type&)</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>get<your_type>()</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">¶</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 several macros to make your life easier as long as you want to use a <abbr title="JavaScript Object Notation">JSON</abbr> object as serialization. The macros are following the naming pattern, and you can choose the macro based on the needed features:</p> <ul> <li>All the macros start with <code>NLOHMANN_DEFINE</code>.</li> <li>If you want a macro for the derived object, use the <a href=../../api/macros/nlohmann_define_derived_type/ ><code>DERIVED_TYPE</code></a> variant, otherwise use <code>TYPE</code>.<ul> <li>The <code>DERIVED_TYPE</code> variant requires an additional parameter of a base type, which should have the <code>to_json</code>/<code>from_json</code> functions defined. For instance, with a macro of its own.</li> </ul> </li> <li>If you need access to the private fields use <a href=../../api/macros/nlohmann_define_type_intrusive/ ><code>INTRUSIVE</code></a> variant, otherwise use <a href=../../api/macros/nlohmann_define_type_non_intrusive/ ><code>NON_INTRUSIVE</code></a>.<ul> <li>The <code>INTRUSIVE</code> macro should be defined <strong>inside</strong> the target class/struct, <code>NON_INTRUSIVE</code> should be defined within the same namespace.</li> </ul> </li> <li>If you want to deserialize the incomplete JSONs, use the <code>WITH_DEFAULTS</code> variant, which will use the default values for the member variables absent in <abbr title="JavaScript Object Notation">JSON</abbr>, the variant without <code>WITH_DEFAULTS</code> will raise an exception.</li> <li>If you do not need deserialization at all and only interested in <code>to_json</code> function, you can use the <code>ONLY_SERIALIZE</code> variant.</li> <li>If you want to use the custom <abbr title="JavaScript Object Notation">JSON</abbr> names for member variables, use <a href=../../api/macros/nlohmann_define_type_with_names/ ><code>WITH_NAMES</code></a> variant, otherwise the <abbr title="JavaScript Object Notation">JSON</abbr> name of the variable will be the same as its regular name. </li> </ul> <p>For all the macros, the first parameter is the name of the class/struct. The <code>DERIVED_TYPE</code> macros require a second parameter of a base class. All the remaining parameters name the member variables. The <code>WITH_NAMES</code> macros require a <abbr title="JavaScript Object Notation">JSON</abbr> name before each of the variables.</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 style="color: green;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m16.28-2.72a.75.75 0 0 0-.018-1.042.75.75 0 0 0-1.042-.018l-5.97 5.97-2.47-2.47a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042l3 3a.75.75 0 0 0 1.06 0Z"/></svg></span></div></td> <td><div style="color: red;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 Line truncated
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@@ -79,25 +79,25 @@
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<span class=w> </span><span class=kt>int</span><span class=w> </span><span class=n>m_postcode</span><span class=p>;</span>
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<span class=w> </span><span class=k>public</span><span class=o>:</span>
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<span class=w> </span><span class=n>NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES</span><span class=p>(</span><span class=n>address</span><span class=p>,</span><span class=w> </span><span class=s>"street"</span><span class=p>,</span><span class=w> </span><span class=n>m_street</span><span class=p>,</span>
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<span class=w> </span><span class=s>"housenumber"</span><span class=p>,</span><span class=w> </span><span class=n>m_housenumber</span><span class=p>,</span>
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<span class=w> </span><span class=s>"postcode"</span><span class=p>,</span><span class=w> </span><span class=n>m_postcode</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=n>NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES</span><span class=p>(</span><span class=n>address</span><span class=p>,</span><span class=w> </span><span class=s>"street"</span><span class=p>,</span><span class=w> </span><span class=n>m_street</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s>"housenumber"</span><span class=p>,</span><span class=w> </span><span class=n>m_housenumber</span><span class=p>,</span>
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||||
<span class=w> </span><span class=s>"postcode"</span><span class=p>,</span><span class=w> </span><span class=n>m_postcode</span><span class=p>)</span>
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||||
<span class=w> </span><span class=p>};</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <div class="admonition warning"> <p class=admonition-title>Overriding conversions for natively-supported types</p> <p>The library already provides built-in <code>to_json</code>/<code>from_json</code> conversions for <abbr title="Standard Template Library">STL</abbr> containers such as <code>std::vector</code>, <code>std::array</code>, and <code>std::map</code>. Defining your own free-function <code>to_json</code>/<code>from_json</code> overload for one of these container types directly (instead of for your own type) can conflict with the built-in overload during overload resolution, producing compiler errors ("no matching overloaded function", "call is ambiguous") that vary by compiler and library version. If you need different conversion behavior for a container type the library already handles, wrap it in your own type (or use <code>adl_serializer</code> specialization, as shown <a href=#how-do-i-convert-third-party-types>above</a> for <code>boost::optional</code>) instead of trying to re-specialize <code>to_json</code>/<code>from_json</code> for the container type itself.</p> </div> <div class="admonition warning"> <p class=admonition-title>Raw C-style arrays</p> <p>Members declared as raw C-style arrays (e.g., <code>char buf[1024]</code>) do not round-trip safely through <code>NLOHMANN_DEFINE_TYPE_*</code> macros or the default (de)serializers: <code>to_json</code> serializes any <code>char</code> array as a <abbr title="JavaScript Object Notation">JSON</abbr> <em>string</em> (matching the <code>std::string</code>-constructible overload), but the <code>from_json</code> overload for fixed-size arrays expects a <abbr title="JavaScript Object Notation">JSON</abbr> <em>array</em> and iterates it element-wise, which fails with a <code>type_error</code> when given a string. Use <code>std::string</code>, <code>std::array<char, N></code>, or a manually written <code>to_json</code>/<code>from_json</code> pair for such members instead.</p> </div> <div class="admonition note"> <p class=admonition-title>Macros and <code>nlohmann::ordered_json</code></p> <p>The <code>NLOHMANN_DEFINE_TYPE_*</code>/<code>NLOHMANN_DEFINE_DERIVED_TYPE_*</code> macros are generic over any <code>basic_json</code> specialization, including <code>nlohmann::ordered_json</code>. Simply use <code>ordered_json</code> as the target type and members are serialized in declaration order -- no separate macro or extra 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>
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||||
<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>
|
||||
|
||||
<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=p>{</span><span class=s>"Ned Flanders"</span><span class=p>,</span><span class=w> </span><span class=s>"744 Evergreen Terrace"</span><span class=p>,</span><span class=w> </span><span class=mi>60</span><span class=p>};</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=p>{</span><span class=s>"Ned Flanders"</span><span class=p>,</span><span class=w> </span><span class=s>"744 Evergreen Terrace"</span><span class=p>,</span><span class=w> </span><span class=mi>60</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>p</span><span class=p>;</span><span class=w> </span><span class=c1>// keys appear in declaration order: name, address, age</span>
|
||||
</code></pre></div> </div> <div class="admonition note"> <p class=admonition-title>No macro for non-default-constructible types</p> <p>There is currently no <code>NLOHMANN_DEFINE_TYPE_*</code>-style macro for types that are not <a href="https://en.cppreference.com/w/cpp/named_req/DefaultConstructible">DefaultConstructible</a>. This is not an intentional omission of documentation -- no such macro exists yet; see <a href=#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> for the manual pattern to use instead.</p> </div> <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">¶</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><</span><span class=k>typename</span><span class=w> </span><span class=nc>T</span><span class=o>></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>&</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>&</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 "to_json" method in T's namespace</span>
|
||||
<span class=w> </span><span class=c1>// calls the "to_json" method in T'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>&</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>&</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 "from_json" method</span>
|
||||
<span class=w> </span><span class=c1>// same thing, but with the "from_json" 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>
|
||||
@@ -109,7 +109,7 @@
|
||||
<span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=nb>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<T>::to_json which will</span>
|
||||
<span class=w> </span><span class=c1>// find the free function to_json in T's namespace!</span>
|
||||
<span class=w> </span><span class=c1>// find the free function to_json in T's namespace!</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
|
||||
@@ -135,13 +135,13 @@
|
||||
<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><></span>
|
||||
<span class=w> </span><span class=k>struct</span><span class=w> </span><span class=nc>adl_serializer</span><span class=o><</span><span class=n>move_only_type</span><span class=o>></span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=c1>// note: the return type is no longer 'void', and the method only takes</span>
|
||||
<span class=w> </span><span class=c1>// note: the return type is no longer 'void', 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>&</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=n>get</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=p>()};</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// Here's the catch! You must provide a to_json method! Otherwise, you</span>
|
||||
<span class=w> </span><span class=c1>// Here'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>&</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>
|
||||
@@ -165,11 +165,11 @@
|
||||
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>a</span><span class=p>.</span><span class=n>type</span><span class=p>()</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=k>typeid</span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=p>))</span><span class=w> </span><span class=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>any_cast</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>(</span><span class=n>a</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>nullptr</span><span class=p>;</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <h2 id=why-does-serializing-a-stdmapstdunordered_map-with-non-string-keys-produce-an-array>Why does serializing a <code>std::map</code>/<code>std::unordered_map</code> with non-string keys produce an array?<a class=headerlink href=#why-does-serializing-a-stdmapstdunordered_map-with-non-string-keys-produce-an-array title="Permanent link">¶</a></h2> <p>A <code>std::map</code>/<code>std::unordered_map</code> whose key type is not string-like (e.g., <code>std::map<int, std::string></code>) is serialized as a <abbr title="JavaScript Object Notation">JSON</abbr> <em>array</em> of 2-element <code>[key, value]</code> arrays, not as a <abbr title="JavaScript Object Notation">JSON</abbr> object -- <abbr title="JavaScript Object Notation">JSON</abbr> object keys must be strings, so the library cannot represent an integer-keyed map as an object.</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>map</span><span class=o><</span><span class=kt>int</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=o>></span><span class=w> </span><span class=n>m</span><span class=p>{{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=s>"one"</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=s>"two"</span><span class=p>}};</span>
|
||||
</code></pre></div> <h2 id=why-does-serializing-a-stdmapstdunordered_map-with-non-string-keys-produce-an-array>Why does serializing a <code>std::map</code>/<code>std::unordered_map</code> with non-string keys produce an array?<a class=headerlink href=#why-does-serializing-a-stdmapstdunordered_map-with-non-string-keys-produce-an-array title="Permanent link">¶</a></h2> <p>A <code>std::map</code>/<code>std::unordered_map</code> whose key type is not string-like (e.g., <code>std::map<int, std::string></code>) is serialized as a <abbr title="JavaScript Object Notation">JSON</abbr> <em>array</em> of 2-element <code>[key, value]</code> arrays, not as a <abbr title="JavaScript Object Notation">JSON</abbr> object -- <abbr title="JavaScript Object Notation">JSON</abbr> object keys must be strings, so the library cannot represent an integer-keyed map as an object.</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>map</span><span class=o><</span><span class=kt>int</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=o>></span><span class=w> </span><span class=n>m</span><span class=p>{{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=s>"one"</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=s>"two"</span><span class=p>}};</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>m</span><span class=p>;</span>
|
||||
<span class=c1>// j is [[1,"one"],[2,"two"]], not {"1":"one","2":"two"}</span>
|
||||
<span class=c1>// j is [[1,"one"],[2,"two"]], not {"1":"one","2":"two"}</span>
|
||||
</code></pre></div> <h2 id=why-does-stdwstring-convert-or-dump-incorrectly>Why does <code>std::wstring</code> convert or dump incorrectly?<a class=headerlink href=#why-does-stdwstring-convert-or-dump-incorrectly title="Permanent link">¶</a></h2> <p>The library assumes <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoding internally, so <code>std::wstring</code> is not supported out of the box -- see the <abbr title="Frequently Asked Questions">FAQ</abbr> entry on <a href=../../home/faq/#wide-string-handling>wide string handling</a> for why, and for a <abbr title="Unicode Transformation Format">UTF</abbr>-8 conversion recipe.</p> <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">¶</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 <= 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't need compile-time checks on T</span>
|
||||
<span class=c1>// if you don't need compile-time checks on T</span>
|
||||
<span class=k>template</span><span class=o><</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><</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><=</span><span class=w> </span><span class=mi>32</span><span class=o>>::</span><span class=n>type</span><span class=o>></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><</span><span class=k>typename</span><span class=w> </span><span class=nc>BasicJsonType</span><span class=o>></span>
|
||||
@@ -204,4 +204,4 @@
|
||||
<span class=w> </span><span class=n>value</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=k>template</span><span class=w> </span><span class=n>get</span><span class=o><</span><span class=n>T</span><span class=o>></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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> <!-- 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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -4,8 +4,8 @@
|
||||
|
||||
<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>"key"</span><span class=p>,</span><span class=w> </span><span class=s>"value"</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>"missing"</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>"key"</span><span class=p>,</span><span class=w> </span><span class=s>"value"</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>"missing"</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->find(key) != m_value.object->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">¶</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><nlohmann/json.hpp></span>
|
||||
@@ -35,7 +35,7 @@
|
||||
|
||||
<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>"nonexistent_file.json"</span><span class=p>,</span><span class=w> </span><span class=s>"r"</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>"nonexistent_file.json"</span><span class=p>,</span><span class=w> </span><span class=s>"r"</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>&</span><span class=w> </span><span class=n>e</span><span class=p>)</span><span class=w> </span><span class=p>{</span>
|
||||
@@ -43,4 +43,4 @@
|
||||
<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">¶</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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@@ -1,7 +1,7 @@
|
||||
<!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-9.7.7"><title>BJData - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#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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|
||||
<span class=w> </span><span class=nt>"_ArrayType_"</span><span class=p>:</span><span class=w> </span><span class=s2>"uint8"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"_ArraySize_"</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>"_ArrayData_"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>1</span><span class=p>,</span><span class=mi>2</span><span class=p>,</span><span class=mi>3</span><span class=p>,</span><span class=mi>4</span><span class=p>,</span><span class=mi>5</span><span class=p>,</span><span class=mi>6</span><span class=p>]</span>
|
||||
<span class=w> </span><span class=nt>"_ArrayType_"</span><span class=p>:</span><span class=w> </span><span class=s2>"uint8"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"_ArraySize_"</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>"_ArrayData_"</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 serialized 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. 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 instead.</p> <p>An object is only converted if the annotation actually describes a packed array; otherwise it is serialized as a regular <abbr title="JavaScript Object Notation">JSON</abbr> object. This requires all of the following:</p> <ul> <li><code>"_ArrayType_"</code> is one of <code>uint8</code>, <code>int8</code>, <code>uint16</code>, <code>int16</code>, <code>uint32</code>, <code>int32</code>, <code>uint64</code>, <code>int64</code>, <code>single</code>, <code>double</code>, <code>char</code>, or <code>byte</code>,</li> <li>every entry of <code>"_ArraySize_"</code> is a non-negative integer, and their product is representable as a <code>std::size_t</code>,</li> <li><code>"_ArrayData_"</code> holds exactly that many elements, and</li> <li>every element of <code>"_ArrayData_"</code> is a number of the kind named by <code>"_ArrayType_"</code> (a floating-point number for <code>single</code> and <code>double</code>, an integer otherwise).</li> </ul> <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><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iomanip></span>
|
||||
@@ -10,31 +10,31 @@
|
||||
<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'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=c1>// function to print BJData'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=kt>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><</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><</span><span class=w> </span><span class=mi>128</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><</span><span class=w> </span><span class=kt>byte</span><span class=w> </span><span class=k>and</span><span class=w> </span><span class=kt>byte</span><span class=w> </span><span class=o><</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><<</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=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=p>(</span><span class=kt>char</span><span class=p>)</span><span class=kt>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><<</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=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=p>)</span><span class=kt>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>"</span><span class=dl>(</span><span class=s>{"compact": true, "schema": false}</span><span class=dl>)</span><span class=s>"</span><span class=n>_json</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=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"compact": true, "schema": false}</span><span class=dl>)</span><span class=s>"</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><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></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>&</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=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&</span><span class=w> </span><span class=kt>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=n>print_byte</span><span class=p>(</span><span class=kt>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><<</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span>
|
||||
|
||||
@@ -49,21 +49,21 @@
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></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>&</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=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&</span><span class=w> </span><span class=kt>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=n>print_byte</span><span class=p>(</span><span class=kt>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><<</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>&</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=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&</span><span class=w> </span><span class=kt>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=n>print_byte</span><span class=p>(</span><span class=kt>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><<</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>&</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=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&</span><span class=w> </span><span class=kt>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=n>print_byte</span><span class=p>(</span><span class=kt>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><<</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>
|
||||
@@ -92,7 +92,7 @@
|
||||
<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><<</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><<</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>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>"compact"</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>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
|
||||
<span class=w> </span><span class=nt>"compact"</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>"schema"</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> </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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -8,15 +8,15 @@
|
||||
<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>"</span><span class=dl>(</span><span class=s>{"compact": true, "schema": 0}</span><span class=dl>)</span><span class=s>"</span><span class=n>_json</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=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"compact": true, "schema": 0}</span><span class=dl>)</span><span class=s>"</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><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></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>&</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=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&</span><span class=w> </span><span class=kt>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><<</span><span class=w> </span><span class=s>"0x"</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>hex</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>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</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>setfill</span><span class=p>(</span><span class=sc>'0'</span><span class=p>)</span><span class=w> </span><span class=o><<</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><<</span><span class=w> </span><span class=s>" "</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><<</span><span class=w> </span><span class=s>"0x"</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>hex</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>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</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>setfill</span><span class=p>(</span><span class=sc>'0'</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=p>)</span><span class=kt>byte</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>" "</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><<</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>
|
||||
@@ -43,7 +43,7 @@
|
||||
<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><<</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><<</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>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>"compact"</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>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
|
||||
<span class=w> </span><span class=nt>"compact"</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>"schema"</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> </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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -160,14 +160,11 @@ The library maps CBOR types to JSON value types as follows:
|
||||
|
||||
The mapping is **incomplete** in the sense that not all CBOR types can be converted to a JSON value. The following CBOR types are not supported and will yield parse errors:
|
||||
|
||||
- date/time (0xC0..0xC1)
|
||||
- bignum (0xC2..0xC3)
|
||||
- decimal fraction (0xC4)
|
||||
- bigfloat (0xC5)
|
||||
- expected conversions (0xD5..0xD7)
|
||||
- simple values (0xE0..0xF3, 0xF8)
|
||||
- undefined (0xF7)
|
||||
|
||||
Tagged items (0xC0..0xDB) are not interpreted either; see the note on tagged items below.
|
||||
|
||||
!!! warning "Negative integer overflow"
|
||||
|
||||
CBOR negative integers (major type 1) are decoded as `-1 - n`. If the encoded magnitude `n` is too large for the
|
||||
@@ -181,7 +178,7 @@ The library maps CBOR types to JSON value types as follows:
|
||||
|
||||
!!! warning "Tagged items"
|
||||
|
||||
Tagged items will throw a parse error by default. They can be ignored by passing `cbor_tag_handler_t::ignore` to function `from_cbor`. They can be stored by passing `cbor_tag_handler_t::store` to function `from_cbor`.
|
||||
Tagged items (0xC0..0xDB) will throw a parse error by default. They can be ignored by passing `cbor_tag_handler_t::ignore` to function `from_cbor`, in which case the tag is skipped and the enclosed data item is parsed on its own. They can be stored by passing `cbor_tag_handler_t::store` to function `from_cbor`. Note that no tag is ever interpreted: for instance, a text string tagged with tag 0 (date/time) stays a string.
|
||||
|
||||
??? example
|
||||
|
||||
|
||||
@@ -8,20 +8,20 @@
|
||||
<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>"</span><span class=dl>(</span><span class=s>{"compact": true, "schema": 0}</span><span class=dl>)</span><span class=s>"</span><span class=n>_json</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=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"compact": true, "schema": 0}</span><span class=dl>)</span><span class=s>"</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><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></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>&</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=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&</span><span class=w> </span><span class=kt>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><<</span><span class=w> </span><span class=s>"0x"</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>hex</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>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</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>setfill</span><span class=p>(</span><span class=sc>'0'</span><span class=p>)</span><span class=w> </span><span class=o><<</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><<</span><span class=w> </span><span class=s>" "</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><<</span><span class=w> </span><span class=s>"0x"</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>hex</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>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</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>setfill</span><span class=p>(</span><span class=sc>'0'</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=p>)</span><span class=kt>byte</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>" "</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><<</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">¶</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>Negative integer overflow</p> <p><abbr title="Concise Binary Object Representation">CBOR</abbr> negative integers (major type 1) are decoded as <code>-1 - n</code>. If the encoded magnitude <code>n</code> is too large for the result to fit into <code>number_integer_t</code> (<code>std::int64_t</code> by default), parsing fails with a <a href=../../../home/exceptions/#jsonexceptionparse_error112><code>parse_error.112</code></a> exception rather than overflowing silently.</p> </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</abLine truncated
|
||||
</code></pre></div> </details> <h2 id=deserialization>Deserialization<a class=headerlink href=#deserialization title="Permanent link">¶</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>simple values (0xE0..0xF3, 0xF8)</li> <li>undefined (0xF7)</li> </ul> <p>Tagged items (0xC0..0xDB) are not interpreted either; see the note on tagged items below.</p> </div> <div class="admonition warning"> <p class=admonition-title>Negative integer overflow</p> <p><abbr title="Concise Binary Object Representation">CBOR</abbr> negative integers (major type 1) are decoded as <code>-1 - n</code>. If the encoded magnitude <code>n</code> is too large for the result to fit into <code>number_integer_t</code> (<code>std::int64_t</code> by default), parsing fails with a <a href=../../../home/exceptions/#jsonexceptionparse_error112><code>parse_error.112</code></a> exception rather than overflowing silently.</p> </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 TransformationLine truncated
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iomanip></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
|
||||
@@ -42,7 +42,7 @@
|
||||
<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><<</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><<</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>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>"compact"</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>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
|
||||
<span class=w> </span><span class=nt>"compact"</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>"schema"</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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||||
</code></pre></div> </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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -174,14 +174,11 @@ Incomplete mapping
|
||||
|
||||
The mapping is **incomplete** in the sense that not all CBOR types can be converted to a JSON value. The following CBOR types are not supported and will yield parse errors:
|
||||
|
||||
- date/time (0xC0..0xC1)
|
||||
- bignum (0xC2..0xC3)
|
||||
- decimal fraction (0xC4)
|
||||
- bigfloat (0xC5)
|
||||
- expected conversions (0xD5..0xD7)
|
||||
- simple values (0xE0..0xF3, 0xF8)
|
||||
- undefined (0xF7)
|
||||
|
||||
Tagged items (0xC0..0xDB) are not interpreted either; see the note on tagged items below.
|
||||
|
||||
Negative integer overflow
|
||||
|
||||
CBOR negative integers (major type 1) are decoded as `-1 - n`. If the encoded magnitude `n` is too large for the result to fit into `number_integer_t` (`std::int64_t` by default), parsing fails with a [`parse_error.112`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error112) exception rather than overflowing silently.
|
||||
@@ -192,7 +189,7 @@ CBOR allows map keys of any type, whereas JSON only allows strings as keys in ob
|
||||
|
||||
Tagged items
|
||||
|
||||
Tagged items will throw a parse error by default. They can be ignored by passing `cbor_tag_handler_t::ignore` to function `from_cbor`. They can be stored by passing `cbor_tag_handler_t::store` to function `from_cbor`.
|
||||
Tagged items (0xC0..0xDB) will throw a parse error by default. They can be ignored by passing `cbor_tag_handler_t::ignore` to function `from_cbor`, in which case the tag is skipped and the enclosed data item is parsed on its own. They can be stored by passing `cbor_tag_handler_t::store` to function `from_cbor`. Note that no tag is ever interpreted: for instance, a text string tagged with tag 0 (date/time) stays a string.
|
||||
|
||||
Example
|
||||
|
||||
|
||||
@@ -1 +1 @@
|
||||
<!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/" rel="canonical"><link href=../arbitrary_types/ rel=prev><link href=bjdata/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Binary Formats - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#binary-formats 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> Binary Formats </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 id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 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> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 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 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 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
|
||||
<!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/" rel="canonical"><link href=../arbitrary_types/ rel=prev><link href=bjdata/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Binary Formats - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#binary-formats 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> Binary Formats </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 id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 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> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 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 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 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
|
||||
@@ -8,15 +8,15 @@
|
||||
<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>"</span><span class=dl>(</span><span class=s>{"compact": true, "schema": 0}</span><span class=dl>)</span><span class=s>"</span><span class=n>_json</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=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"compact": true, "schema": 0}</span><span class=dl>)</span><span class=s>"</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><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></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>&</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=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&</span><span class=w> </span><span class=kt>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><<</span><span class=w> </span><span class=s>"0x"</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>hex</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>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</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>setfill</span><span class=p>(</span><span class=sc>'0'</span><span class=p>)</span><span class=w> </span><span class=o><<</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><<</span><span class=w> </span><span class=s>" "</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><<</span><span class=w> </span><span class=s>"0x"</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>hex</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>setw</span><span class=p>(</span><span class=mi>2</span><span class=p>)</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>setfill</span><span class=p>(</span><span class=sc>'0'</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=p>)</span><span class=kt>byte</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>" "</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><<</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>
|
||||
@@ -42,7 +42,7 @@
|
||||
<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><<</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><<</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>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>"compact"</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>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
|
||||
<span class=w> </span><span class=nt>"compact"</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>"schema"</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> </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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -5,31 +5,31 @@
|
||||
<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'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=c1>// function to print UBJSON'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=kt>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><</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><</span><span class=w> </span><span class=mi>128</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><</span><span class=w> </span><span class=kt>byte</span><span class=w> </span><span class=k>and</span><span class=w> </span><span class=kt>byte</span><span class=w> </span><span class=o><</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><<</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=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=p>(</span><span class=kt>char</span><span class=p>)</span><span class=kt>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><<</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=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=p>)</span><span class=kt>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>"</span><span class=dl>(</span><span class=s>{"compact": true, "schema": false}</span><span class=dl>)</span><span class=s>"</span><span class=n>_json</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=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"compact": true, "schema": false}</span><span class=dl>)</span><span class=s>"</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><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></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>&</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=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&</span><span class=w> </span><span class=kt>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=n>print_byte</span><span class=p>(</span><span class=kt>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><<</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span>
|
||||
|
||||
@@ -44,21 +44,21 @@
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></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>&</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=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&</span><span class=w> </span><span class=kt>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=n>print_byte</span><span class=p>(</span><span class=kt>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><<</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>&</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=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&</span><span class=w> </span><span class=kt>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=n>print_byte</span><span class=p>(</span><span class=kt>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><<</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>&</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=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&</span><span class=w> </span><span class=kt>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=n>print_byte</span><span class=p>(</span><span class=kt>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><<</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>
|
||||
@@ -87,7 +87,7 @@
|
||||
<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><<</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><<</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>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>"compact"</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>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
|
||||
<span class=w> </span><span class=nt>"compact"</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>"schema"</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> </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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -36,122 +36,122 @@ vector <|-- binary_t</code></pre> <p>By default, binary values are stored as
|
||||
<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">¶</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">¶</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>"binary"</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=n>j</span><span class=p>[</span><span class=s>"binary"</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><<</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><<</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>"binary"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"bytes"</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>"subtype"</span><span class=p>:</span><span class=w> </span><span class=mi>42</span>
|
||||
<span class=w> </span><span class=nt>"binary"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"bytes"</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>"subtype"</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">¶</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>"binary"</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=n>j</span><span class=p>[</span><span class=s>"binary"</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<std::uint8_t></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>// '{'</span>
|
||||
</code></pre></div> <p><code>v</code> is a <code>std::vector<std::uint8_t></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>// '{'</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>// "binary"</span>
|
||||
<span class=w> </span><span class=mh>0x5B</span><span class=w> </span><span class=c1>// '['</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 'U')</span>
|
||||
<span class=w> </span><span class=mh>0x5D</span><span class=w> </span><span class=c1>// ']'</span>
|
||||
<span class=mh>0x7D</span><span class=w> </span><span class=c1>// '}'</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>// "binary"</span>
|
||||
<span class=w> </span><span class=mh>0x5B</span><span class=w> </span><span class=c1>// '['</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 'U')</span>
|
||||
<span class=w> </span><span class=mh>0x5D</span><span class=w> </span><span class=c1>// ']'</span>
|
||||
<span class=mh>0x7D</span><span class=w> </span><span class=c1>// '}'</span>
|
||||
</code></pre></div> <p>The following code uses the type and size optimization for <abbr title="Binary JData">BJData</abbr>:</p> <div class=highlight><pre><span></span><code><span class=c1>// convert to BJData 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>// '{'</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>// '#' 'i' type of the array elements: unsigned integers</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>// '{'</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>// '#' 'i' 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>// "binary"</span>
|
||||
<span class=w> </span><span class=mh>0x5B</span><span class=w> </span><span class=c1>// '[' 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>// '$' 'U' 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>// '#' i 4 number of array elements</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>// "binary"</span>
|
||||
<span class=w> </span><span class=mh>0x5B</span><span class=w> </span><span class=c1>// '[' 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>// '$' 'U' 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>// '#' 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="Binary JData">BJData</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>"binary"</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>"binary"</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">¶</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>"binary"</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=n>j</span><span class=p>[</span><span class=s>"binary"</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<std::uint8_t></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 "binary" + null byte</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 "binary" + 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>"binary"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"bytes"</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>"subtype"</span><span class=p>:</span><span class=w> </span><span class=mi>42</span>
|
||||
<span class=w> </span><span class=nt>"binary"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"bytes"</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>"subtype"</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">¶</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>"binary"</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=n>j</span><span class=p>[</span><span class=s>"binary"</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<std::uint8_t></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>// "binary"</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>// "binary"</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>"binary"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"bytes"</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>"subtype"</span><span class=p>:</span><span class=w> </span><span class=kc>null</span>
|
||||
<span class=w> </span><span class=nt>"binary"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"bytes"</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>"subtype"</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">¶</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>"binary"</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=n>j</span><span class=p>[</span><span class=s>"binary"</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<std::uint8_t></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>// "binary"</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>// "binary"</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>"binary"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"bytes"</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>"subtype"</span><span class=p>:</span><span class=w> </span><span class=mi>42</span>
|
||||
<span class=w> </span><span class=nt>"binary"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"bytes"</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>"subtype"</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">¶</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>"binary"</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=n>j</span><span class=p>[</span><span class=s>"binary"</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<std::uint8_t></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>// '{'</span>
|
||||
</code></pre></div> <p><code>v</code> is a <code>std::vector<std::uint8_t></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>// '{'</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>// "binary"</span>
|
||||
<span class=w> </span><span class=mh>0x5B</span><span class=w> </span><span class=c1>// '['</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 'U')</span>
|
||||
<span class=w> </span><span class=mh>0x5D</span><span class=w> </span><span class=c1>// ']'</span>
|
||||
<span class=mh>0x7D</span><span class=w> </span><span class=c1>// '}'</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>// "binary"</span>
|
||||
<span class=w> </span><span class=mh>0x5B</span><span class=w> </span><span class=c1>// '['</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 'U')</span>
|
||||
<span class=w> </span><span class=mh>0x5D</span><span class=w> </span><span class=c1>// ']'</span>
|
||||
<span class=mh>0x7D</span><span class=w> </span><span class=c1>// '}'</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>// '{'</span>
|
||||
<span class=w> </span><span class=mh>0x24</span><span class=w> </span><span class=c1>// '$' type of the object elements</span>
|
||||
<span class=w> </span><span class=mh>0x5B</span><span class=w> </span><span class=c1>// '[' 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>// '#' i 1 number of object elements</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>// '{'</span>
|
||||
<span class=w> </span><span class=mh>0x24</span><span class=w> </span><span class=c1>// '$' type of the object elements</span>
|
||||
<span class=w> </span><span class=mh>0x5B</span><span class=w> </span><span class=c1>// '[' array</span>
|
||||
<span class=w> </span><span class=mh>0x23</span><span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x01</span><span class=w> </span><span class=c1>// '#' i 1 number of object elements</span>
|
||||
<span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x06</span><span class=w> </span><span class=c1>// i 6 (length of the key)</span>
|
||||
<span class=w> </span><span class=mh>0x62</span><span class=w> </span><span class=mh>0x69</span><span class=w> </span><span class=mh>0x6E</span><span class=w> </span><span class=mh>0x61</span><span class=w> </span><span class=mh>0x72</span><span class=w> </span><span class=mh>0x79</span><span class=w> </span><span class=c1>// "binary"</span>
|
||||
<span class=w> </span><span class=mh>0x24</span><span class=w> </span><span class=mh>0x55</span><span class=w> </span><span class=c1>// '$' 'U' 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>// '#' i 4 number of array elements</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>// "binary"</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>// '$' 'U' 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>// '#' 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>"binary"</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>"binary"</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> </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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -1,7 +1,7 @@
|
||||
<!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=../conversions/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Comments - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#comments 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> Comments </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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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 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 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-search data-Line truncated
|
||||
<span class=w> </span><span class=c1>// update in 2006: removed Pluto</span>
|
||||
<span class=w> </span><span class=nt>"planets"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"Mercury"</span><span class=p>,</span><span class=w> </span><span class=s2>"Venus"</span><span class=p>,</span><span class=w> </span><span class=s2>"Earth"</span><span class=p>,</span><span class=w> </span><span class=s2>"Mars"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Jupiter"</span><span class=p>,</span><span class=w> </span><span class=s2>"Uranus"</span><span class=p>,</span><span class=w> </span><span class=s2>"Neptune"</span><span class=w> </span><span class=cm>/*, "Pluto" */</span><span class=p>]</span>
|
||||
<span class=w> </span><span class=nt>"planets"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"Mercury"</span><span class=p>,</span><span class=w> </span><span class=s2>"Venus"</span><span class=p>,</span><span class=w> </span><span class=s2>"Earth"</span><span class=p>,</span><span class=w> </span><span class=s2>"Mars"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Jupiter"</span><span class=p>,</span><span class=w> </span><span class=s2>"Uranus"</span><span class=p>,</span><span class=w> </span><span class=s2>"Neptune"</span><span class=w> </span><span class=cm>/*, "Pluto" */</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 class=highlight><span class=nb>true</span></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><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
@@ -10,13 +10,13 @@
|
||||
|
||||
<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>"</span><span class=dl>(</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>"</span><span class=dl>(</span>
|
||||
<span class=s> {</span>
|
||||
<span class=s> // update in 2006: removed Pluto</span>
|
||||
<span class=s> "planets": ["Mercury", "Venus", "Earth", "Mars",</span>
|
||||
<span class=s> "Jupiter", "Uranus", "Neptune" /*, "Pluto" */]</span>
|
||||
<span class=s> "planets": ["Mercury", "Venus", "Earth", "Mars",</span>
|
||||
<span class=s> "Jupiter", "Uranus", "Neptune" /*, "Pluto" */]</span>
|
||||
<span class=s> }</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</span><span class=p>;</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=k>try</span>
|
||||
<span class=w> </span><span class=p>{</span>
|
||||
@@ -31,18 +31,18 @@
|
||||
<span class=w> </span><span class=cm>/* callback */</span><span class=w> </span><span class=nb>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><<</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><<</span><span class=w> </span><span class=sc>'\n'</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><<</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><<</span><span class=w> </span><span class=sc>'\n'</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: '<U+000A> {<U+000A> /'; expected string literal
|
||||
</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: '<U+000A> {<U+000A> /'; expected string literal
|
||||
{
|
||||
"planets": [
|
||||
"Mercury",
|
||||
"Venus",
|
||||
"Earth",
|
||||
"Mars",
|
||||
"Jupiter",
|
||||
"Uranus",
|
||||
"Neptune"
|
||||
"planets": [
|
||||
"Mercury",
|
||||
"Venus",
|
||||
"Earth",
|
||||
"Mars",
|
||||
"Jupiter",
|
||||
"Uranus",
|
||||
"Neptune"
|
||||
]
|
||||
}
|
||||
</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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<span class=k>auto</span><span class=w> </span><span class=n>name</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>"name"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>();</span><span class=w> </span><span class=c1>// "Mary"</span>
|
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<span class=k>auto</span><span class=w> </span><span class=n>age</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>"age"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=p>();</span><span class=w> </span><span class=c1>// 42</span>
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<span class=k>auto</span><span class=w> </span><span class=n>hobbies</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>"hobbies"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>>></span><span class=p>();</span><span class=w> </span><span class=c1>// {"hiking", "reading"}</span>
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</code></pre></div> <div class="admonition note"> <p class=admonition-title>Getting a string without quotes</p> <p>A frequent point of confusion: use <a href=../../api/basic_json/get/ ><code>get</code></a>, <strong>not</strong> <a href=../serialization/ ><code>dump</code></a>, to read a string value. <code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>()</span></code> yields <code class=highlight><span class=n>Mary</span></code>, whereas <code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>].</span><span class=n>dump</span><span class=p>()</span></code> yields the <abbr title="JavaScript Object Notation">JSON</abbr> text <code class=highlight><span class=s>"Mary"</span></code> (<strong>with</strong> quotes), because <code>dump</code> always produces a <abbr title="JavaScript Object Notation">JSON</abbr> text.</p> </div> <p>Alternatively, <a href=../../api/basic_json/get_to/ ><code>get_to</code></a> writes into an existing variable and deduces the target type, which avoids repeating it:</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><iostream></span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>name</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>"name"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>();</span><span class=w> </span><span class=c1>// "Mary"</span>
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||||
<span class=k>auto</span><span class=w> </span><span class=n>age</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>"age"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=p>();</span><span class=w> </span><span class=c1>// 42</span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>hobbies</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>"hobbies"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>>></span><span class=p>();</span><span class=w> </span><span class=c1>// {"hiking", "reading"}</span>
|
||||
</code></pre></div> <div class="admonition note"> <p class=admonition-title>Getting a string without quotes</p> <p>A frequent point of confusion: use <a href=../../api/basic_json/get/ ><code>get</code></a>, <strong>not</strong> <a href=../serialization/ ><code>dump</code></a>, to read a string value. <code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>()</span></code> yields <code class=highlight><span class=n>Mary</span></code>, whereas <code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>].</span><span class=n>dump</span><span class=p>()</span></code> yields the <abbr title="JavaScript Object Notation">JSON</abbr> text <code class=highlight><span class=s>"Mary"</span></code> (<strong>with</strong> quotes), because <code>dump</code> always produces a <abbr title="JavaScript Object Notation">JSON</abbr> text.</p> </div> <p>Alternatively, <a href=../../api/basic_json/get_to/ ><code>get_to</code></a> writes into an existing variable and deduces the target type, which avoids repeating it:</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><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><unordered_map></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
|
||||
@@ -14,16 +14,16 @@
|
||||
<span class=w> </span><span class=c1>// create a JSON value with different types</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>json_types</span><span class=w> </span><span class=o>=</span>
|
||||
<span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"boolean"</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"boolean"</span><span class=p>,</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=s>"number"</span><span class=p>,</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"integer"</span><span class=p>,</span><span class=w> </span><span class=mi>42</span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"floating-point"</span><span class=p>,</span><span class=w> </span><span class=mf>17.23</span><span class=p>}</span>
|
||||
<span class=w> </span><span class=s>"number"</span><span class=p>,</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"integer"</span><span class=p>,</span><span class=w> </span><span class=mi>42</span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"floating-point"</span><span class=p>,</span><span class=w> </span><span class=mf>17.23</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=s>"string"</span><span class=p>,</span><span class=w> </span><span class=s>"Hello, world!"</span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"array"</span><span class=p>,</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=p>{</span><span class=s>"null"</span><span class=p>,</span><span class=w> </span><span class=nb>nullptr</span><span class=p>}</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"string"</span><span class=p>,</span><span class=w> </span><span class=s>"Hello, world!"</span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"array"</span><span class=p>,</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=p>{</span><span class=s>"null"</span><span class=p>,</span><span class=w> </span><span class=nb>nullptr</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>v1</span><span class=p>;</span>
|
||||
@@ -36,30 +36,30 @@
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>unordered_map</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>></span><span class=w> </span><span class=n>v8</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// use explicit conversions</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"boolean"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v1</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"number"</span><span class=p>][</span><span class=s>"integer"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v2</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"number"</span><span class=p>][</span><span class=s>"integer"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v3</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"number"</span><span class=p>][</span><span class=s>"floating-point"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v4</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"number"</span><span class=p>][</span><span class=s>"floating-point"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v5</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"string"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v6</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"array"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v7</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"boolean"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v1</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"number"</span><span class=p>][</span><span class=s>"integer"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v2</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"number"</span><span class=p>][</span><span class=s>"integer"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v3</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"number"</span><span class=p>][</span><span class=s>"floating-point"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v4</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"number"</span><span class=p>][</span><span class=s>"floating-point"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v5</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"string"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v6</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>[</span><span class=s>"array"</span><span class=p>].</span><span class=n>get_to</span><span class=p>(</span><span class=n>v7</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>json_types</span><span class=p>.</span><span class=n>get_to</span><span class=p>(</span><span class=n>v8</span><span class=p>);</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// print the conversion results</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><<</span><span class=w> </span><span class=n>v1</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=n>v2</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>' '</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>v3</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=n>v4</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>' '</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>v5</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=n>v6</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=n>v1</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=n>v2</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>' '</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>v3</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=n>v4</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>' '</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>v5</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=n>v6</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>v7</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><<</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>' '</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><<</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>' '</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>v8</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><<</span><span class=w> </span><span class=n>i</span><span class=p>.</span><span class=n>first</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>": "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>i</span><span class=p>.</span><span class=n>second</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=n>i</span><span class=p>.</span><span class=n>first</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>": "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>i</span><span class=p>.</span><span class=n>second</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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=mi>1</span>
|
||||
@@ -68,19 +68,19 @@
|
||||
<span class=err>Hello</span><span class=p>,</span><span class=w> </span><span class=err>world!</span>
|
||||
<span class=mi>1</span><span class=w> </span><span class=mi>2</span><span class=w> </span><span class=mi>3</span><span class=w> </span><span class=mi>4</span><span class=w> </span><span class=mi>5</span><span class=w> </span>
|
||||
|
||||
<span class=err>s</span><span class=kc>tr</span><span class=err>i</span><span class=kc>n</span><span class=err>g</span><span class=p>:</span><span class=w> </span><span class=s2>"Hello, world!"</span>
|
||||
<span class=kc>nu</span><span class=err>mber</span><span class=p>:</span><span class=w> </span><span class=p>{</span><span class=nt>"floating-point"</span><span class=p>:</span><span class=mf>17.23</span><span class=p>,</span><span class=nt>"integer"</span><span class=p>:</span><span class=mi>42</span><span class=p>}</span>
|
||||
<span class=err>s</span><span class=kc>tr</span><span class=err>i</span><span class=kc>n</span><span class=err>g</span><span class=p>:</span><span class=w> </span><span class=s2>"Hello, world!"</span>
|
||||
<span class=kc>nu</span><span class=err>mber</span><span class=p>:</span><span class=w> </span><span class=p>{</span><span class=nt>"floating-point"</span><span class=p>:</span><span class=mf>17.23</span><span class=p>,</span><span class=nt>"integer"</span><span class=p>:</span><span class=mi>42</span><span class=p>}</span>
|
||||
<span class=kc>null</span><span class=p>:</span><span class=w> </span><span class=kc>null</span>
|
||||
<span class=err>boolea</span><span class=kc>n</span><span class=p>:</span><span class=w> </span><span class=kc>true</span>
|
||||
<span class=err>array</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>
|
||||
</code></pre></div> </details> <p>The library already knows how to convert to and from the scalar types and the <abbr title="Standard Template Library">STL</abbr> containers (such as <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>array</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>optional</span></code>, and many more). Converting a <abbr title="JavaScript Object Notation">JSON</abbr> object back to a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code> or a <abbr title="JavaScript Object Notation">JSON</abbr> array back to a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code> therefore works without any extra 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=p>{{</span><span class=s>"one"</span><span class=p>,</span><span class=w> </span><span class=mi>1</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"two"</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>}};</span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>m</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>map</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=p>,</span><span class=w> </span><span class=kt>int</span><span class=o>>></span><span class=p>();</span><span class=w> </span><span class=c1>// {{"one", 1}, {"two", 2}}</span>
|
||||
</code></pre></div> <p><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>pair</span></code> and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>tuple</span></code> are also supported, converting positionally to and from a <abbr title="JavaScript Object Notation">JSON</abbr> array:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mf>1.0</span><span class=p>,</span><span class=w> </span><span class=s>"hello"</span><span class=p>,</span><span class=w> </span><span class=mi>42</span><span class=p>};</span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>t</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>tuple</span><span class=o><</span><span class=kt>double</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=p>,</span><span class=w> </span><span class=kt>int</span><span class=o>>></span><span class=p>();</span><span class=w> </span><span class=c1>// {1.0, "hello", 42}</span>
|
||||
</code></pre></div> <div class="admonition info"> <p class=admonition-title>Extracting references into a tuple</p> <p>A tuple type may also hold references (e.g. <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>tuple</span><span class=o><</span><span class=kt>double</span><span class=o>&</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=o>&></span></code>) to avoid copying: <code>get</code> then returns a tuple of references pointing directly at the elements stored inside the <code>basic_json</code> array, rather than a tuple of copies:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mf>1.0</span><span class=p>,</span><span class=w> </span><span class=s>"hello"</span><span class=p>};</span>
|
||||
</code></pre></div> </details> <p>The library already knows how to convert to and from the scalar types and the <abbr title="Standard Template Library">STL</abbr> containers (such as <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>array</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>optional</span></code>, and many more). Converting a <abbr title="JavaScript Object Notation">JSON</abbr> object back to a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code> or a <abbr title="JavaScript Object Notation">JSON</abbr> array back to a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code> therefore works without any extra 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=p>{{</span><span class=s>"one"</span><span class=p>,</span><span class=w> </span><span class=mi>1</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"two"</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>}};</span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>m</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>map</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=p>,</span><span class=w> </span><span class=kt>int</span><span class=o>>></span><span class=p>();</span><span class=w> </span><span class=c1>// {{"one", 1}, {"two", 2}}</span>
|
||||
</code></pre></div> <p><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>pair</span></code> and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>tuple</span></code> are also supported, converting positionally to and from a <abbr title="JavaScript Object Notation">JSON</abbr> array:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mf>1.0</span><span class=p>,</span><span class=w> </span><span class=s>"hello"</span><span class=p>,</span><span class=w> </span><span class=mi>42</span><span class=p>};</span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>t</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>tuple</span><span class=o><</span><span class=kt>double</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=p>,</span><span class=w> </span><span class=kt>int</span><span class=o>>></span><span class=p>();</span><span class=w> </span><span class=c1>// {1.0, "hello", 42}</span>
|
||||
</code></pre></div> <div class="admonition info"> <p class=admonition-title>Extracting references into a tuple</p> <p>A tuple type may also hold references (e.g. <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>tuple</span><span class=o><</span><span class=kt>double</span><span class=o>&</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=o>&></span></code>) to avoid copying: <code>get</code> then returns a tuple of references pointing directly at the elements stored inside the <code>basic_json</code> array, rather than a tuple of copies:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mf>1.0</span><span class=p>,</span><span class=w> </span><span class=s>"hello"</span><span class=p>};</span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>refs</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>tuple</span><span class=o><</span><span class=kt>double</span><span class=o>&</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=o>&>></span><span class=p>();</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>get</span><span class=o><</span><span class=mi>1</span><span class=o>></span><span class=p>(</span><span class=n>refs</span><span class=p>)</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"world"</span><span class=p>;</span><span class=w> </span><span class=c1>// modifies j[1] in place</span>
|
||||
</code></pre></div> <p>A referenced element must name the type the library actually <em>stores</em> — one of <a href=../../api/basic_json/boolean_t/ ><code>boolean_t</code></a>, <a href=../../api/basic_json/number_integer_t/ ><code>number_integer_t</code></a>, <a href=../../api/basic_json/number_unsigned_t/ ><code>number_unsigned_t</code></a>, <a href=../../api/basic_json/number_float_t/ ><code>number_float_t</code></a>, <a href=../../api/basic_json/string_t/ ><code>string_t</code></a>, <a href=../../api/basic_json/binary_t/ ><code>binary_t</code></a>, <a href=../../api/basic_json/array_t/ ><code>array_t</code></a>, or <a href=../../api/basic_json/object_t/ ><code>object_t</code></a>. There is nothing else to refer to, so a reference to any other type is a compile error even when a conversion would exist: <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>tuple</span><span class=o><</span><span class=kt>int</span><span class=o>&></span></code> is rejected, because the library stores a <code class=highlight><span class=n>number_integer_t</span></code> (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>int64_t</span></code> by default) and not an <code class=highlight><span class=kt>int</span></code>. This restriction applies only to reference elements — a plain <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>tuple</span><span class=o><</span><span class=kt>int</span><span class=o>></span></code> converts by value as usual.</p> </div> <h2 id=implicit-conversions>Implicit conversions<a class=headerlink href=#implicit-conversions title="Permanent link">¶</a></h2> <p>By default, a <abbr title="JavaScript Object Notation">JSON</abbr> value implicitly converts to a compatible C++ type, so the explicit <code>get</code> call can often be omitted:</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=s>"Hello"</span><span class=p>;</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>get</span><span class=o><</span><span class=mi>1</span><span class=o>></span><span class=p>(</span><span class=n>refs</span><span class=p>)</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"world"</span><span class=p>;</span><span class=w> </span><span class=c1>// modifies j[1] in place</span>
|
||||
</code></pre></div> <p>A referenced element must name the type the library actually <em>stores</em> — one of <a href=../../api/basic_json/boolean_t/ ><code>boolean_t</code></a>, <a href=../../api/basic_json/number_integer_t/ ><code>number_integer_t</code></a>, <a href=../../api/basic_json/number_unsigned_t/ ><code>number_unsigned_t</code></a>, <a href=../../api/basic_json/number_float_t/ ><code>number_float_t</code></a>, <a href=../../api/basic_json/string_t/ ><code>string_t</code></a>, <a href=../../api/basic_json/binary_t/ ><code>binary_t</code></a>, <a href=../../api/basic_json/array_t/ ><code>array_t</code></a>, or <a href=../../api/basic_json/object_t/ ><code>object_t</code></a>. There is nothing else to refer to, so a reference to any other type is a compile error even when a conversion would exist: <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>tuple</span><span class=o><</span><span class=kt>int</span><span class=o>&></span></code> is rejected, because the library stores a <code class=highlight><span class=n>number_integer_t</span></code> (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>int64_t</span></code> by default) and not an <code class=highlight><span class=kt>int</span></code>. This restriction applies only to reference elements — a plain <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>tuple</span><span class=o><</span><span class=kt>int</span><span class=o>></span></code> converts by value as usual.</p> </div> <h2 id=implicit-conversions>Implicit conversions<a class=headerlink href=#implicit-conversions title="Permanent link">¶</a></h2> <p>By default, a <abbr title="JavaScript Object Notation">JSON</abbr> value implicitly converts to a compatible C++ type, so the explicit <code>get</code> call can often be omitted:</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=s>"Hello"</span><span class=p>;</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>s</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>// implicit conversion, same as j.get<std::string>()</span>
|
||||
</code></pre></div> <p>Implicit conversions are convenient but can be surprising (for example, in overload resolution or with <code>auto</code>). They can be disabled by defining <a href=../../api/macros/json_use_implicit_conversions/ ><code>JSON_USE_IMPLICIT_CONVERSIONS</code></a> to <code class=highlight><span class=mi>0</span></code>, which forces the explicit <code>get</code> form and can catch unintended conversions at compile time.</p> <div class="admonition warning"> <p class=admonition-title>Conversions do not range-check numbers</p> <p>Just like C++ itself, the <code>get</code> family performs numeric conversions without range checks — retrieving a floating-point value as an integer truncates it, and narrowing conversions may overflow. See <a href=../types/number_handling/#number-conversion>number conversion</a> for details and how to guard against it.</p> </div> <div class="admonition warning"> <p class=admonition-title>std::optional direct construction from <abbr title="JavaScript Object Notation">JSON</abbr> null throws</p> <p>Constructing or assigning <code>std::optional<T></code> directly from a <abbr title="JavaScript Object Notation">JSON</abbr> value does not correctly produce <code>std::nullopt</code> for a <abbr title="JavaScript Object Notation">JSON</abbr> <code>null</code>:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j_null</span><span class=p>;</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>optional</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=w> </span><span class=n>opt</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j_null</span><span class=p>;</span><span class=w> </span><span class=c1>// ❌ throws type_error 302</span>
|
||||
@@ -99,9 +99,9 @@
|
||||
<span class=p>};</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>&</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>&</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=s>"name"</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=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=s>"name"</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=k>if</span><span class=w> </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=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"age"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=o>*</span><span class=n>p</span><span class=p>.</span><span class=n>age</span><span class=p>;</span><span class=w> </span><span class=c1>// key is only inserted when the optional has a value</span>
|
||||
<span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"age"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=o>*</span><span class=n>p</span><span class=p>.</span><span class=n>age</span><span class=p>;</span><span class=w> </span><span class=c1>// key is only inserted when the optional has a value</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <h2 id=putting-values-in>Putting values in<a class=headerlink href=#putting-values-in title="Permanent link">¶</a></h2> <p>The reverse direction works the same way: assigning or constructing a <code>json</code> from a C++ value converts it to <abbr title="JavaScript Object Notation">JSON</abbr>.</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=w> </span><span class=n>numbers</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>
|
||||
@@ -109,4 +109,4 @@
|
||||
</code></pre></div> <div class="admonition info"> <p class=admonition-title>Constructing from a C++20 range view</p> <p>A <code>json</code> array can also be constructed directly from a C++20 range view (<code>std::ranges::view</code>), such as the result of <code>std::views::filter</code> or <code>std::views::transform</code> -- no intermediate container is needed:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=w> </span><span class=n>nums</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>37</span><span class=p>,</span><span class=w> </span><span class=mi>42</span><span class=p>,</span><span class=w> </span><span class=mi>21</span><span class=p>};</span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>filtered</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nums</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>views</span><span class=o>::</span><span class=n>filter</span><span class=p>([](</span><span class=kt>int</span><span class=w> </span><span class=n>i</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>></span><span class=w> </span><span class=mi>10</span><span class=p>;</span><span class=w> </span><span class=p>});</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=nf>j</span><span class=p>(</span><span class=n>filtered</span><span class=p>);</span><span class=w> </span><span class=c1>// [37,42,21]</span>
|
||||
</code></pre></div> <p>This requires <a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a> to be enabled and is unavailable on MinGW due to incomplete C++20 ranges support there.</p> </div> <h2 id=your-own-types>Your own types<a class=headerlink href=#your-own-types title="Permanent link">¶</a></h2> <p>The conversions above are built in for standard types. To make the same syntax work for <strong>your own</strong> types, provide <code>to_json</code>/<code>from_json</code> functions (or use one of the convenience macros). This is described in detail on the <a href=../arbitrary_types/ >arbitrary types conversions</a> page. Enums can be mapped to strings as described in <a href=../enum_conversion/ >specializing enum conversion</a>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/get/ ><code>get</code></a> - get a copy converted to a given type</li> <li><a href=../../api/basic_json/get_to/ ><code>get_to</code></a> - convert into an existing variable</li> <li><a href=../../api/basic_json/get_ref/ ><code>get_ref</code></a> / <a href=../../api/basic_json/get_ptr/ ><code>get_ptr</code></a> - access the stored value without copying</li> <li><a href=../arbitrary_types/ >Arbitrary types conversions</a> - support your own types</li> <li><a href=../../api/macros/json_use_implicit_conversions/ ><code>JSON_USE_IMPLICIT_CONVERSIONS</code></a> - toggle implicit conversions</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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://wLine truncated
|
||||
</code></pre></div> <p>This requires <a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a> to be enabled and is unavailable on MinGW due to incomplete C++20 ranges support there.</p> </div> <h2 id=your-own-types>Your own types<a class=headerlink href=#your-own-types title="Permanent link">¶</a></h2> <p>The conversions above are built in for standard types. To make the same syntax work for <strong>your own</strong> types, provide <code>to_json</code>/<code>from_json</code> functions (or use one of the convenience macros). This is described in detail on the <a href=../arbitrary_types/ >arbitrary types conversions</a> page. Enums can be mapped to strings as described in <a href=../enum_conversion/ >specializing enum conversion</a>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/get/ ><code>get</code></a> - get a copy converted to a given type</li> <li><a href=../../api/basic_json/get_to/ ><code>get_to</code></a> - convert into an existing variable</li> <li><a href=../../api/basic_json/get_ref/ ><code>get_ref</code></a> / <a href=../../api/basic_json/get_ptr/ ><code>get_ptr</code></a> - access the stored value without copying</li> <li><a href=../arbitrary_types/ >Arbitrary types conversions</a> - support your own types</li> <li><a href=../../api/macros/json_use_implicit_conversions/ ><code>JSON_USE_IMPLICIT_CONVERSIONS</code></a> - toggle implicit conversions</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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:/Line truncated
|
||||
@@ -1,6 +1,6 @@
|
||||
<!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/creating_values/" rel="canonical"><link href=../conversions/ 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-9.7.7"><title>Creating JSON values - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#creating-json-values 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> Creating JSON values </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 id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 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> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 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 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 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"Line truncated
|
||||
<span class=n>json</span><span class=w> </span><span class=n>j_float</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mf>3.141</span><span class=p>;</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>j_string</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"Hello"</span><span class=p>;</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>j_string</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"Hello"</span><span class=p>;</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>j_boolean</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=nb>true</span><span class=p>;</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>j_null</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=nb>nullptr</span><span class=p>;</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>j_vector</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=kt>int</span><span class=o>></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=c1>// array</span>
|
||||
@@ -9,22 +9,22 @@
|
||||
|
||||
<span class=c1>// an object (a list of key/value pairs)</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>object</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"pi"</span><span class=p>,</span><span class=w> </span><span class=mf>3.141</span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"happy"</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"name"</span><span class=p>,</span><span class=w> </span><span class=s>"Niels"</span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"nothing"</span><span class=p>,</span><span class=w> </span><span class=nb>nullptr</span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"list"</span><span class=p>,</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>0</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>}},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"object"</span><span class=p>,</span><span class=w> </span><span class=p>{{</span><span class=s>"currency"</span><span class=p>,</span><span class=w> </span><span class=s>"USD"</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"value"</span><span class=p>,</span><span class=w> </span><span class=mf>42.99</span><span class=p>}}}</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"pi"</span><span class=p>,</span><span class=w> </span><span class=mf>3.141</span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"happy"</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"name"</span><span class=p>,</span><span class=w> </span><span class=s>"Niels"</span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"nothing"</span><span class=p>,</span><span class=w> </span><span class=nb>nullptr</span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"list"</span><span class=p>,</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>0</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>}},</span>
|
||||
<span class=w> </span><span class=p>{</span><span class=s>"object"</span><span class=p>,</span><span class=w> </span><span class=p>{{</span><span class=s>"currency"</span><span class=p>,</span><span class=w> </span><span class=s>"USD"</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"value"</span><span class=p>,</span><span class=w> </span><span class=mf>42.99</span><span class=p>}}}</span>
|
||||
<span class=p>};</span>
|
||||
</code></pre></div> <p>The library decides between an array and an object based on the content: a list whose elements are all two-element lists with a string as the first element is treated as an object, everything else as an array.</p> <div class="admonition warning"> <p class=admonition-title>Ambiguous cases: <code class=highlight><span class=p>{}</span></code> vs. <code class=highlight><span class=p>[]</span></code></p> <p>Because the same <code class=highlight><span class=p>{}</span></code> syntax is used for both arrays and objects, some cases are ambiguous. To force a particular type, use the explicit factory functions <a href=../../api/basic_json/array/ ><code>json::array</code></a> and <a href=../../api/basic_json/object/ ><code>json::object</code></a>:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>empty_array_explicit</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>array</span><span class=p>();</span><span class=w> </span><span class=c1>// []</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>empty_object_explicit</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>object</span><span class=p>();</span><span class=w> </span><span class=c1>// {}</span>
|
||||
|
||||
<span class=c1>// a JSON array with one object, not an object with one member</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>array_of_objects</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>array</span><span class=p>({{</span><span class=s>"key"</span><span class=p>,</span><span class=w> </span><span class=s>"value"</span><span class=p>}});</span><span class=w> </span><span class=c1>// [{"key":"value"}]</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>array_of_objects</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>array</span><span class=p>({{</span><span class=s>"key"</span><span class=p>,</span><span class=w> </span><span class=s>"value"</span><span class=p>}});</span><span class=w> </span><span class=c1>// [{"key":"value"}]</span>
|
||||
</code></pre></div> <p>Related to this, single-element brace initialization such as <code class=highlight><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>{</span><span class=n>value</span><span class=p>};</span></code> wraps the element in a single-element <strong>array</strong> by default, and its behavior even differs between compilers. See the <a href=../../home/faq/#brace-initialization-yields-arrays><abbr title="Frequently Asked Questions">FAQ</abbr></a> for details and the opt-in <a href=../../api/macros/json_brace_init_copy_semantics/ ><code>JSON_BRACE_INIT_COPY_SEMANTICS</code></a> macro.</p> </div> <h2 id=building-incrementally>Building incrementally<a class=headerlink href=#building-incrementally title="Permanent link">¶</a></h2> <p>A value can also be built up piece by piece. Accessing a non-existing object key or array index with <a href=../element_access/unchecked_access/ ><code>operator[]</code></a> creates the element on the fly:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span><span class=w> </span><span class=c1>// null</span>
|
||||
<span class=n>j</span><span class=p>[</span><span class=s>"answer"</span><span class=p>][</span><span class=s>"everything"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>42</span><span class=p>;</span><span class=w> </span><span class=c1>// becomes an object</span>
|
||||
<span class=n>j</span><span class=p>[</span><span class=s>"list"</span><span class=p>]</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>0</span><span class=p>,</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>"list"</span><span class=p>].</span><span class=n>push_back</span><span class=p>(</span><span class=mi>3</span><span class=p>);</span><span class=w> </span><span class=c1>// [1,0,2,3]</span>
|
||||
<span class=n>j</span><span class=p>[</span><span class=s>"answer"</span><span class=p>][</span><span class=s>"everything"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>42</span><span class=p>;</span><span class=w> </span><span class=c1>// becomes an object</span>
|
||||
<span class=n>j</span><span class=p>[</span><span class=s>"list"</span><span class=p>]</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>0</span><span class=p>,</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>"list"</span><span class=p>].</span><span class=n>push_back</span><span class=p>(</span><span class=mi>3</span><span class=p>);</span><span class=w> </span><span class=c1>// [1,0,2,3]</span>
|
||||
</code></pre></div> <p>See <a href=../modifying_values/ >modifying values</a> for <a href=../../api/basic_json/push_back/ ><code>push_back</code></a>, <a href=../../api/basic_json/emplace/ ><code>emplace</code></a>, and related functions.</p> <h2 id=with-the-_json-literal>With the <code>_json</code> literal<a class=headerlink href=#with-the-_json-literal title="Permanent link">¶</a></h2> <p>The <code>_json</code> <a href=../../api/operator_literal_json/ >user-defined literal</a> parses a string at the call site and is a convenient way to write a <abbr title="JavaScript Object Notation">JSON</abbr> value inline:</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><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iomanip></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
@@ -34,12 +34,12 @@
|
||||
|
||||
<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=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s> {"hello": "world", "answer": 42} </span><span class=dl>)</span><span class=s>"</span><span class=n>_json</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=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s> {"hello": "world", "answer": 42} </span><span class=dl>)</span><span class=s>"</span><span class=n>_json</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><<</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><<</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</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><<</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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>"answer"</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>"hello"</span><span class=p>:</span><span class=w> </span><span class=s2>"world"</span>
|
||||
<span class=w> </span><span class=nt>"answer"</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>"hello"</span><span class=p>:</span><span class=w> </span><span class=s2>"world"</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <p>Note this <strong>parses</strong> the string, so <code class=highlight><span class=s>"42"</span><span class=n>_json</span></code> is the number <code class=highlight><span class=mi>42</span></code>, whereas <code class=highlight><span class=n>json</span><span class=p>(</span><span class=s>"42"</span><span class=p>)</span></code> is the <abbr title="JavaScript Object Notation">JSON</abbr> string <code class=highlight><span class=s2>"42"</span></code>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/basic_json/ ><code>basic_json</code> constructors</a> - all ways to construct a value</li> <li><a href=../../api/basic_json/array/ ><code>array</code></a> / <a href=../../api/basic_json/object/ ><code>object</code></a> - force array or object type</li> <li><a href=../../api/operator_literal_json/ ><code>operator""_json</code></a> - the <code>_json</code> literal</li> <li><a href=../conversions/ >Converting values</a> - which C++ types can be used</li> <li><a href=../parsing/ >Parsing</a> - create a value from <abbr title="JavaScript Object Notation">JSON</abbr> text</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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 7.1.0 by @fontawesome - https://fontawesome.com License - https://foLine truncated
|
||||
</code></pre></div> </details> <p>Note this <strong>parses</strong> the string, so <code class=highlight><span class=s>"42"</span><span class=n>_json</span></code> is the number <code class=highlight><span class=mi>42</span></code>, whereas <code class=highlight><span class=n>json</span><span class=p>(</span><span class=s>"42"</span><span class=p>)</span></code> is the <abbr title="JavaScript Object Notation">JSON</abbr> string <code class=highlight><span class=s2>"42"</span></code>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/basic_json/ ><code>basic_json</code> constructors</a> - all ways to construct a value</li> <li><a href=../../api/basic_json/array/ ><code>array</code></a> / <a href=../../api/basic_json/object/ ><code>object</code></a> - force array or object type</li> <li><a href=../../api/operator_literal_json/ ><code>operator""_json</code></a> - the <code>_json</code> literal</li> <li><a href=../conversions/ >Converting values</a> - which C++ types can be used</li> <li><a href=../parsing/ >Parsing</a> - create a value from <abbr title="JavaScript Object Notation">JSON</abbr> text</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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<!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-9.7.7"><title>Checked access: at - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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<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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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 14Line truncated
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<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</span><span class=p>]</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span><span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"name"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"Mary Smith"</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>"age"</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>"hobbies"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"hobbies"</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>"hiking"</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>"hobbies"</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>"reading"</span></code></td> </tr> </tbody> </table> </details> <p>The return value is a reference, so it can be used to modify 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>"name"</span><span class=p>)</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"John Smith"</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>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span><span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"name"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"Mary Smith"</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>"age"</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>"hobbies"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"hobbies"</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>"hiking"</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>"hobbies"</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>"reading"</span></code></td> </tr> </tbody> </table> </details> <p>The return value is a reference, so it can be used to modify 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>"name"</span><span class=p>)</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"John Smith"</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>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"John Smith"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</span><span class=p>]</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"John Smith"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"hobbies"</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>"cooking"</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>"hobbies"</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>"cooking"</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 <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">¶</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">¶</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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 Line truncated
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</code></pre></div> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <ul> <li><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">¶</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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.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/default_value/" rel="canonical"><link href=../checked_access/ rel=prev><link href=../../iterators/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Access with default value: value - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Access with default value: value </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo 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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 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 Line truncated
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<span class=w> </span><span class=nt>"logOutput"</span><span class=p>:</span><span class=w> </span><span class=s2>"result.log"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"append"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span>
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||||
<span class=w> </span><span class=nt>"logOutput"</span><span class=p>:</span><span class=w> </span><span class=s2>"result.log"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"append"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span>
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<span class=p>}</span>
|
||||
</code></pre></div> <p>Assume the value is parsed to a <code>json</code> variable <code>j</code>.</p> <table> <thead> <tr> <th>expression</th> <th>value</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>j</span></code></td> <td><code class=highlight><span class=p>{</span><span class=nt>"logOutput"</span><span class=p>:</span><span class=w> </span><span class=s2>"result.log"</span><span class=p>,</span><span class=w> </span><span class=nt>"append"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>}</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"logOutput"</span><span class=p>,</span><span class=w> </span><span class=s>"logfile.log"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"result.log"</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"append"</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>)</span></code></td> <td><code class=highlight><span class=kc>true</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"append"</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>)</span></code></td> <td><code class=highlight><span class=kc>true</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"logLevel"</span><span class=p>,</span><span class=w> </span><span class=s>"verbose"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"verbose"</span></code></td> </tr> </tbody> </table> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <ul> <li>With string keys, <code>value</code> can only be used with objects. For other types, a <a href=../../../home/exceptions/#jsonexceptiontype_error306><code>basic_json::type_error</code></a> is thrown.</li> <li>With <abbr title="JavaScript Object Notation">JSON</abbr> Pointers, <code>value</code> can be used with both objects and arrays. For other types (null, boolean, number, string), a <a href=../../../home/exceptions/#jsonexceptiontype_error306><code>basic_json::type_error</code></a> is thrown.</li> </ul> </div> <div class="admonition warning"> <p class=admonition-title>Return type</p> <p>The value function is a template, and the return type of the function is determined by the type of the provided default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit unsigned integer. We get exactly that value when using <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a>. However, when we call <code>value</code> and provide <code class=highlight><span class=mi>0</span></code> as default value, then <code class=highlight><span class=mi>-1</span></code> is returned. This occurs, because <code class=highlight><span class=mi>0</span></code> has type <code class=highlight><span class=kt>int</span></code> which overflows when handling the value <code class=highlight><span class=mi>18446744073709551615</span></code>.</p> <p>To address this issue, either provide a correctly typed default value or use the template parameter to specify the desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default value is not used as the return value.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
</code></pre></div> <p>Assume the value is parsed to a <code>json</code> variable <code>j</code>.</p> <table> <thead> <tr> <th>expression</th> <th>value</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>j</span></code></td> <td><code class=highlight><span class=p>{</span><span class=nt>"logOutput"</span><span class=p>:</span><span class=w> </span><span class=s2>"result.log"</span><span class=p>,</span><span class=w> </span><span class=nt>"append"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>}</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"logOutput"</span><span class=p>,</span><span class=w> </span><span class=s>"logfile.log"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"result.log"</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"append"</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>)</span></code></td> <td><code class=highlight><span class=kc>true</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"append"</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>)</span></code></td> <td><code class=highlight><span class=kc>true</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"logLevel"</span><span class=p>,</span><span class=w> </span><span class=s>"verbose"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"verbose"</span></code></td> </tr> </tbody> </table> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <ul> <li>With string keys, <code>value</code> can only be used with objects. For other types, a <a href=../../../home/exceptions/#jsonexceptiontype_error306><code>basic_json::type_error</code></a> is thrown.</li> <li>With <abbr title="JavaScript Object Notation">JSON</abbr> Pointers, <code>value</code> can be used with both objects and arrays. For other types (null, boolean, number, string), a <a href=../../../home/exceptions/#jsonexceptiontype_error306><code>basic_json::type_error</code></a> is thrown.</li> </ul> </div> <div class="admonition warning"> <p class=admonition-title>Return type</p> <p>The value function is a template, and the return type of the function is determined by the type of the provided default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit unsigned integer. We get exactly that value when using <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a>. However, when we call <code>value</code> and provide <code class=highlight><span class=mi>0</span></code> as default value, then <code class=highlight><span class=mi>-1</span></code> is returned. This occurs, because <code class=highlight><span class=mi>0</span></code> has type <code class=highlight><span class=kt>int</span></code> which overflows when handling the value <code class=highlight><span class=mi>18446744073709551615</span></code>.</p> <p>To address this issue, either provide a correctly typed default value or use the template parameter to specify the desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default value is not used as the return value.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
|
||||
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
|
||||
|
||||
<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>"</span><span class=dl>(</span><span class=s>{"uint64": 18446744073709551615}</span><span class=dl>)</span><span class=s>"</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>"</span><span class=dl>(</span><span class=s>{"uint64": 18446744073709551615}</span><span class=dl>)</span><span class=s>"</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><<</span><span class=w> </span><span class=s>"operator[]: "</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>"uint64"</span><span class=p>]</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"default value (int): "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"uint64"</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><<</span><span class=w> </span><span class=sc>'\n'</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"default value (uint64_t): "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"uint64"</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><<</span><span class=w> </span><span class=sc>'\n'</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"explicit return value type: "</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=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</span><span class=o>></span><span class=p>(</span><span class=s>"uint64"</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><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=s>"operator[]: "</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>"uint64"</span><span class=p>]</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"default value (int): "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"uint64"</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><<</span><span class=w> </span><span class=sc>'\n'</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"default value (uint64_t): "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"uint64"</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><<</span><span class=w> </span><span class=sc>'\n'</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"explicit return value type: "</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=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</span><span class=o>></span><span class=p>(</span><span class=s>"uint64"</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><<</span><span class=w> </span><span class=sc>'\n'</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>explici</span><span class=kc>t</span><span class=w> </span><span class=err>re</span><span class=kc>turn</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=kc>t</span><span class=err>ype</span><span class=p>:</span><span class=w> </span><span class=mi>18446744073709551615</span>
|
||||
</code></pre></div> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../../api/basic_json/value/ ><code>value</code></a> for access with default value</li> <li>documentation on <a href=../checked_access/ >checked access</a></li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="August 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../../api/basic_json/value/ ><code>value</code></a> for access with default value</li> <li>documentation on <a href=../checked_access/ >checked access</a></li> </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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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<!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-9.7.7"><title>Unchecked access: operator[] - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Unchecked access: operator[] </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo 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6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 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 5 Line truncated
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</span><span class=p>]</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span><span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"name"</span><span class=p>]</span></code></td> <td><code class=highlight><span class=s2>"Mary Smith"</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"age"</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>"hobbies"</span><span class=p>]</span></code></td> <td><code class=highlight><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"hobbies"</span><span class=p>][</span><span class=mi>0</span><span class=p>]</span></code></td> <td><code class=highlight><span class=s2>"hiking"</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"hobbies"</span><span class=p>][</span><span class=mi>1</span><span class=p>]</span></code></td> <td><code class=highlight><span class=s2>"reading"</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>"name"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"John Smith"</span><span class=p>;</span>
|
||||
<span class=n>j</span><span class=p>[</span><span class=s>"maidenName"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"Jones"</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>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span><span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"name"</span><span class=p>]</span></code></td> <td><code class=highlight><span class=s2>"Mary Smith"</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"age"</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>"hobbies"</span><span class=p>]</span></code></td> <td><code class=highlight><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"hobbies"</span><span class=p>][</span><span class=mi>0</span><span class=p>]</span></code></td> <td><code class=highlight><span class=s2>"hiking"</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"hobbies"</span><span class=p>][</span><span class=mi>1</span><span class=p>]</span></code></td> <td><code class=highlight><span class=s2>"reading"</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>"name"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"John Smith"</span><span class=p>;</span>
|
||||
<span class=n>j</span><span class=p>[</span><span class=s>"maidenName"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"Jones"</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>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"John Smith"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"maidenName"</span><span class=p>:</span><span class=w> </span><span class=s2>"Jones"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</span><span class=p>]</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"John Smith"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"maidenName"</span><span class=p>:</span><span class=w> </span><span class=s2>"Jones"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"hobbies"</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>"running"</span><span class=p>;</span>
|
||||
<span class=n>j</span><span class=p>[</span><span class=s>"hobbies"</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>"cooking"</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>"hobbies"</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>"running"</span><span class=p>;</span>
|
||||
<span class=n>j</span><span class=p>[</span><span class=s>"hobbies"</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>"cooking"</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>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"John Smith"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"maidenName"</span><span class=p>:</span><span class=w> </span><span class=s2>"Jones"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"running"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"cooking"</span><span class=p>]</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"John Smith"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"maidenName"</span><span class=p>:</span><span class=w> </span><span class=s2>"Jones"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"age"</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>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"running"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</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>"cooking"</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">¶</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=performance-reserving-array-capacity>Performance: reserving array capacity<a class=headerlink href=#performance-reserving-array-capacity title="Permanent link">¶</a></h2> <p>There is no public <code>reserve(count)</code> member on <code>basic_json</code> for pre-allocating array capacity. If you are building a large array incrementally (e.g., via repeated <code>push_back()</code>) and know its final size ahead of time, you can reserve capacity via <code>get_ref()</code> to access the underlying <code>array_t</code> directly:</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>array</span><span class=p>();</span>
|
||||
<span class=n>j</span><span class=p>.</span><span class=n>get_ref</span><span class=o><</span><span class=n>json</span><span class=o>::</span><span class=n>array_t</span><span class=o>&></span><span class=p>().</span><span class=n>reserve</span><span class=p>(</span><span class=mi>1000</span><span class=p>);</span>
|
||||
<span class=k>for</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>0</span><span class=p>;</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o><</span><span class=w> </span><span class=mi>1000</span><span class=p>;</span><span class=w> </span><span class=o>++</span><span class=n>i</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>push_back</span><span class=p>(</span><span class=n>i</span><span class=p>);</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">¶</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/licenseLine truncated
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</code></pre></div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">¶</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/licenLine truncated
|
||||
@@ -9,19 +9,19 @@
|
||||
<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=nb>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>"stopped"</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>"running"</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>"completed"</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>"stopped"</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>"running"</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>"completed"</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">¶</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>"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>"stopped"</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>"running"</span><span class=p>;</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>"running"</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=n>get</span><span class=o><</span><span class=n>TaskState</span><span class=o>></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=n>get</span><span class=o><</span><span class=n>TaskState</span><span class=o>></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">¶</a></h2> <p>Just as in <a href=../arbitrary_types/ >Arbitrary Type Conversions</a> above,</p> <ul> <li><a href=../../api/macros/nlohmann_json_serialize_enum/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM()</code></a> MUST be declared in your enum type's namespace (which can be the global namespace), or the library will not be able to locate it, and it will default to integer serialization.</li> <li>It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.</li> </ul> <p>Other Important points:</p> <ul> <li>When using <code>get<ENUM_TYPE>()</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. If you desire an exception in this circumstance use <a href=../../api/macros/nlohmann_json_serialize_enum_strict/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM_STRICT()</code></a> which behaves identically except for throwing an exception on unrecognized values.</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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 Line truncated
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</code></pre></div> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <p>Just as in <a href=../arbitrary_types/ >Arbitrary Type Conversions</a> above,</p> <ul> <li><a href=../../api/macros/nlohmann_json_serialize_enum/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM()</code></a> MUST be declared in your enum type's namespace (which can be the global namespace), or the library will not be able to locate it, and it will default to integer serialization.</li> <li>It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.</li> </ul> <p>Other Important points:</p> <ul> <li>When using <code>get<ENUM_TYPE>()</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. If you desire an exception in this circumstance use <a href=../../api/macros/nlohmann_json_serialize_enum_strict/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM_STRICT()</code></a> which behaves identically except for throwing an exception on unrecognized values.</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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.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/" rel="canonical"><link href=../home/debugging/ rel=prev><link href=arbitrary_types/ rel=next><link rel=icon href=../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Features - JSON for Modern C++</title><link rel=stylesheet href=../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../css/custom.css><script>__md_scope=new 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<!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/" rel="canonical"><link href=../home/debugging/ rel=prev><link href=arbitrary_types/ rel=next><link rel=icon href=../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Features - JSON for Modern C++</title><link rel=stylesheet href=../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../css/custom.css><script>__md_scope=new 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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-search data-md-component=search role=diaLine truncated
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@@ -1,8 +1,8 @@
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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 clLine 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/iterators/" rel="canonical"><link href=../element_access/default_value/ rel=prev><link href=../merge_patch/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Iterators - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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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 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 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div clLine truncated
|
||||
<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>"one"</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>"two"</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>"three"</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=n>j</span><span class=p>[</span><span class=s>"one"</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>"two"</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>"three"</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>
|
||||
@@ -11,31 +11,31 @@
|
||||
</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">¶</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 {"one": 1, "two": 2, "three": 3}</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">¶</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 {"one": 1, "two": 2, "three": 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>"one"</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>"two"</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>"three"</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=n>j</span><span class=p>[</span><span class=s>"one"</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>"two"</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>"three"</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><<</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><<</span><span class=w> </span><span class=s>" : "</span><span class=w> </span><span class=o><<</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><<</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>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</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><<</span><span class=w> </span><span class=s>" : "</span><span class=w> </span><span class=o><<</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><<</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">¶</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>// "it" 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><<</span><span class=w> </span><span class=s>"value: "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=c1>// "it" 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><<</span><span class=w> </span><span class=s>"value: "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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>&</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><<</span><span class=w> </span><span class=s>"key: "</span><span class=w> </span><span class=o><<</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><<</span><span class=w> </span><span class=s>", value:"</span><span class=w> </span><span class=o><<</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><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=s>"key: "</span><span class=w> </span><span class=o><<</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><<</span><span class=w> </span><span class=s>", value:"</span><span class=w> </span><span class=o><<</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><<</span><span class=w> </span><span class=sc>'\n'</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>&</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><<</span><span class=w> </span><span class=s>"key: "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>key</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>", value:"</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>val</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=s>"key: "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>key</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>", value:"</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>val</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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">¶</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>
|
||||
|
||||
@@ -47,10 +47,10 @@
|
||||
<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">¶</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 if 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>"Hello, world"</span><span class=p>;</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">¶</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 if 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>"Hello, world"</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><<</span><span class=w> </span><span class=o>*</span><span class=n>it</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>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>"Hello, world"</span>
|
||||
</code></pre></div> </details> <h2 id=iterator-invalidation>Iterator invalidation<a class=headerlink href=#iterator-invalidation title="Permanent link">¶</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=s2>"Hello, world"</span>
|
||||
</code></pre></div> </details> <h2 id=iterator-invalidation>Iterator invalidation<a class=headerlink href=#iterator-invalidation title="Permanent link">¶</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -8,38 +8,38 @@
|
||||
<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>"</span><span class=dl>(</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>"</span><span class=dl>(</span>
|
||||
<span class=s> {</span>
|
||||
<span class=s> "baz": "qux",</span>
|
||||
<span class=s> "foo": "bar"</span>
|
||||
<span class=s> "baz": "qux",</span>
|
||||
<span class=s> "foo": "bar"</span>
|
||||
<span class=s> }</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</span><span class=n>_json</span><span class=p>;</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</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>"</span><span class=dl>(</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>"</span><span class=dl>(</span>
|
||||
<span class=s> [</span>
|
||||
<span class=s> { "op": "replace", "path": "/baz", "value": "boo" },</span>
|
||||
<span class=s> { "op": "add", "path": "/hello", "value": ["world"] },</span>
|
||||
<span class=s> { "op": "remove", "path": "/foo"}</span>
|
||||
<span class=s> { "op": "replace", "path": "/baz", "value": "boo" },</span>
|
||||
<span class=s> { "op": "add", "path": "/hello", "value": ["world"] },</span>
|
||||
<span class=s> { "op": "remove", "path": "/foo"}</span>
|
||||
<span class=s> ]</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</span><span class=n>_json</span><span class=p>;</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</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><<</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><<</span><span class=w> </span><span class=n>doc</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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><<</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><<</span><span class=w> </span><span class=n>doc</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o><<</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>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>"baz"</span><span class=p>:</span><span class=w> </span><span class=s2>"qux"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"foo"</span><span class=p>:</span><span class=w> </span><span class=s2>"bar"</span>
|
||||
<span class=w> </span><span class=nt>"baz"</span><span class=p>:</span><span class=w> </span><span class=s2>"qux"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"foo"</span><span class=p>:</span><span class=w> </span><span class=s2>"bar"</span>
|
||||
<span class=p>}</span>
|
||||
|
||||
<span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"baz"</span><span class=p>:</span><span class=w> </span><span class=s2>"boo"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"hello"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=s2>"world"</span>
|
||||
<span class=w> </span><span class=nt>"baz"</span><span class=p>:</span><span class=w> </span><span class=s2>"boo"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"hello"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=s2>"world"</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">¶</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><span class=n>source</span><span class=p>.</span><span class=n>patch</span><span class=p>(</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=o>==</span><span class=w> </span><span class=n>target</span><span class=p>;</span>
|
||||
@@ -53,22 +53,22 @@
|
||||
<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>"</span><span class=dl>(</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>"</span><span class=dl>(</span>
|
||||
<span class=s> {</span>
|
||||
<span class=s> "baz": "qux",</span>
|
||||
<span class=s> "foo": "bar"</span>
|
||||
<span class=s> "baz": "qux",</span>
|
||||
<span class=s> "foo": "bar"</span>
|
||||
<span class=s> }</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</span><span class=n>_json</span><span class=p>;</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</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>"</span><span class=dl>(</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>"</span><span class=dl>(</span>
|
||||
<span class=s> {</span>
|
||||
<span class=s> "baz": "boo",</span>
|
||||
<span class=s> "hello": [</span>
|
||||
<span class=s> "world"</span>
|
||||
<span class=s> "baz": "boo",</span>
|
||||
<span class=s> "hello": [</span>
|
||||
<span class=s> "world"</span>
|
||||
<span class=s> ]</span>
|
||||
<span class=s> }</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</span><span class=n>_json</span><span class=p>;</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</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>
|
||||
@@ -77,32 +77,32 @@
|
||||
<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><<</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><<</span><span class=w> </span><span class=n>patch</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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><<</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><<</span><span class=w> </span><span class=n>patch</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o><<</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>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>"op"</span><span class=p>:</span><span class=w> </span><span class=s2>"replace"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"path"</span><span class=p>:</span><span class=w> </span><span class=s2>"/baz"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"value"</span><span class=p>:</span><span class=w> </span><span class=s2>"boo"</span>
|
||||
<span class=w> </span><span class=nt>"op"</span><span class=p>:</span><span class=w> </span><span class=s2>"replace"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"path"</span><span class=p>:</span><span class=w> </span><span class=s2>"/baz"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"value"</span><span class=p>:</span><span class=w> </span><span class=s2>"boo"</span>
|
||||
<span class=w> </span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"op"</span><span class=p>:</span><span class=w> </span><span class=s2>"remove"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"path"</span><span class=p>:</span><span class=w> </span><span class=s2>"/foo"</span>
|
||||
<span class=w> </span><span class=nt>"op"</span><span class=p>:</span><span class=w> </span><span class=s2>"remove"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"path"</span><span class=p>:</span><span class=w> </span><span class=s2>"/foo"</span>
|
||||
<span class=w> </span><span class=p>},</span>
|
||||
<span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"op"</span><span class=p>:</span><span class=w> </span><span class=s2>"add"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"path"</span><span class=p>:</span><span class=w> </span><span class=s2>"/hello"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"value"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=s2>"world"</span>
|
||||
<span class=w> </span><span class=nt>"op"</span><span class=p>:</span><span class=w> </span><span class=s2>"add"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"path"</span><span class=p>:</span><span class=w> </span><span class=s2>"/hello"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"value"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=s2>"world"</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>"baz"</span><span class=p>:</span><span class=w> </span><span class=s2>"boo"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"hello"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=s2>"world"</span>
|
||||
<span class=w> </span><span class=nt>"baz"</span><span class=p>:</span><span class=w> </span><span class=s2>"boo"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"hello"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=s2>"world"</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> </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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -1,48 +1,48 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/json_pointer/" rel="canonical"><link href=../json_patch/ rel=prev><link href=../modifying_values/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>JSON Pointer - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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|
||||
<span class=w> </span><span class=nt>"array"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"A"</span><span class=p>,</span><span class=w> </span><span class=s2>"B"</span><span class=p>,</span><span class=w> </span><span class=s2>"C"</span><span class=p>],</span>
|
||||
<span class=w> </span><span class=nt>"nested"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"one"</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>"two"</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>"three"</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=nt>"array"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"A"</span><span class=p>,</span><span class=w> </span><span class=s2>"B"</span><span class=p>,</span><span class=w> </span><span class=s2>"C"</span><span class=p>],</span>
|
||||
<span class=w> </span><span class=nt>"nested"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"one"</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>"two"</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>"three"</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>"array"</span><span class=p>:[</span><span class=s2>"A"</span><span class=p>,</span><span class=s2>"B"</span><span class=p>,</span><span class=s2>"C"</span><span class=p>],</span><span class=nt>"nested"</span><span class=p>:{</span><span class=nt>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>,</span><span class=nt>"three"</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>"A"</span><span class=p>,</span><span class=s2>"B"</span><span class=p>,</span><span class=s2>"C"</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>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>,</span><span class=nt>"three"</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">¶</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>"/nested/one"</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>"/nested/one"</span><span class=n>_json_pointer</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>"array"</span><span class=p>:[</span><span class=s2>"A"</span><span class=p>,</span><span class=s2>"B"</span><span class=p>,</span><span class=s2>"C"</span><span class=p>],</span><span class=nt>"nested"</span><span class=p>:{</span><span class=nt>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>,</span><span class=nt>"three"</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>"A"</span><span class=p>,</span><span class=s2>"B"</span><span class=p>,</span><span class=s2>"C"</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>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>,</span><span class=nt>"three"</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">¶</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>"/nested/one"</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>"/nested/one"</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">¶</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>"</span><span class=dl>(</span><span class=s>{</span>
|
||||
<span class=s> "array": ["A", "B", "C"],</span>
|
||||
<span class=s> "nested": {</span>
|
||||
<span class=s> "one": 1,</span>
|
||||
<span class=s> "two": 2,</span>
|
||||
<span class=s> "three": [true, false]</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>"</span><span class=dl>(</span><span class=s>{</span>
|
||||
<span class=s> "array": ["A", "B", "C"],</span>
|
||||
<span class=s> "nested": {</span>
|
||||
<span class=s> "one": 1,</span>
|
||||
<span class=s> "two": 2,</span>
|
||||
<span class=s> "three": [true, false]</span>
|
||||
<span class=s> }</span>
|
||||
<span class=s>}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
|
||||
<span class=s>}</span><span class=dl>)</span><span class=s>"</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>""</span><span class=n>_json_pointer</span><span class=p>];</span><span class=w> </span><span class=c1>// {"array":["A","B","C"],...}</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>"/nested/one"</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>"/nested/three/1"</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>"/nested/four"</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>
|
||||
<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>""</span><span class=n>_json_pointer</span><span class=p>];</span><span class=w> </span><span class=c1>// {"array":["A","B","C"],...}</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>"/nested/one"</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>"/nested/three/1"</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>"/nested/four"</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> <div class="admonition note"> <p class=admonition-title>Creating intermediate levels that don't exist</p> <p>See the <a href=../../api/basic_json/operator%5B%5D/#return-value><code>operator[]</code> notes</a> for how array vs. object is decided when a pointer creates intermediate levels that don't exist yet.</p> </div> <h2 id=flatten-unflatten>Flatten / unflatten<a class=headerlink href=#flatten-unflatten title="Permanent link">¶</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>"</span><span class=dl>(</span><span class=s>{</span>
|
||||
<span class=s> "array": ["A", "B", "C"],</span>
|
||||
<span class=s> "nested": {</span>
|
||||
<span class=s> "one": 1,</span>
|
||||
<span class=s> "two": 2,</span>
|
||||
<span class=s> "three": [true, false]</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>"</span><span class=dl>(</span><span class=s>{</span>
|
||||
<span class=s> "array": ["A", "B", "C"],</span>
|
||||
<span class=s> "nested": {</span>
|
||||
<span class=s> "one": 1,</span>
|
||||
<span class=s> "two": 2,</span>
|
||||
<span class=s> "three": [true, false]</span>
|
||||
<span class=s> }</span>
|
||||
<span class=s>}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
|
||||
<span class=s>}</span><span class=dl>)</span><span class=s>"</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>"/array/0"</span><span class=p>:</span><span class=w> </span><span class=s2>"A"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"/array/1"</span><span class=p>:</span><span class=w> </span><span class=s2>"B"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"/array/2"</span><span class=p>:</span><span class=w> </span><span class=s2>"C"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"/nested/one"</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>"/nested/two"</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>"/nested/three/0"</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>"/nested/three/1"</span><span class=p>:</span><span class=w> </span><span class=kc>false</span>
|
||||
<span class=w> </span><span class=nt>"/array/0"</span><span class=p>:</span><span class=w> </span><span class=s2>"A"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"/array/1"</span><span class=p>:</span><span class=w> </span><span class=s2>"B"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"/array/2"</span><span class=p>:</span><span class=w> </span><span class=s2>"C"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"/nested/one"</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>"/nested/two"</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>"/nested/three/0"</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>"/nested/three/1"</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">¶</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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<!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/macros/" rel="canonical"><link href=../enum_conversion/ 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-9.7.7"><title>Supported Macros - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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<!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/macros/" rel="canonical"><link href=../enum_conversion/ 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-9.7.7"><title>Supported Macros - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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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> </labelLine truncated
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@@ -1,6 +1,6 @@
|
||||
<!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=../iterators/ rel=prev><link href=../json_patch/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>JSON Merge Patch - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#json-merge-patch 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> 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> <divLine truncated
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iomanip></span><span class=c1> // for std::setw</span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iomanip></span><span class=w> </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>
|
||||
@@ -8,30 +8,30 @@
|
||||
<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>"</span><span class=dl>(</span><span class=s>{</span>
|
||||
<span class=s> "title": "Goodbye!",</span>
|
||||
<span class=s> "author": {</span>
|
||||
<span class=s> "givenName": "John",</span>
|
||||
<span class=s> "familyName": "Doe"</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>"</span><span class=dl>(</span><span class=s>{</span>
|
||||
<span class=s> "title": "Goodbye!",</span>
|
||||
<span class=s> "author": {</span>
|
||||
<span class=s> "givenName": "John",</span>
|
||||
<span class=s> "familyName": "Doe"</span>
|
||||
<span class=s> },</span>
|
||||
<span class=s> "tags": [</span>
|
||||
<span class=s> "example",</span>
|
||||
<span class=s> "sample"</span>
|
||||
<span class=s> "tags": [</span>
|
||||
<span class=s> "example",</span>
|
||||
<span class=s> "sample"</span>
|
||||
<span class=s> ],</span>
|
||||
<span class=s> "content": "This will be unchanged"</span>
|
||||
<span class=s> }</span><span class=dl>)</span><span class=s>"</span><span class=n>_json</span><span class=p>;</span>
|
||||
<span class=s> "content": "This will be unchanged"</span>
|
||||
<span class=s> }</span><span class=dl>)</span><span class=s>"</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>"</span><span class=dl>(</span><span class=s>{</span>
|
||||
<span class=s> "title": "Hello!",</span>
|
||||
<span class=s> "phoneNumber": "+01-123-456-7890",</span>
|
||||
<span class=s> "author": {</span>
|
||||
<span class=s> "familyName": null</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>"</span><span class=dl>(</span><span class=s>{</span>
|
||||
<span class=s> "title": "Hello!",</span>
|
||||
<span class=s> "phoneNumber": "+01-123-456-7890",</span>
|
||||
<span class=s> "author": {</span>
|
||||
<span class=s> "familyName": null</span>
|
||||
<span class=s> },</span>
|
||||
<span class=s> "tags": [</span>
|
||||
<span class=s> "example"</span>
|
||||
<span class=s> "tags": [</span>
|
||||
<span class=s> "example"</span>
|
||||
<span class=s> ]</span>
|
||||
<span class=s> }</span><span class=dl>)</span><span class=s>"</span><span class=n>_json</span><span class=p>;</span>
|
||||
<span class=s> }</span><span class=dl>)</span><span class=s>"</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>
|
||||
@@ -40,14 +40,14 @@
|
||||
<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><<</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><<</span><span class=w> </span><span class=n>document</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>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>"author"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"givenName"</span><span class=p>:</span><span class=w> </span><span class=s2>"John"</span>
|
||||
<span class=w> </span><span class=nt>"author"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"givenName"</span><span class=p>:</span><span class=w> </span><span class=s2>"John"</span>
|
||||
<span class=w> </span><span class=p>},</span>
|
||||
<span class=w> </span><span class=nt>"content"</span><span class=p>:</span><span class=w> </span><span class=s2>"This will be unchanged"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"phoneNumber"</span><span class=p>:</span><span class=w> </span><span class=s2>"+01-123-456-7890"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"tags"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=s2>"example"</span>
|
||||
<span class=w> </span><span class=nt>"content"</span><span class=p>:</span><span class=w> </span><span class=s2>"This will be unchanged"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"phoneNumber"</span><span class=p>:</span><span class=w> </span><span class=s2>"+01-123-456-7890"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"tags"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=s2>"example"</span>
|
||||
<span class=w> </span><span class=p>],</span>
|
||||
<span class=w> </span><span class=nt>"title"</span><span class=p>:</span><span class=w> </span><span class=s2>"Hello!"</span>
|
||||
<span class=w> </span><span class=nt>"title"</span><span class=p>:</span><span class=w> </span><span class=s2>"Hello!"</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> </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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -6,8 +6,8 @@
|
||||
<span class=c1>// operator+= is a shorthand for push_back</span>
|
||||
<span class=n>j</span><span class=w> </span><span class=o>+=</span><span class=w> </span><span class=mi>4</span><span class=p>;</span><span class=w> </span><span class=c1>// [1,2,3,4]</span>
|
||||
</code></pre></div> <h2 id=adding-to-objects>Adding to objects<a class=headerlink href=#adding-to-objects title="Permanent link">¶</a></h2> <p>The most common way to add or replace a member is <a href=../element_access/unchecked_access/ ><code>operator[]</code></a>, which inserts the key if it does not exist yet:</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=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"Mary"</span><span class=p>;</span><span class=w> </span><span class=c1>// {"name":"Mary"}</span>
|
||||
<span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"John"</span><span class=p>;</span><span class=w> </span><span class=c1>// {"name":"John"} (replaced)</span>
|
||||
<span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"Mary"</span><span class=p>;</span><span class=w> </span><span class=c1>// {"name":"Mary"}</span>
|
||||
<span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"John"</span><span class=p>;</span><span class=w> </span><span class=c1>// {"name":"John"} (replaced)</span>
|
||||
</code></pre></div> <p><a href=../../api/basic_json/emplace/ ><code>emplace</code></a> inserts a member only if the key is not already present, and reports whether the insertion happened — useful for "add if absent" semantics.</p> <h2 id=merging-objects>Merging objects<a class=headerlink href=#merging-objects title="Permanent link">¶</a></h2> <p>To merge one object into another, <a href=../../api/basic_json/update/ ><code>update</code></a> copies all members from another object, overwriting existing keys (similar to Python's <code>dict.update</code>). This is the idiomatic way to combine two objects.</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><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iomanip></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
@@ -18,40 +18,40 @@
|
||||
<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 two JSON objects</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>o1</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s> {"color": "red", "price": 17.99, "names": {"de": "Flugzeug"}} </span><span class=dl>)</span><span class=s>"</span><span class=n>_json</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>o2</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s> {"color": "blue", "speed": 100, "names": {"en": "plane"}} </span><span class=dl>)</span><span class=s>"</span><span class=n>_json</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>o1</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s> {"color": "red", "price": 17.99, "names": {"de": "Flugzeug"}} </span><span class=dl>)</span><span class=s>"</span><span class=n>_json</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>o2</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s> {"color": "blue", "speed": 100, "names": {"en": "plane"}} </span><span class=dl>)</span><span class=s>"</span><span class=n>_json</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>o3</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>o1</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// add all keys from o2 to o1 (updating "color", replacing "names")</span>
|
||||
<span class=w> </span><span class=c1>// add all keys from o2 to o1 (updating "color", replacing "names")</span>
|
||||
<span class=w> </span><span class=n>o1</span><span class=p>.</span><span class=n>update</span><span class=p>(</span><span class=n>o2</span><span class=p>);</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// add all keys from o2 to o1 (updating "color", merging "names")</span>
|
||||
<span class=w> </span><span class=c1>// add all keys from o2 to o1 (updating "color", merging "names")</span>
|
||||
<span class=w> </span><span class=n>o3</span><span class=p>.</span><span class=n>update</span><span class=p>(</span><span class=n>o2</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>// output updated object o1 and o3</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><<</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><<</span><span class=w> </span><span class=n>o1</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</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><<</span><span class=w> </span><span class=n>o3</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</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><<</span><span class=w> </span><span class=n>o1</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</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><<</span><span class=w> </span><span class=n>o3</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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>"color"</span><span class=p>:</span><span class=w> </span><span class=s2>"blue"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"names"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"en"</span><span class=p>:</span><span class=w> </span><span class=s2>"plane"</span>
|
||||
<span class=w> </span><span class=nt>"color"</span><span class=p>:</span><span class=w> </span><span class=s2>"blue"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"names"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"en"</span><span class=p>:</span><span class=w> </span><span class=s2>"plane"</span>
|
||||
<span class=w> </span><span class=p>},</span>
|
||||
<span class=w> </span><span class=nt>"price"</span><span class=p>:</span><span class=w> </span><span class=mf>17.99</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"speed"</span><span class=p>:</span><span class=w> </span><span class=mi>100</span>
|
||||
<span class=w> </span><span class=nt>"price"</span><span class=p>:</span><span class=w> </span><span class=mf>17.99</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"speed"</span><span class=p>:</span><span class=w> </span><span class=mi>100</span>
|
||||
<span class=p>}</span>
|
||||
<span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"color"</span><span class=p>:</span><span class=w> </span><span class=s2>"blue"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"names"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"de"</span><span class=p>:</span><span class=w> </span><span class=s2>"Flugzeug"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"en"</span><span class=p>:</span><span class=w> </span><span class=s2>"plane"</span>
|
||||
<span class=w> </span><span class=nt>"color"</span><span class=p>:</span><span class=w> </span><span class=s2>"blue"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"names"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"de"</span><span class=p>:</span><span class=w> </span><span class=s2>"Flugzeug"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"en"</span><span class=p>:</span><span class=w> </span><span class=s2>"plane"</span>
|
||||
<span class=w> </span><span class=p>},</span>
|
||||
<span class=w> </span><span class=nt>"price"</span><span class=p>:</span><span class=w> </span><span class=mf>17.99</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"speed"</span><span class=p>:</span><span class=w> </span><span class=mi>100</span>
|
||||
<span class=w> </span><span class=nt>"price"</span><span class=p>:</span><span class=w> </span><span class=mf>17.99</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"speed"</span><span class=p>:</span><span class=w> </span><span class=mi>100</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <p>For a recursive merge that follows <a href="https://tools.ietf.org/html/rfc7386"><abbr title="Request for Comments">RFC</abbr> 7386</a>, see <a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a>. To apply a sequence of well-defined edit operations, see <a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch</a>.</p> <h2 id=removing-elements>Removing elements<a class=headerlink href=#removing-elements title="Permanent link">¶</a></h2> <p>Elements are removed with <a href=../../api/basic_json/erase/ ><code>erase</code></a>, which accepts an object key, an array index, or an iterator. <a href=../../api/basic_json/clear/ ><code>clear</code></a> empties a value while keeping its type, and <a href=../element_access/unchecked_access/ ><code>operator[]</code></a> combined with assignment can overwrite a value entirely.</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>"a"</span><span class=p>,</span><span class=w> </span><span class=mi>1</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"b"</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"c"</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>}};</span>
|
||||
<span class=n>j</span><span class=p>.</span><span class=n>erase</span><span class=p>(</span><span class=s>"b"</span><span class=p>);</span><span class=w> </span><span class=c1>// {"a":1,"c":3}</span>
|
||||
</code></pre></div> </details> <p>For a recursive merge that follows <a href="https://tools.ietf.org/html/rfc7386"><abbr title="Request for Comments">RFC</abbr> 7386</a>, see <a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a>. To apply a sequence of well-defined edit operations, see <a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch</a>.</p> <h2 id=removing-elements>Removing elements<a class=headerlink href=#removing-elements title="Permanent link">¶</a></h2> <p>Elements are removed with <a href=../../api/basic_json/erase/ ><code>erase</code></a>, which accepts an object key, an array index, or an iterator. <a href=../../api/basic_json/clear/ ><code>clear</code></a> empties a value while keeping its type, and <a href=../element_access/unchecked_access/ ><code>operator[]</code></a> combined with assignment can overwrite a value entirely.</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>"a"</span><span class=p>,</span><span class=w> </span><span class=mi>1</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"b"</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"c"</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>}};</span>
|
||||
<span class=n>j</span><span class=p>.</span><span class=n>erase</span><span class=p>(</span><span class=s>"b"</span><span class=p>);</span><span class=w> </span><span class=c1>// {"a":1,"c":3}</span>
|
||||
|
||||
<span class=n>json</span><span class=w> </span><span class=n>a</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=n>a</span><span class=p>.</span><span class=n>erase</span><span class=p>(</span><span class=mi>1</span><span class=p>);</span><span class=w> </span><span class=c1>// [1,3,4] (erase by index)</span>
|
||||
</code></pre></div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/push_back/ ><code>push_back</code></a> / <a href=../../api/basic_json/emplace_back/ ><code>emplace_back</code></a> - append to an array</li> <li><a href=../../api/basic_json/emplace/ ><code>emplace</code></a> - insert into an object if the key is absent</li> <li><a href=../../api/basic_json/update/ ><code>update</code></a> - merge objects</li> <li><a href=../../api/basic_json/erase/ ><code>erase</code></a> / <a href=../../api/basic_json/clear/ ><code>clear</code></a> - remove elements</li> <li><a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch and Diff</a> and <a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a> - structured modifications</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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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 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.Line truncated
|
||||
</code></pre></div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/push_back/ ><code>push_back</code></a> / <a href=../../api/basic_json/emplace_back/ ><code>emplace_back</code></a> - append to an array</li> <li><a href=../../api/basic_json/emplace/ ><code>emplace</code></a> - insert into an object if the key is absent</li> <li><a href=../../api/basic_json/update/ ><code>update</code></a> - merge objects</li> <li><a href=../../api/basic_json/erase/ ><code>erase</code></a> / <a href=../../api/basic_json/clear/ ><code>clear</code></a> - remove elements</li> <li><a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch and Diff</a> and <a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a> - structured modifications</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -5,6 +5,6 @@
|
||||
|
||||
<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>"example.json"</span><span class=p>);</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>"example.json"</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">¶</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.</p> <h2 id=exported-symbols>Exported symbols<a class=headerlink href=#exported-symbols title="Permanent link">¶</a></h2> <p>Only the following symbols are exported from <code>nlohmann.json</code>:</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_json</code></li> <li><code>nlohmann::ordered_map</code></li> <li><code>nlohmann::to_string</code></li> <li><code>nlohmann::literals::json_literals::operator""_json</code></li> <li><code>nlohmann::literals::json_literals::operator""_json_pointer</code></li> </ul> <p>Additionally, the following <code>nlohmann::detail</code> symbols are exported, solely to work around an <abbr title="Microsoft Visual C++">MSVC</abbr> compilation issue (<a href="https://github.com/nlohmann/json/issues/3970">#3970</a>). They are implementation details, not part of the public <abbr title="Application Programming Interfaces">API</abbr>, and should not be used directly:</p> <ul> <li><code>nlohmann::detail::json_sax_dom_callback_parser</code></li> <li><code>nlohmann::detail::unknown_size</code></li> </ul> <h2 id=known-issues>Known issues<a class=headerlink href=#known-issues title="Permanent link">¶</a></h2> <p>C++20 modules support is exercised in <abbr title="Continuous Integration">CI</abbr> against current <abbr title="GNU Compiler Collection">GCC</abbr> and Clang on Ubuntu, and the default <abbr title="Microsoft Visual C++">MSVC</abbr> toolset on Windows Server 2022 — there is no documented minimum compiler version, unlike feature-test-macro-gated features such as <a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a>.</p> <div class="admonition info"> <p class=admonition-title>Known compiler issues</p> <ul> <li><strong><abbr title="GNU Compiler Collection">GCC</abbr></strong> may emit "redefinition" errors when <code>#include <nlohmann/json.hpp></code> appears in a module preamble together with other imports. This is an upstream <abbr title="GNU Compiler Collection">GCC</abbr> bug, not yet resolved as of <abbr title="GNU Compiler Collection">GCC</abbr> 16. Workarounds: include <code>nlohmann/json.hpp</code> before other <code>#include</code>s, use <code>import nlohmann.json;</code> instead, or upgrade <abbr title="GNU Compiler Collection">GCC</abbr>. (<a href="https://github.com/nlohmann/json/issues/5103">issue #5103</a>)</li> <li><strong><abbr title="Microsoft Visual C++">MSVC</abbr></strong> could fail with <code>C2039: 'json_sax_dom_callback_parser' is not a member of ... detail</code>; fixed by exporting the required internal symbols from <code>json.cppm</code> (see <a href=#exported-symbols>Exported symbols</a> above). (<a href="https://github.com/nlohmann/json/issues/3970">issue #3970</a>)</li> </ul> <p>If you hit a different module-related build failure, search <a href="https://github.com/nlohmann/json/issues?q=is%3Aissue+modules">existing issues</a> before filing a new 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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__innLine truncated
|
||||
</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">¶</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.</p> <h2 id=exported-symbols>Exported symbols<a class=headerlink href=#exported-symbols title="Permanent link">¶</a></h2> <p>Only the following symbols are exported from <code>nlohmann.json</code>:</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_json</code></li> <li><code>nlohmann::ordered_map</code></li> <li><code>nlohmann::to_string</code></li> <li><code>nlohmann::literals::json_literals::operator""_json</code></li> <li><code>nlohmann::literals::json_literals::operator""_json_pointer</code></li> </ul> <p>Additionally, the following <code>nlohmann::detail</code> symbols are exported, solely to work around an <abbr title="Microsoft Visual C++">MSVC</abbr> compilation issue (<a href="https://github.com/nlohmann/json/issues/3970">#3970</a>). They are implementation details, not part of the public <abbr title="Application Programming Interfaces">API</abbr>, and should not be used directly:</p> <ul> <li><code>nlohmann::detail::json_sax_dom_callback_parser</code></li> <li><code>nlohmann::detail::unknown_size</code></li> </ul> <h2 id=known-issues>Known issues<a class=headerlink href=#known-issues title="Permanent link">¶</a></h2> <p>C++20 modules support is exercised in <abbr title="Continuous Integration">CI</abbr> against current <abbr title="GNU Compiler Collection">GCC</abbr> and Clang on Ubuntu, and the default <abbr title="Microsoft Visual C++">MSVC</abbr> toolset on Windows Server 2022 — there is no documented minimum compiler version, unlike feature-test-macro-gated features such as <a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a>.</p> <div class="admonition info"> <p class=admonition-title>Known compiler issues</p> <ul> <li><strong><abbr title="GNU Compiler Collection">GCC</abbr></strong> may emit "redefinition" errors when <code>#include <nlohmann/json.hpp></code> appears in a module preamble together with other imports. This is an upstream <abbr title="GNU Compiler Collection">GCC</abbr> bug, not yet resolved as of <abbr title="GNU Compiler Collection">GCC</abbr> 16. Workarounds: include <code>nlohmann/json.hpp</code> before other <code>#include</code>s, use <code>import nlohmann.json;</code> instead, or upgrade <abbr title="GNU Compiler Collection">GCC</abbr>. (<a href="https://github.com/nlohmann/json/issues/5103">issue #5103</a>)</li> <li><strong><abbr title="Microsoft Visual C++">MSVC</abbr></strong> could fail with <code>C2039: 'json_sax_dom_callback_parser' is not a member of ... detail</code>; fixed by exporting the required internal symbols from <code>json.cppm</code> (see <a href=#exported-symbols>Exported symbols</a> above). (<a href="https://github.com/nlohmann/json/issues/3970">issue #3970</a>)</li> </ul> <p>If you hit a different module-related build failure, search <a href="https://github.com/nlohmann/json/issues?q=is%3Aissue+modules">existing issues</a> before filing a new 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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__iLine truncated
|
||||
@@ -9,4 +9,4 @@
|
||||
app --> json_diag
|
||||
app --> 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">¶</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">¶</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 inline 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">¶</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">¶</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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 7.1.0 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 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/svg viewbox="0 0 384 512"><!-- Font AweLine truncated
|
||||
</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">¶</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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 7.1.0 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 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/svg viewbox="0 0 384 512"><!-- Font ALine truncated
|
||||
@@ -6,16 +6,16 @@
|
||||
<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>"one"</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>"two"</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>"three"</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>j</span><span class=p>[</span><span class=s>"one"</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>"two"</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>"three"</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><<</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><<</span><span class=w> </span><span class=sc>'\n'</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><<</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><<</span><span class=w> </span><span class=sc>'\n'</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>"one"</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>"three"</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>"two"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span>
|
||||
<span class=w> </span><span class=nt>"one"</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>"three"</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>"two"</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">¶</a></h2> <p>If you do want to preserve the <strong>insertion order</strong>, you can use the type <a href=../../api/ordered_json/ ><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><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
@@ -25,36 +25,36 @@
|
||||
<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>"one"</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>"two"</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>"three"</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>j</span><span class=p>[</span><span class=s>"one"</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>"two"</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>"three"</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><<</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><<</span><span class=w> </span><span class=sc>'\n'</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><<</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><<</span><span class=w> </span><span class=sc>'\n'</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>"one"</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>"two"</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>"three"</span><span class=p>:</span><span class=w> </span><span class=mi>3</span>
|
||||
<span class=w> </span><span class=nt>"one"</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>"two"</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>"three"</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> <p>The <a href=../../api/ordered_map/ ><code>ordered_map</code></a> behind <code>nlohmann::ordered_json</code> is deliberately minimal and has no lookup index, so every key access is a linear scan and building an object of <code>n</code> keys costs O(n²). This is unnoticeable at typical object sizes but becomes significant for objects with many thousands of keys; see <a href=../../api/ordered_map/#complexity><code>ordered_map</code> complexity</a>. The alternatives above keep a lookup index and do not have this cost.</p> <h3 id=notes-on-parsing>Notes on parsing<a class=headerlink href=#notes-on-parsing title="Permanent link">¶</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>"one"</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>"two"</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>"three"</span><span class=p>:</span><span class=w> </span><span class=mi>3</span>
|
||||
<span class=w> </span><span class=nt>"one"</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>"two"</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>"three"</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>"input.json"</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>"input.json"</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><<</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><<</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>"one"</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>"two"</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>"three"</span><span class=p>:</span><span class=w> </span><span class=mi>3</span>
|
||||
<span class=w> </span><span class=nt>"one"</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>"two"</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>"three"</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>"input.json"</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>"input.json"</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><<</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><<</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>"one"</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>"three"</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>"two"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span>
|
||||
<span class=w> </span><span class=nt>"one"</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>"three"</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>"two"</span><span class=p>:</span><span class=w> </span><span class=mi>2</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> </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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -1,7 +1,7 @@
|
||||
<!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/" rel="canonical"><link href=../object_order/ rel=prev><link href=json_lines/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Parsing - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#parsing 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> Parsing </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 id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 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> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 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 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 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-search data-md-componLine 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>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"happy": true, "pi": 3.141}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</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>"</span><span class=dl>(</span><span class=s>{"happy": true, "pi": 3.141}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
|
||||
|
||||
<span class=c1>// parse from a file</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>"example.json"</span><span class=p>);</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>"example.json"</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>The input must be encoded in <abbr title="Unicode Transformation Format">UTF</abbr>-8; other encodings are not supported. A single input may contain only one <abbr title="JavaScript Object Notation">JSON</abbr> value. Inputs consisting of multiple values separated by newlines are handled by the <a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> format.</p> <p>By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the <code>parse</code> function — see <a href=../comments/ >comments</a> and <a href=../trailing_commas/ >trailing commas</a>.</p> <h2 id=strictness-and-trailing-data>Strictness and trailing data<a class=headerlink href=#strictness-and-trailing-data title="Permanent link">¶</a></h2> <p><a href=../../api/basic_json/parse/ ><code>parse</code></a> reads a single <abbr title="JavaScript Object Notation">JSON</abbr> value and requires the whole input to be consumed: any non-whitespace data after the value is reported as a parse error. Use it when you want to guarantee that an input is exactly one complete <abbr title="JavaScript Object Notation">JSON</abbr> document.</p> <p><a href=../../api/operator_gtgt/ ><code>operator>></code></a> follows relaxed <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> semantics instead: it parses one <abbr title="JavaScript Object Notation">JSON</abbr> value and leaves the stream positioned right after it, without requiring the rest of the stream to be consumed. This is what makes it possible to read several concatenated values from the same stream, but it also means that "a valid document followed by trailing bytes" is accepted rather than rejected. If you are validating conformance, or need to reject any input that is not exactly one <abbr title="JavaScript Object Notation">JSON</abbr> document, prefer <code>parse</code>.</p> <p>When using <code>operator>></code> to read several concatenated values this way, a value that is a number must be followed by whitespace, because <code>operator>></code> consumes the character that terminates a number — see the <a href=../../api/operator_gtgt/#notes><code>operator>></code> notes</a> for details and examples.</p> <h2 id=sax-vs-dom-parsing><abbr title="Simple API for XML">SAX</abbr> vs. <abbr title="Document Object Model">DOM</abbr> parsing<a class=headerlink href=#sax-vs-dom-parsing title="Permanent link">¶</a></h2> <p>The library offers two parsing models:</p> <ul> <li><strong><abbr title="Document Object Model">DOM</abbr> parsing</strong> (the default): the complete input is read and stored as an in-memory <code>basic_json</code> value that can be traversed and modified freely. This is what <a href=../../api/basic_json/parse/ ><code>parse</code></a> does, and it is the right choice for most use cases.</li> <li><strong><abbr title="Simple API for XML">SAX</abbr> parsing</strong>: instead of building a value, the parser reports events (such as "a string was read" or "an object started") to a handler that you implement. This avoids building the full value in memory and is useful for very large inputs or when you only need to extract parts of the input. See the <a href=sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> for details and <a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> for the <abbr title="Application Programming Interfaces">API</abbr>.</li> </ul> <p>You can influence a <abbr title="Document Object Model">DOM</abbr> parse without switching to the <abbr title="Simple API for XML">SAX</abbr> interface by passing a <a href=parser_callbacks/ >parser callback</a>, which is called during parsing and can, for example, discard parts of the input.</p> <h2 id=exceptions>Exceptions<a class=headerlink href=#exceptions title="Permanent link">¶</a></h2> <p>When the input is not valid <abbr title="JavaScript Object Notation">JSON</abbr>, the <code>parse</code> function throws an exception by default. If exceptions are undesired or unavailable, the parser can instead return a discarded value, or <a href=../../api/basic_json/accept/ ><code>accept</code></a> can be used to only check whether an input is valid <abbr title="JavaScript Object Notation">JSON</abbr>. See <a href=parse_exceptions/ >parsing and exceptions</a> for the available options.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../api/basic_json/accept/ ><code>accept</code></a> - check if the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=json_lines/ ><abbr title="JavaScript ObjectLine truncated
|
||||
</code></pre></div> <p>The input must be encoded in <abbr title="Unicode Transformation Format">UTF</abbr>-8; other encodings are not supported. A single input may contain only one <abbr title="JavaScript Object Notation">JSON</abbr> value. Inputs consisting of multiple values separated by newlines are handled by the <a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> format.</p> <p>By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the <code>parse</code> function — see <a href=../comments/ >comments</a> and <a href=../trailing_commas/ >trailing commas</a>.</p> <h2 id=strictness-and-trailing-data>Strictness and trailing data<a class=headerlink href=#strictness-and-trailing-data title="Permanent link">¶</a></h2> <p><a href=../../api/basic_json/parse/ ><code>parse</code></a> reads a single <abbr title="JavaScript Object Notation">JSON</abbr> value and requires the whole input to be consumed: any non-whitespace data after the value is reported as a parse error. Use it when you want to guarantee that an input is exactly one complete <abbr title="JavaScript Object Notation">JSON</abbr> document.</p> <p><a href=../../api/operator_gtgt/ ><code>operator>></code></a> follows relaxed <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> semantics instead: it parses one <abbr title="JavaScript Object Notation">JSON</abbr> value and leaves the stream positioned right after it, without requiring the rest of the stream to be consumed. This is what makes it possible to read several concatenated values from the same stream, but it also means that "a valid document followed by trailing bytes" is accepted rather than rejected. If you are validating conformance, or need to reject any input that is not exactly one <abbr title="JavaScript Object Notation">JSON</abbr> document, prefer <code>parse</code>.</p> <p>When using <code>operator>></code> to read several concatenated values this way, a value that is a number must be followed by whitespace, because <code>operator>></code> consumes the character that terminates a number — see the <a href=../../api/operator_gtgt/#notes><code>operator>></code> notes</a> for details and examples.</p> <h2 id=sax-vs-dom-parsing><abbr title="Simple API for XML">SAX</abbr> vs. <abbr title="Document Object Model">DOM</abbr> parsing<a class=headerlink href=#sax-vs-dom-parsing title="Permanent link">¶</a></h2> <p>The library offers two parsing models:</p> <ul> <li><strong><abbr title="Document Object Model">DOM</abbr> parsing</strong> (the default): the complete input is read and stored as an in-memory <code>basic_json</code> value that can be traversed and modified freely. This is what <a href=../../api/basic_json/parse/ ><code>parse</code></a> does, and it is the right choice for most use cases.</li> <li><strong><abbr title="Simple API for XML">SAX</abbr> parsing</strong>: instead of building a value, the parser reports events (such as "a string was read" or "an object started") to a handler that you implement. This avoids building the full value in memory and is useful for very large inputs or when you only need to extract parts of the input. See the <a href=sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> for details and <a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> for the <abbr title="Application Programming Interfaces">API</abbr>.</li> </ul> <p>You can influence a <abbr title="Document Object Model">DOM</abbr> parse without switching to the <abbr title="Simple API for XML">SAX</abbr> interface by passing a <a href=parser_callbacks/ >parser callback</a>, which is called during parsing and can, for example, discard parts of the input.</p> <h2 id=exceptions>Exceptions<a class=headerlink href=#exceptions title="Permanent link">¶</a></h2> <p>When the input is not valid <abbr title="JavaScript Object Notation">JSON</abbr>, the <code>parse</code> function throws an exception by default. If exceptions are undesired or unavailable, the parser can instead return a discarded value, or <a href=../../api/basic_json/accept/ ><code>accept</code></a> can be used to only check whether an input is valid <abbr title="JavaScript Object Notation">JSON</abbr>. See <a href=parse_exceptions/ >parsing and exceptions</a> for the available options.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../api/basic_json/accept/ ><code>accept</code></a> - check if the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=json_lines/ ><abbr title="JavaScript ObjectLine truncated
|
||||
@@ -1,7 +1,7 @@
|
||||
<!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-9.7.7"><title>JSON Lines - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#json-lines 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> 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 name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 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> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 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 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 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </labelLine truncated
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Alexa"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=w> </span><span class=s2>"4♠"</span><span class=p>],</span><span class=w> </span><span class=p>[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=w> </span><span class=s2>"9♠"</span><span class=p>]]}</span>
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"May"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[]}</span>
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Deloise"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>"three of a kind"</span><span class=p>,</span><span class=w> </span><span class=s2>"5♣"</span><span class=p>]]}</span>
|
||||
<!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-9.7.7"><title>JSON Lines - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#json-lines 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> 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 name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 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> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 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 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 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </labelLine truncated
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Alexa"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=w> </span><span class=s2>"4♠"</span><span class=p>],</span><span class=w> </span><span class=p>[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=w> </span><span class=s2>"9♠"</span><span class=p>]]}</span>
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"May"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[]}</span>
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Deloise"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>"three of a kind"</span><span class=p>,</span><span class=w> </span><span class=s2>"5♣"</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><sstream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
@@ -12,11 +12,11 @@
|
||||
<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><<</span><span class=w> </span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"name": "Gilbert", "wins": [["straight", "7♣"], ["one pair", "10♥"]]}</span>
|
||||
<span class=s>{"name": "Alexa", "wins": [["two pair", "4♠"], ["two pair", "9♠"]]}</span>
|
||||
<span class=s>{"name": "May", "wins": []}</span>
|
||||
<span class=s>{"name": "Deloise", "wins": [["three of a kind", "5♣"]]}</span>
|
||||
<span class=dl>)</span><span class=s>"</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>input</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"name": "Gilbert", "wins": [["straight", "7♣"], ["one pair", "10♥"]]}</span>
|
||||
<span class=s>{"name": "Alexa", "wins": [["two pair", "4♠"], ["two pair", "9♠"]]}</span>
|
||||
<span class=s>{"name": "May", "wins": []}</span>
|
||||
<span class=s>{"name": "Deloise", "wins": [["three of a kind", "5♣"]]}</span>
|
||||
<span class=dl>)</span><span class=s>"</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>
|
||||
@@ -24,13 +24,13 @@
|
||||
<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><<</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><<</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>"name"</span><span class=p>:</span><span class=s2>"Gilbert"</span><span class=p>,</span><span class=nt>"wins"</span><span class=p>:[[</span><span class=s2>"straight"</span><span class=p>,</span><span class=s2>"7♣"</span><span class=p>],[</span><span class=s2>"one pair"</span><span class=p>,</span><span class=s2>"10♥"</span><span class=p>]]}</span>
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"Alexa"</span><span class=p>,</span><span class=nt>"wins"</span><span class=p>:[[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=s2>"4♠"</span><span class=p>],[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=s2>"9♠"</span><span class=p>]]}</span>
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"May"</span><span class=p>,</span><span class=nt>"wins"</span><span class=p>:[]}</span>
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"Deloise"</span><span class=p>,</span><span class=nt>"wins"</span><span class=p>:[[</span><span class=s2>"three of a kind"</span><span class=p>,</span><span class=s2>"5♣"</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>"name"</span><span class=p>:</span><span class=s2>"Gilbert"</span><span class=p>,</span><span class=nt>"wins"</span><span class=p>:[[</span><span class=s2>"straight"</span><span class=p>,</span><span class=s2>"7♣"</span><span class=p>],[</span><span class=s2>"one pair"</span><span class=p>,</span><span class=s2>"10♥"</span><span class=p>]]}</span>
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"Alexa"</span><span class=p>,</span><span class=nt>"wins"</span><span class=p>:[[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=s2>"4♠"</span><span class=p>],[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=s2>"9♠"</span><span class=p>]]}</span>
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"May"</span><span class=p>,</span><span class=nt>"wins"</span><span class=p>:[]}</span>
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"Deloise"</span><span class=p>,</span><span class=nt>"wins"</span><span class=p>:[[</span><span class=s2>"three of a kind"</span><span class=p>,</span><span class=s2>"5♣"</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>></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>>></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><<</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>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> <p>This is different from parsing a stream of <em>concatenated</em> (non-newline-delimited) <abbr title="JavaScript Object Notation">JSON</abbr> values, for which <code>operator>></code> does work, provided that a value that is a number is followed by whitespace -- see its <a href=../../../api/operator_gtgt/#notes>notes</a> for details.</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> <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> <p>This is different from parsing a stream of <em>concatenated</em> (non-newline-delimited) <abbr title="JavaScript Object Notation">JSON</abbr> values, for which <code>operator>></code> does work, provided that a value that is a number is followed by whitespace -- see its <a href=../../../api/operator_gtgt/#notes>notes</a> for details.</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -5,16 +5,16 @@
|
||||
<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>&</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><<</span><span class=w> </span><span class=s>"parse error at byte "</span><span class=w> </span><span class=o><<</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><<</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>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"parse error at byte "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>ex</span><span class=p>.</span><span class=kt>byte</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>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">¶</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=nb>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><<</span><span class=w> </span><span class=s>"parse error"</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>endl</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><<</span><span class=w> </span><span class=s>"parse error"</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>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">¶</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><<</span><span class=w> </span><span class=s>"parse error"</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>endl</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><<</span><span class=w> </span><span class=s>"parse error"</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>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">¶</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>&</span><span class=w> </span><span class=n>last_token</span><span class=p>,</span>
|
||||
@@ -35,9 +35,9 @@
|
||||
<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>&</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>&</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><<</span><span class=w> </span><span class=s>"parse error at input byte "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>position</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n</span><span class=s>"</span>
|
||||
<span class=w> </span><span class=o><<</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><<</span><span class=w> </span><span class=s>"</span><span class=se>\n</span><span class=s>"</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"last read: </span><span class=se>\"</span><span class=s>"</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>last_token</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\"</span><span class=s>"</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><<</span><span class=w> </span><span class=s>"parse error at input byte "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>position</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n</span><span class=s>"</span>
|
||||
<span class=w> </span><span class=o><<</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><<</span><span class=w> </span><span class=s>"</span><span class=se>\n</span><span class=s>"</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"last read: </span><span class=se>\"</span><span class=s>"</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>last_token</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\"</span><span class=s>"</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>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>
|
||||
@@ -45,7 +45,7 @@
|
||||
|
||||
<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>"[1,2,3,]"</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>"[1,2,3,]"</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>
|
||||
@@ -53,14 +53,14 @@
|
||||
<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>&</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><<</span><span class=w> </span><span class=s>"parsing unsuccessful!"</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>endl</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><<</span><span class=w> </span><span class=s>"parsing unsuccessful!"</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>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><<</span><span class=w> </span><span class=s>"parsed value: "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>result</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>endl</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><<</span><span class=w> </span><span class=s>"parsed value: "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>result</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>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 ']'; expected '[', '{', or a literal
|
||||
last read: "3,]"
|
||||
[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
|
||||
last read: "3,]"
|
||||
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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> </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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -2,13 +2,13 @@
|
||||
<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><</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>&</span><span class=w> </span><span class=n>parsed</span><span class=p>)</span><span class=o>></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">¶</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>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Berlin"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"location"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Berlin"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"location"</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>"name"</span></code></td> </tr> <tr> <td><code>value</code></td> <td>1</td> <td><code class=highlight><span class=s2>"Berlin"</span></code></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>"location"</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>"location"</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>"name"</span><span class=p>:</span><span class=s2>"Berlin"</span><span class=p>}</span></code></td> </tr> </tbody> </table> </details> <div class="admonition note"> <p class=admonition-title>No built-in nesting depth limit</p> <p>The library has no built-in limit on recursion/nesting depth while parsing. A parser callback can only <em>discard</em> content it has already parsed (by returning <code class=highlight><span class=nb>false</span></code>); it cannot make parsing fail once a nesting limit is exceeded partway through reading a deeply nested value. If you need to reject over-deep untrusted input outright, track <code>depth</code> in a callback and <code>throw</code> from it once your limit is exceeded (a thrown exception propagates out of <code>parse()</code> as usual).</p> </div> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">¶</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><iostream></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>"name"</span></code></td> </tr> <tr> <td><code>value</code></td> <td>1</td> <td><code class=highlight><span class=s2>"Berlin"</span></code></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>"location"</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>"location"</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>"name"</span><span class=p>:</span><span class=s2>"Berlin"</span><span class=p>}</span></code></td> </tr> </tbody> </table> </details> <div class="admonition note"> <p class=admonition-title>No built-in nesting depth limit</p> <p>The library has no built-in limit on recursion/nesting depth while parsing. A parser callback can only <em>discard</em> content it has already parsed (by returning <code class=highlight><span class=nb>false</span></code>); it cannot make parsing fail once a nesting limit is exceeded partway through reading a deeply nested value. If you need to reject over-deep untrusted input outright, track <code>depth</code> in a callback and <code>throw</code> from it once your limit is exceeded (a thrown exception propagates out of <code>parse()</code> as usual).</p> </div> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">¶</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><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iomanip></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
|
||||
@@ -17,32 +17,32 @@
|
||||
<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>"</span><span class=dl>(</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>"</span><span class=dl>(</span>
|
||||
<span class=s> {</span>
|
||||
<span class=s> "Image": {</span>
|
||||
<span class=s> "Width": 800,</span>
|
||||
<span class=s> "Height": 600,</span>
|
||||
<span class=s> "Title": "View from 15th Floor",</span>
|
||||
<span class=s> "Thumbnail": {</span>
|
||||
<span class=s> "Url": "http://www.example.com/image/481989943",</span>
|
||||
<span class=s> "Height": 125,</span>
|
||||
<span class=s> "Width": 100</span>
|
||||
<span class=s> "Image": {</span>
|
||||
<span class=s> "Width": 800,</span>
|
||||
<span class=s> "Height": 600,</span>
|
||||
<span class=s> "Title": "View from 15th Floor",</span>
|
||||
<span class=s> "Thumbnail": {</span>
|
||||
<span class=s> "Url": "http://www.example.com/image/481989943",</span>
|
||||
<span class=s> "Height": 125,</span>
|
||||
<span class=s> "Width": 100</span>
|
||||
<span class=s> },</span>
|
||||
<span class=s> "Animated" : false,</span>
|
||||
<span class=s> "IDs": [116, 943, 234, 38793]</span>
|
||||
<span class=s> "Animated" : false,</span>
|
||||
<span class=s> "IDs": [116, 943, 234, 38793]</span>
|
||||
<span class=s> }</span>
|
||||
<span class=s> }</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</span><span class=p>;</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</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><<</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><<</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=s>"</span><span class=se>\n\n</span><span class=s>"</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><<</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><<</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=s>"</span><span class=se>\n\n</span><span class=s>"</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>&</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 "Thumbnail"</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>"Thumbnail"</span><span class=p>))</span>
|
||||
<span class=w> </span><span class=c1>// skip object elements with key "Thumbnail"</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>"Thumbnail"</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>
|
||||
@@ -54,40 +54,40 @@
|
||||
|
||||
<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><<</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><<</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=sc>'\n'</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><<</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><<</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=sc>'\n'</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>"Image"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"Animated"</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>"Height"</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>"IDs"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=nt>"Image"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"Animated"</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>"Height"</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>"IDs"</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>"Thumbnail"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"Height"</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>"Url"</span><span class=p>:</span><span class=w> </span><span class=s2>"http://www.example.com/image/481989943"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"Width"</span><span class=p>:</span><span class=w> </span><span class=mi>100</span>
|
||||
<span class=w> </span><span class=nt>"Thumbnail"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"Height"</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>"Url"</span><span class=p>:</span><span class=w> </span><span class=s2>"http://www.example.com/image/481989943"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"Width"</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>"Title"</span><span class=p>:</span><span class=w> </span><span class=s2>"View from 15th Floor"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"Width"</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
|
||||
<span class=w> </span><span class=nt>"Title"</span><span class=p>:</span><span class=w> </span><span class=s2>"View from 15th Floor"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"Width"</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>"Image"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"Animated"</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>"Height"</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>"IDs"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=nt>"Image"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"Animated"</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>"Height"</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>"IDs"</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>"Title"</span><span class=p>:</span><span class=w> </span><span class=s2>"View from 15th Floor"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"Width"</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
|
||||
<span class=w> </span><span class=nt>"Title"</span><span class=p>:</span><span class=w> </span><span class=s2>"View from 15th Floor"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"Width"</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> <h2 id=recipe-rejecting-duplicate-object-keys>Recipe: rejecting duplicate object keys<a class=headerlink href=#recipe-rejecting-duplicate-object-keys title="Permanent link">¶</a></h2> <p>The <abbr title="JavaScript Object Notation">JSON</abbr> specification leaves the handling of objects with repeated keys up to the implementation. As described in <a href=../../../api/basic_json/object_t/#behavior><code>object_t</code></a>, it is unspecified which value for a repeated key ends up in the resulting <code class=highlight><span class=n>json</span></code> value -- once parsing has produced that value, the duplicate is already gone, because object storage maps each key to a single value. If duplicate keys should instead be treated as an error, a parser callback can detect them while the object is still being read, before that ambiguity ever applies.</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><iostream></span>
|
||||
@@ -124,7 +124,7 @@
|
||||
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=o>&</span><span class=w> </span><span class=n>key</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>parsed</span><span class=p>.</span><span class=n>get_ref</span><span class=o><</span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>&></span><span class=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>seen</span><span class=p>.</span><span class=n>insert</span><span class=p>(</span><span class=n>key</span><span class=p>).</span><span class=n>second</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>"duplicate JSON object key: "</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=n>key</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>"duplicate JSON object key: "</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=n>key</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>true</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
@@ -138,22 +138,22 @@
|
||||
<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>// parsing succeeds when all keys are unique</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>parse_strict</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"one": 1, "two": 2}</span><span class=dl>)</span><span class=s>"</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><<</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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>parse_strict</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"one": 1, "two": 2}</span><span class=dl>)</span><span class=s>"</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><<</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// parsing throws when a key is repeated</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>parse_strict</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"one": 1, "one": 2}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=n>parse_strict</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"one": 1, "one": 2}</span><span class=dl>)</span><span class=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=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>exception</span><span class=o>&</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><<</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><<</span><span class=w> </span><span class=sc>'\n'</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><<</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><<</span><span class=w> </span><span class=sc>'\n'</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>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</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>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>}</span>
|
||||
<span class=err>duplica</span><span class=kc>te</span><span class=w> </span><span class=err>JSON</span><span class=w> </span><span class=err>objec</span><span class=kc>t</span><span class=w> </span><span class=err>key</span><span class=p>:</span><span class=w> </span><span class=err>o</span><span class=kc>ne</span>
|
||||
</code></pre></div> </details> <p>This approach has two limitations:</p> <ul> <li>The depth-indexed bookkeeping must account for the fact that <code>object_start</code> reports the depth of the <em>parent</em> of the object, while the <code>key</code> events inside that object are reported one depth deeper (see the event table above); it is easy to get this off by one for nested objects.</li> <li>The thrown exception cannot carry a <code>parse_error</code>-style byte offset, because position tracking only exists inside the parser and lexer, not at the callback layer.</li> </ul> <p>For strict validation with precise error positions, implementing a <a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> instead gives access to the parser's position information directly.</p> <h2 id=recipe-streaming-a-large-homogeneous-array>Recipe: streaming a large homogeneous array<a class=headerlink href=#recipe-streaming-a-large-homogeneous-array title="Permanent link">¶</a></h2> <p>A common use case is a huge top-level array of many similarly-shaped objects, too large to hold entirely in memory as a <code class=highlight><span class=n>json</span></code> value. A parser callback can hand off each completed element to a user function and then discard it, so memory usage stays bounded by a single element (plus the not-yet-parsed tail of the input) rather than the whole document. Since the top-level array's <code>array_start</code>/<code>array_end</code> are reported at <code>depth == 0</code> (its parent is the document root), the object elements it contains are reported at <code>depth == 1</code>:</p> <details class=example> <summary>Example</summary> <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>input</span><span class=p>(</span><span class=s>"large_array.json"</span><span class=p>);</span>
|
||||
</code></pre></div> </details> <p>This approach has two limitations:</p> <ul> <li>The depth-indexed bookkeeping must account for the fact that <code>object_start</code> reports the depth of the <em>parent</em> of the object, while the <code>key</code> events inside that object are reported one depth deeper (see the event table above); it is easy to get this off by one for nested objects.</li> <li>The thrown exception cannot carry a <code>parse_error</code>-style byte offset, because position tracking only exists inside the parser and lexer, not at the callback layer.</li> </ul> <p>For strict validation with precise error positions, implementing a <a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> instead gives access to the parser's position information directly.</p> <h2 id=recipe-streaming-a-large-homogeneous-array>Recipe: streaming a large homogeneous array<a class=headerlink href=#recipe-streaming-a-large-homogeneous-array title="Permanent link">¶</a></h2> <p>A common use case is a huge top-level array of many similarly-shaped objects, too large to hold entirely in memory as a <code class=highlight><span class=n>json</span></code> value. A parser callback can hand off each completed element to a user function and then discard it, so memory usage stays bounded by a single element (plus the not-yet-parsed tail of the input) rather than the whole document. Since the top-level array's <code>array_start</code>/<code>array_end</code> are reported at <code>depth == 0</code> (its parent is the document root), the object elements it contains are reported at <code>depth == 1</code>:</p> <details class=example> <summary>Example</summary> <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>input</span><span class=p>(</span><span class=s>"large_array.json"</span><span class=p>);</span>
|
||||
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>callback</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=o>&</span><span class=w> </span><span class=n>parsed</span><span class=p>)</span><span class=w> </span><span class=o>-></span><span class=w> </span><span class=kt>bool</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>depth</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=mi>1</span><span class=w> </span><span class=o>&&</span><span class=w> </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>object_end</span><span class=p>)</span><span class=w> </span><span class=p>{</span>
|
||||
@@ -168,10 +168,10 @@
|
||||
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>callback</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=cm>/*event*/</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>&</span><span class=w> </span><span class=cm>/*parsed*/</span><span class=p>)</span><span class=w> </span><span class=o>-></span><span class=w> </span><span class=kt>bool</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>depth</span><span class=w> </span><span class=o>></span><span class=w> </span><span class=n>max_depth</span><span class=p>)</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>"maximum nesting depth exceeded"</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>"maximum nesting depth exceeded"</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>true</span><span class=p>;</span>
|
||||
<span class=p>};</span>
|
||||
|
||||
<span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>input</span><span class=p>,</span><span class=w> </span><span class=n>callback</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> </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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -49,4 +49,4 @@ class sax_t ["json::sax_t"] {
|
||||
|
||||
<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>&</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>&</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<json></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>&</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">¶</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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 23.6-58.9-2Line truncated
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||||
</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<json></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>&</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">¶</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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 23.6-58.9Line 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/serialization/" rel="canonical"><link href=../assertions/ 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-9.7.7"><title>Serialization - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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<!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/serialization/" rel="canonical"><link href=../assertions/ 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-9.7.7"><title>Serialization - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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|
||||
<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=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>();</span><span class=w> </span><span class=c1>// {"happy":true,"pi":3.141}</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=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>();</span><span class=w> </span><span class=c1>// {"happy":true,"pi":3.141}</span>
|
||||
</code></pre></div> <p>To write a value directly to a stream (for example, a file or <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>cout</span></code>), the <a href=../../api/operator_ltlt/ ><code>operator<<</code></a> is provided:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</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>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span>
|
||||
</code></pre></div> <div class="admonition note"> <p class=admonition-title>String, not raw value</p> <p><code>dump</code> always returns a <strong><abbr title="JavaScript Object Notation">JSON</abbr> text</strong>. Serializing a <abbr title="JavaScript Object Notation">JSON</abbr> string therefore includes the surrounding quotes and escapes special characters. To obtain the <em>contained</em> string value without quotes, use <a href=../conversions/ ><code>get<std::string>()</code></a> instead of <code>dump</code>. See the <a href=../conversions/ >converting values</a> page.</p> </div> <h2 id=pretty-printing>Pretty-printing<a class=headerlink href=#pretty-printing title="Permanent link">¶</a></h2> <p>By default, <code>dump</code> produces the most compact representation without any superfluous whitespace. Passing a non-negative <code>indent</code> argument pretty-prints the output with the given number of spaces per level:</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><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
@@ -10,31 +10,31 @@
|
||||
<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 JSON values</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_object</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>"one"</span><span class=p>,</span><span class=w> </span><span class=mi>1</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"two"</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>}};</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_object</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>"one"</span><span class=p>,</span><span class=w> </span><span class=mi>1</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"two"</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>}};</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_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>4</span><span class=p>,</span><span class=w> </span><span class=mi>8</span><span class=p>,</span><span class=w> </span><span class=mi>16</span><span class=p>};</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_string</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"Hellö 😀!"</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_string</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"Hellö 😀!"</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// call dump()</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><<</span><span class=w> </span><span class=s>"objects:"</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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>dump</span><span class=p>()</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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>dump</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>"</span><span class=se>\n\n</span><span class=s>"</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>dump</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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>dump</span><span class=p>(</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=sc>'\t'</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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><<</span><span class=w> </span><span class=s>"objects:"</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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>dump</span><span class=p>()</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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>dump</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>"</span><span class=se>\n\n</span><span class=s>"</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>dump</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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>dump</span><span class=p>(</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=sc>'\t'</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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><<</span><span class=w> </span><span class=s>"arrays:"</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_array</span><span class=p>.</span><span class=n>dump</span><span class=p>()</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_array</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_array</span><span class=p>.</span><span class=n>dump</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>"</span><span class=se>\n\n</span><span class=s>"</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_array</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_array</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=sc>'\t'</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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><<</span><span class=w> </span><span class=s>"arrays:"</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_array</span><span class=p>.</span><span class=n>dump</span><span class=p>()</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_array</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_array</span><span class=p>.</span><span class=n>dump</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>"</span><span class=se>\n\n</span><span class=s>"</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_array</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_array</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=sc>'\t'</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</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><<</span><span class=w> </span><span class=s>"strings:"</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_string</span><span class=p>.</span><span class=n>dump</span><span class=p>()</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_string</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=s>"strings:"</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_string</span><span class=p>.</span><span class=n>dump</span><span class=p>()</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_string</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// create JSON value with invalid UTF-8 byte sequence</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_invalid</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"ä</span><span class=se>\xA9</span><span class=s>ü"</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_invalid</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"ä</span><span class=se>\xA9</span><span class=s>ü"</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>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_invalid</span><span class=p>.</span><span class=n>dump</span><span class=p>()</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>endl</span><span class=p>;</span>
|
||||
@@ -44,30 +44,30 @@
|
||||
<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><<</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><<</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><<</span><span class=w> </span><span class=s>"string with replaced invalid characters: "</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_invalid</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>error_handler_t</span><span class=o>::</span><span class=n>replace</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n</span><span class=s>string with ignored invalid characters: "</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_invalid</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>error_handler_t</span><span class=o>::</span><span class=n>ignore</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=s>"string with replaced invalid characters: "</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_invalid</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>error_handler_t</span><span class=o>::</span><span class=n>replace</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n</span><span class=s>string with ignored invalid characters: "</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_invalid</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>error_handler_t</span><span class=o>::</span><span class=n>ignore</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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>objec</span><span class=kc>ts</span><span class=p>:</span>
|
||||
<span class=p>{</span><span class=nt>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>}</span>
|
||||
<span class=p>{</span><span class=nt>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>}</span>
|
||||
|
||||
<span class=p>{</span><span class=nt>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>}</span>
|
||||
<span class=p>{</span><span class=nt>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>}</span>
|
||||
|
||||
<span class=p>{</span>
|
||||
<span class=nt>"one"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span>
|
||||
<span class=nt>"two"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span>
|
||||
<span class=nt>"one"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span>
|
||||
<span class=nt>"two"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span>
|
||||
<span class=p>}</span>
|
||||
|
||||
<span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"one"</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>"two"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span>
|
||||
<span class=w> </span><span class=nt>"one"</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>"two"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span>
|
||||
<span class=p>}</span>
|
||||
|
||||
<span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"one"</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>"two"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span>
|
||||
<span class=w> </span><span class=nt>"one"</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>"two"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span>
|
||||
<span class=p>}</span>
|
||||
|
||||
<span class=err>arrays</span><span class=p>:</span>
|
||||
@@ -100,14 +100,14 @@
|
||||
<span class=p>]</span>
|
||||
|
||||
<span class=err>s</span><span class=kc>tr</span><span class=err>i</span><span class=kc>n</span><span class=err>gs</span><span class=p>:</span>
|
||||
<span class=s2>"Hellö 😀!"</span>
|
||||
<span class=s2>"Hell\u00f6 \ud83d\ude00!"</span>
|
||||
<span class=s2>"Hellö 😀!"</span>
|
||||
<span class=s2>"Hell\u00f6 \ud83d\ude00!"</span>
|
||||
<span class=p>[</span><span class=err>jso</span><span class=kc>n</span><span class=err>.excep</span><span class=kc>t</span><span class=err>io</span><span class=kc>n</span><span class=err>.</span><span class=kc>t</span><span class=err>ype_error.</span><span class=mi>316</span><span class=p>]</span><span class=w> </span><span class=err>i</span><span class=kc>n</span><span class=err>valid</span><span class=w> </span><span class=err>UTF</span><span class=mi>-8</span><span class=w> </span><span class=err>by</span><span class=kc>te</span><span class=w> </span><span class=err>a</span><span class=kc>t</span><span class=w> </span><span class=err>i</span><span class=kc>n</span><span class=err>dex</span><span class=w> </span><span class=mi>2</span><span class=p>:</span><span class=w> </span><span class=mi>0</span><span class=err>xA</span><span class=mi>9</span>
|
||||
<span class=err>s</span><span class=kc>tr</span><span class=err>i</span><span class=kc>n</span><span class=err>g</span><span class=w> </span><span class=err>wi</span><span class=kc>t</span><span class=err>h</span><span class=w> </span><span class=err>replaced</span><span class=w> </span><span class=err>i</span><span class=kc>n</span><span class=err>valid</span><span class=w> </span><span class=err>charac</span><span class=kc>ters</span><span class=p>:</span><span class=w> </span><span class=s2>"ä�ü"</span>
|
||||
<span class=err>s</span><span class=kc>tr</span><span class=err>i</span><span class=kc>n</span><span class=err>g</span><span class=w> </span><span class=err>wi</span><span class=kc>t</span><span class=err>h</span><span class=w> </span><span class=err>ig</span><span class=kc>n</span><span class=err>ored</span><span class=w> </span><span class=err>i</span><span class=kc>n</span><span class=err>valid</span><span class=w> </span><span class=err>charac</span><span class=kc>ters</span><span class=p>:</span><span class=w> </span><span class=s2>"äü"</span>
|
||||
</code></pre></div> </details> <p>The indentation character can be changed with the second argument (e.g., a tab <code class=highlight><span class=sc>'\t'</span></code>). An <code>indent</code> of <code>0</code> inserts newlines but no leading spaces, and the default of <code class=highlight><span class=mi>-1</span></code> selects the compact single-line form.</p> <h2 id=non-ascii-characters>Non-<abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters<a class=headerlink href=#non-ascii-characters title="Permanent link">¶</a></h2> <p>Strings are stored and serialized as <abbr title="Unicode Transformation Format">UTF</abbr>-8 (see <a href=../types/#strings>types</a>). By default, <code>dump</code> copies valid non-<abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters as-is. Setting the third argument <code>ensure_ascii</code> to <code class=highlight><span class=nb>true</span></code> escapes all non-<abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters with <code>\uXXXX</code> sequences, so that the output contains only <abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters:</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=s>"苹果"</span><span class=p>;</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=c1>// "苹果"</span>
|
||||
<span class=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</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>// "苹果"</span>
|
||||
<span class=err>s</span><span class=kc>tr</span><span class=err>i</span><span class=kc>n</span><span class=err>g</span><span class=w> </span><span class=err>wi</span><span class=kc>t</span><span class=err>h</span><span class=w> </span><span class=err>replaced</span><span class=w> </span><span class=err>i</span><span class=kc>n</span><span class=err>valid</span><span class=w> </span><span class=err>charac</span><span class=kc>ters</span><span class=p>:</span><span class=w> </span><span class=s2>"ä�ü"</span>
|
||||
<span class=err>s</span><span class=kc>tr</span><span class=err>i</span><span class=kc>n</span><span class=err>g</span><span class=w> </span><span class=err>wi</span><span class=kc>t</span><span class=err>h</span><span class=w> </span><span class=err>ig</span><span class=kc>n</span><span class=err>ored</span><span class=w> </span><span class=err>i</span><span class=kc>n</span><span class=err>valid</span><span class=w> </span><span class=err>charac</span><span class=kc>ters</span><span class=p>:</span><span class=w> </span><span class=s2>"äü"</span>
|
||||
</code></pre></div> </details> <p>The indentation character can be changed with the second argument (e.g., a tab <code class=highlight><span class=sc>'\t'</span></code>). An <code>indent</code> of <code>0</code> inserts newlines but no leading spaces, and the default of <code class=highlight><span class=mi>-1</span></code> selects the compact single-line form.</p> <h2 id=non-ascii-characters>Non-<abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters<a class=headerlink href=#non-ascii-characters title="Permanent link">¶</a></h2> <p>Strings are stored and serialized as <abbr title="Unicode Transformation Format">UTF</abbr>-8 (see <a href=../types/#strings>types</a>). By default, <code>dump</code> copies valid non-<abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters as-is. Setting the third argument <code>ensure_ascii</code> to <code class=highlight><span class=nb>true</span></code> escapes all non-<abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters with <code>\uXXXX</code> sequences, so that the output contains only <abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters:</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=s>"苹果"</span><span class=p>;</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=c1>// "苹果"</span>
|
||||
<span class=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</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>// "苹果"</span>
|
||||
</code></pre></div> <h2 id=handling-invalid-utf-8>Handling invalid <abbr title="Unicode Transformation Format">UTF</abbr>-8<a class=headerlink href=#handling-invalid-utf-8 title="Permanent link">¶</a></h2> <p>If a string contains invalid <abbr title="Unicode Transformation Format">UTF</abbr>-8 sequences (for example, because it holds data in another encoding such as Latin-1), serialization fails by default. The fourth argument of <code>dump</code> selects an <a href=../../api/basic_json/error_handler_t/ ><code>error_handler</code></a>:</p> <ul> <li><code>strict</code> (default) — throw a <a href=../../home/exceptions/#jsonexceptiontype_error316><code>type_error.316</code></a> exception.</li> <li><code>replace</code> — replace invalid bytes with the Unicode replacement character U+FFFD (<code>�</code>).</li> <li><code>ignore</code> — silently drop invalid bytes.</li> </ul> <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><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
|
||||
@@ -116,7 +116,7 @@
|
||||
<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 JSON value with invalid UTF-8 byte sequence</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_invalid</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"ä</span><span class=se>\xA9</span><span class=s>ü"</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_invalid</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"ä</span><span class=se>\xA9</span><span class=s>ü"</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>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_invalid</span><span class=p>.</span><span class=n>dump</span><span class=p>()</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>endl</span><span class=p>;</span>
|
||||
@@ -126,16 +126,16 @@
|
||||
<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><<</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><<</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><<</span><span class=w> </span><span class=s>"string with replaced invalid characters: "</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_invalid</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>error_handler_t</span><span class=o>::</span><span class=n>replace</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n</span><span class=s>string with ignored invalid characters: "</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_invalid</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>error_handler_t</span><span class=o>::</span><span class=n>ignore</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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><<</span><span class=w> </span><span class=s>"string with replaced invalid characters: "</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_invalid</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>error_handler_t</span><span class=o>::</span><span class=n>replace</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"</span><span class=se>\n</span><span class=s>string with ignored invalid characters: "</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j_invalid</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>-1</span><span class=p>,</span><span class=w> </span><span class=sc>' '</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>,</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>error_handler_t</span><span class=o>::</span><span class=n>ignore</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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=err>jso</span><span class=kc>n</span><span class=err>.excep</span><span class=kc>t</span><span class=err>io</span><span class=kc>n</span><span class=err>.</span><span class=kc>t</span><span class=err>ype_error.</span><span class=mi>316</span><span class=p>]</span><span class=w> </span><span class=err>i</span><span class=kc>n</span><span class=err>valid</span><span class=w> </span><span class=err>UTF</span><span class=mi>-8</span><span class=w> </span><span class=err>by</span><span class=kc>te</span><span class=w> </span><span class=err>a</span><span class=kc>t</span><span class=w> </span><span class=err>i</span><span class=kc>n</span><span class=err>dex</span><span class=w> </span><span class=mi>2</span><span class=p>:</span><span class=w> </span><span class=mi>0</span><span class=err>xA</span><span class=mi>9</span>
|
||||
<span class=err>s</span><span class=kc>tr</span><span class=err>i</span><span class=kc>n</span><span class=err>g</span><span class=w> </span><span class=err>wi</span><span class=kc>t</span><span class=err>h</span><span class=w> </span><span class=err>replaced</span><span class=w> </span><span class=err>i</span><span class=kc>n</span><span class=err>valid</span><span class=w> </span><span class=err>charac</span><span class=kc>ters</span><span class=p>:</span><span class=w> </span><span class=s2>"ä�ü"</span>
|
||||
<span class=err>s</span><span class=kc>tr</span><span class=err>i</span><span class=kc>n</span><span class=err>g</span><span class=w> </span><span class=err>wi</span><span class=kc>t</span><span class=err>h</span><span class=w> </span><span class=err>ig</span><span class=kc>n</span><span class=err>ored</span><span class=w> </span><span class=err>i</span><span class=kc>n</span><span class=err>valid</span><span class=w> </span><span class=err>charac</span><span class=kc>ters</span><span class=p>:</span><span class=w> </span><span class=s2>"äü"</span>
|
||||
</code></pre></div> </details> <div class="admonition tip"> <p class=admonition-title>Avoiding invalid <abbr title="Unicode Transformation Format">UTF</abbr>-8</p> <p>The best fix is to ensure that all strings are <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoded before storing them. See the <a href=../../home/faq/#parse-errors-reading-non-ascii-characters><abbr title="Frequently Asked Questions">FAQ</abbr> on non-<abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters</a> for how to convert wide or Latin-1 strings.</p> </div> <h2 id=numbers-nan-and-binary-values>Numbers, <abbr title="Not a Number">NaN</abbr>, and binary values<a class=headerlink href=#numbers-nan-and-binary-values title="Permanent link">¶</a></h2> <ul> <li><strong>Numbers</strong> are serialized with enough precision to round-trip; see <a href=../types/number_handling/#number-serialization>number serialization</a>.</li> <li><strong><abbr title="Not a Number">NaN</abbr> and infinity</strong> cannot be represented in <abbr title="JavaScript Object Notation">JSON</abbr> and are serialized as <code class=highlight><span class=kc>null</span></code>; see <a href=../types/number_handling/#nan-handling><abbr title="Not a Number">NaN</abbr> handling</a>. The <a href=../binary_formats/ >binary formats</a> can preserve them.</li> <li><strong>Binary values</strong> have no <abbr title="JavaScript Object Notation">JSON</abbr> representation and are serialized as a helper object for debugging only; see <a href=../binary_values/#serialization>binary values</a>.</li> </ul> <h2 id=using-stdformat-stdprint-and-fmt>Using <code>std::format</code>, <code>std::print</code>, and <code>fmt</code><a class=headerlink href=#using-stdformat-stdprint-and-fmt title="Permanent link">¶</a></h2> <p>Since version 3.12.0, <abbr title="JavaScript Object Notation">JSON</abbr> values can be formatted directly with C++20's <a href="https://en.cppreference.com/w/cpp/utility/format/format"><code>std::format</code></a> whenever the standard library provides the <code><format></code> header (controlled by <a href=../../api/macros/json_has_std_format/ ><code>JSON_HAS_STD_FORMAT</code></a>). This is enabled by the <a href=../../api/basic_json/std_formatter/ ><code>std::formatter<basic_json></code></a> specialization, which also makes <abbr title="JavaScript Object Notation">JSON</abbr> values work with <code>std::format_to</code> and with C++23's <code>std::print</code>/<code>std::println</code>:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>print</span><span class=p>(</span><span class=s>"{}"</span><span class=p>,</span><span class=w> </span><span class=n>j</span><span class=p>);</span><span class=w> </span><span class=c1>// compact, like j.dump()</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>print</span><span class=p>(</span><span class=s>"{:2}"</span><span class=p>,</span><span class=w> </span><span class=n>j</span><span class=p>);</span><span class=w> </span><span class=c1>// pretty-printed with indent 2 (like j.dump(2))</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>println</span><span class=p>(</span><span class=s>"{:#}"</span><span class=p>,</span><span class=w> </span><span class=n>j</span><span class=p>);</span><span class=w> </span><span class=c1>// pretty-printed with the default indent</span>
|
||||
</code></pre></div> <p>The format spec mirrors the <code>dump</code> parameters: <code class=highlight><span class=s>"{:#}"</span></code> pretty-prints, a width such as <code class=highlight><span class=s>"{:2}"</span></code> sets the indent, and a fill-and-align prefix such as <code class=highlight><span class=s>"{:.>#}"</span></code> sets the indent character.</p> <p>For the <a href="https://github.com/fmtlib/fmt">{fmt}</a> library, the library ships a <a href=../../api/basic_json/format_as/ ><code>format_as</code></a> helper. Note its behavior depends on the <code>fmt</code> version; see the <a href=../../home/faq/#using-json-values-with-stdformat-or-fmt><abbr title="Frequently Asked Questions">FAQ</abbr> entry</a> for the details and a recipe for a full <code>fmt::formatter</code> specialization.</p> <h2 id=serializing-to-other-formats>Serializing to other formats<a class=headerlink href=#serializing-to-other-formats title="Permanent link">¶</a></h2> <p>Besides <abbr title="JavaScript Object Notation">JSON</abbr> text, a value can also be serialized to the more compact <a href=../binary_formats/ >binary formats</a> (<abbr title="Binary JData">BJData</abbr>, <abbr title="Binary JSON">BSON</abbr>, <abbr title="Concise Binary Object Representation">CBOR</abbr>, MessagePack, <abbr title="Universal Binary JSON">UBJSON</abbr>).</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/dump/ ><code>dump</code></a> - serialize to a <abbr title="JavaScript Object Notation">JSON</abbr>-formatted string</li> <li><a href=../../api/operator_ltlt/ ><code>operator<<</code></a> - serialize to a stream</li> <li><a href=../../api/basic_json/to_string/ ><code>to_string</code></a> - user-defined-conversion helper</li> <li><a href=../../api/basic_json/std_formatter/ ><code>std::formatter<basic_json></code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> values with <code>std::format</code> and <code>std::print</code></li> <li><a href=../../api/basic_json/format_as/ ><code>format_as</code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> values with the {fmt} library</li> <li><a href=../parsing/ >Parsing</a> - the reverse operation</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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 20Line truncated
|
||||
<span class=err>s</span><span class=kc>tr</span><span class=err>i</span><span class=kc>n</span><span class=err>g</span><span class=w> </span><span class=err>wi</span><span class=kc>t</span><span class=err>h</span><span class=w> </span><span class=err>replaced</span><span class=w> </span><span class=err>i</span><span class=kc>n</span><span class=err>valid</span><span class=w> </span><span class=err>charac</span><span class=kc>ters</span><span class=p>:</span><span class=w> </span><span class=s2>"ä�ü"</span>
|
||||
<span class=err>s</span><span class=kc>tr</span><span class=err>i</span><span class=kc>n</span><span class=err>g</span><span class=w> </span><span class=err>wi</span><span class=kc>t</span><span class=err>h</span><span class=w> </span><span class=err>ig</span><span class=kc>n</span><span class=err>ored</span><span class=w> </span><span class=err>i</span><span class=kc>n</span><span class=err>valid</span><span class=w> </span><span class=err>charac</span><span class=kc>ters</span><span class=p>:</span><span class=w> </span><span class=s2>"äü"</span>
|
||||
</code></pre></div> </details> <div class="admonition tip"> <p class=admonition-title>Avoiding invalid <abbr title="Unicode Transformation Format">UTF</abbr>-8</p> <p>The best fix is to ensure that all strings are <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoded before storing them. See the <a href=../../home/faq/#parse-errors-reading-non-ascii-characters><abbr title="Frequently Asked Questions">FAQ</abbr> on non-<abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters</a> for how to convert wide or Latin-1 strings.</p> </div> <h2 id=numbers-nan-and-binary-values>Numbers, <abbr title="Not a Number">NaN</abbr>, and binary values<a class=headerlink href=#numbers-nan-and-binary-values title="Permanent link">¶</a></h2> <ul> <li><strong>Numbers</strong> are serialized with enough precision to round-trip; see <a href=../types/number_handling/#number-serialization>number serialization</a>.</li> <li><strong><abbr title="Not a Number">NaN</abbr> and infinity</strong> cannot be represented in <abbr title="JavaScript Object Notation">JSON</abbr> and are serialized as <code class=highlight><span class=kc>null</span></code>; see <a href=../types/number_handling/#nan-handling><abbr title="Not a Number">NaN</abbr> handling</a>. The <a href=../binary_formats/ >binary formats</a> can preserve them.</li> <li><strong>Binary values</strong> have no <abbr title="JavaScript Object Notation">JSON</abbr> representation and are serialized as a helper object for debugging only; see <a href=../binary_values/#serialization>binary values</a>.</li> </ul> <h2 id=using-stdformat-stdprint-and-fmt>Using <code>std::format</code>, <code>std::print</code>, and <code>fmt</code><a class=headerlink href=#using-stdformat-stdprint-and-fmt title="Permanent link">¶</a></h2> <p>Since version 3.12.0, <abbr title="JavaScript Object Notation">JSON</abbr> values can be formatted directly with C++20's <a href="https://en.cppreference.com/w/cpp/utility/format/format"><code>std::format</code></a> whenever the standard library provides the <code><format></code> header (controlled by <a href=../../api/macros/json_has_std_format/ ><code>JSON_HAS_STD_FORMAT</code></a>). This is enabled by the <a href=../../api/basic_json/std_formatter/ ><code>std::formatter<basic_json></code></a> specialization, which also makes <abbr title="JavaScript Object Notation">JSON</abbr> values work with <code>std::format_to</code> and with C++23's <code>std::print</code>/<code>std::println</code>:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>print</span><span class=p>(</span><span class=s>"{}"</span><span class=p>,</span><span class=w> </span><span class=n>j</span><span class=p>);</span><span class=w> </span><span class=c1>// compact, like j.dump()</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>print</span><span class=p>(</span><span class=s>"{:2}"</span><span class=p>,</span><span class=w> </span><span class=n>j</span><span class=p>);</span><span class=w> </span><span class=c1>// pretty-printed with indent 2 (like j.dump(2))</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>println</span><span class=p>(</span><span class=s>"{:#}"</span><span class=p>,</span><span class=w> </span><span class=n>j</span><span class=p>);</span><span class=w> </span><span class=c1>// pretty-printed with the default indent</span>
|
||||
</code></pre></div> <p>The format spec mirrors the <code>dump</code> parameters: <code class=highlight><span class=s>"{:#}"</span></code> pretty-prints, a width such as <code class=highlight><span class=s>"{:2}"</span></code> sets the indent, and a fill-and-align prefix such as <code class=highlight><span class=s>"{:.>#}"</span></code> sets the indent character.</p> <p>For the <a href="https://github.com/fmtlib/fmt">{fmt}</a> library, the library ships a <a href=../../api/basic_json/format_as/ ><code>format_as</code></a> helper. Note its behavior depends on the <code>fmt</code> version; see the <a href=../../home/faq/#using-json-values-with-stdformat-or-fmt><abbr title="Frequently Asked Questions">FAQ</abbr> entry</a> for the details and a recipe for a full <code>fmt::formatter</code> specialization.</p> <h2 id=serializing-to-other-formats>Serializing to other formats<a class=headerlink href=#serializing-to-other-formats title="Permanent link">¶</a></h2> <p>Besides <abbr title="JavaScript Object Notation">JSON</abbr> text, a value can also be serialized to the more compact <a href=../binary_formats/ >binary formats</a> (<abbr title="Binary JData">BJData</abbr>, <abbr title="Binary JSON">BSON</abbr>, <abbr title="Concise Binary Object Representation">CBOR</abbr>, MessagePack, <abbr title="Universal Binary JSON">UBJSON</abbr>).</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/dump/ ><code>dump</code></a> - serialize to a <abbr title="JavaScript Object Notation">JSON</abbr>-formatted string</li> <li><a href=../../api/operator_ltlt/ ><code>operator<<</code></a> - serialize to a stream</li> <li><a href=../../api/basic_json/to_string/ ><code>to_string</code></a> - user-defined-conversion helper</li> <li><a href=../../api/basic_json/std_formatter/ ><code>std::formatter<basic_json></code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> values with <code>std::format</code> and <code>std::print</code></li> <li><a href=../../api/basic_json/format_as/ ><code>format_as</code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> values with the {fmt} library</li> <li><a href=../parsing/ >Parsing</a> - the reverse operation</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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 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 16-33.7-5Line truncated
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@@ -1,12 +1,12 @@
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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=../macros/ rel=prev><link href=../types/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Trailing Commas - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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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 class=Line truncated
|
||||
<span class=w> </span><span class=nt>"planets"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=s2>"Mercury"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Venus"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Earth"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Mars"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Jupiter"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Uranus"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Neptune"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"planets"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
|
||||
<span class=w> </span><span class=s2>"Mercury"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Venus"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Earth"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Mars"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Jupiter"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Uranus"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Neptune"</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 class=highlight><span class=nb>true</span></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><iostream></span>
|
||||
@@ -16,19 +16,19 @@
|
||||
|
||||
<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>"</span><span class=dl>(</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>"</span><span class=dl>(</span>
|
||||
<span class=s> {</span>
|
||||
<span class=s> "planets": [</span>
|
||||
<span class=s> "Mercury",</span>
|
||||
<span class=s> "Venus",</span>
|
||||
<span class=s> "Earth",</span>
|
||||
<span class=s> "Mars",</span>
|
||||
<span class=s> "Jupiter",</span>
|
||||
<span class=s> "Uranus",</span>
|
||||
<span class=s> "Neptune",</span>
|
||||
<span class=s> "planets": [</span>
|
||||
<span class=s> "Mercury",</span>
|
||||
<span class=s> "Venus",</span>
|
||||
<span class=s> "Earth",</span>
|
||||
<span class=s> "Mars",</span>
|
||||
<span class=s> "Jupiter",</span>
|
||||
<span class=s> "Uranus",</span>
|
||||
<span class=s> "Neptune",</span>
|
||||
<span class=s> ]</span>
|
||||
<span class=s> }</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</span><span class=p>;</span>
|
||||
<span class=s> </span><span class=dl>)</span><span class=s>"</span><span class=p>;</span>
|
||||
|
||||
<span class=w> </span><span class=k>try</span>
|
||||
<span class=w> </span><span class=p>{</span>
|
||||
@@ -44,18 +44,18 @@
|
||||
<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><<</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><<</span><span class=w> </span><span class=sc>'\n'</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><<</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><<</span><span class=w> </span><span class=sc>'\n'</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 ']'; expected '[', '{', or a literal
|
||||
</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 ']'; expected '[', '{', or a literal
|
||||
{
|
||||
"planets": [
|
||||
"Mercury",
|
||||
"Venus",
|
||||
"Earth",
|
||||
"Mars",
|
||||
"Jupiter",
|
||||
"Uranus",
|
||||
"Neptune"
|
||||
"planets": [
|
||||
"Mercury",
|
||||
"Venus",
|
||||
"Earth",
|
||||
"Mars",
|
||||
"Jupiter",
|
||||
"Uranus",
|
||||
"Neptune"
|
||||
]
|
||||
}
|
||||
</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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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||||
</code></pre></div> </div> <!-- 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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -73,11 +73,11 @@ basic_json .. value_t</code></pre> <h2 id=template-arguments>Template arguments<
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>less</span><span class=o><></span><span class=p>,</span><span class=w> </span><span class=c1>// key_compare</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>allocator</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>pair</span><span class=o><</span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=p>,</span><span class=w> </span><span class=n>basic_json</span><span class=o>>></span><span class=w> </span><span class=c1>// allocator_type</span>
|
||||
<span class=o>></span>
|
||||
</code></pre></div> <h3 id=behavior>Behavior<a class=headerlink href=#behavior title="Permanent link">¶</a></h3> <p>The choice of <code>object_t</code> influences the behavior of the <abbr title="JavaScript Object Notation">JSON</abbr> class. With the default type, objects have the following behavior:</p> <ul> <li>When all names are unique, objects will be interoperable in the sense that all software implementations receiving that object will agree on the name-value mappings.</li> <li>When the names within an object are not unique, it is unspecified which one of the values for a given key will be chosen. For instance, <code class=highlight><span class=p>{</span><span class=nt>"key"</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>"key"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span></code> could be equal to either <code class=highlight><span class=p>{</span><span class=nt>"key"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span></code> or <code class=highlight><span class=p>{</span><span class=nt>"key"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>}</span></code>. To reject duplicate keys instead of silently resolving them one way or another, see <a href=../parsing/parser_callbacks/#recipe-rejecting-duplicate-object-keys>this parsing recipe</a>.</li> <li>Internally, name/value pairs are stored in lexicographical order of the names. Objects will also be serialized (see <code>dump</code>) in this order. For instance, both <code class=highlight><span class=p>{</span><span class=nt>"b"</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>"a"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>}</span></code> and <code class=highlight><span class=p>{</span><span class=nt>"a"</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>"b"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span></code> will be stored and serialized as <code class=highlight><span class=p>{</span><span class=nt>"a"</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>"b"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span></code>.</li> <li>When comparing objects, the order of the name/value pairs is irrelevant. This makes objects interoperable in the sense that they will not be affected by these differences. For instance, <code class=highlight><span class=p>{</span><span class=nt>"b"</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>"a"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>}</span></code> and <code class=highlight><span class=p>{</span><span class=nt>"a"</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>"b"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span></code> will be treated as equal.</li> </ul> <h3 id=key-order>Key order<a class=headerlink href=#key-order title="Permanent link">¶</a></h3> <p>The order in which name/value pairs are added to the object is <em>not</em> preserved by the library. Therefore, iterating an object may return name/value pairs in a different order than they were originally stored. In fact, keys will be traversed in alphabetical order as <code>std::map</code> with <code>std::less</code> is used by default. Please note this behavior conforms to <a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a>, because any order implements the specified "unordered" nature of <abbr title="JavaScript Object Notation">JSON</abbr> objects.</p> <h3 id=limits>Limits<a class=headerlink href=#limits title="Permanent link">¶</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 object's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the <code>max_size</code> function of a <abbr title="JavaScript Object Notation">JSON</abbr> object.</p> <h3 id=storage_1>Storage<a class=headerlink href=#storageLine truncated
|
||||
</code></pre></div> <h3 id=behavior>Behavior<a class=headerlink href=#behavior title="Permanent link">¶</a></h3> <p>The choice of <code>object_t</code> influences the behavior of the <abbr title="JavaScript Object Notation">JSON</abbr> class. With the default type, objects have the following behavior:</p> <ul> <li>When all names are unique, objects will be interoperable in the sense that all software implementations receiving that object will agree on the name-value mappings.</li> <li>When the names within an object are not unique, it is unspecified which one of the values for a given key will be chosen. For instance, <code class=highlight><span class=p>{</span><span class=nt>"key"</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>"key"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span></code> could be equal to either <code class=highlight><span class=p>{</span><span class=nt>"key"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span></code> or <code class=highlight><span class=p>{</span><span class=nt>"key"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>}</span></code>. To reject duplicate keys instead of silently resolving them one way or another, see <a href=../parsing/parser_callbacks/#recipe-rejecting-duplicate-object-keys>this parsing recipe</a>.</li> <li>Internally, name/value pairs are stored in lexicographical order of the names. Objects will also be serialized (see <code>dump</code>) in this order. For instance, both <code class=highlight><span class=p>{</span><span class=nt>"b"</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>"a"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>}</span></code> and <code class=highlight><span class=p>{</span><span class=nt>"a"</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>"b"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span></code> will be stored and serialized as <code class=highlight><span class=p>{</span><span class=nt>"a"</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>"b"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span></code>.</li> <li>When comparing objects, the order of the name/value pairs is irrelevant. This makes objects interoperable in the sense that they will not be affected by these differences. For instance, <code class=highlight><span class=p>{</span><span class=nt>"b"</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>"a"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>}</span></code> and <code class=highlight><span class=p>{</span><span class=nt>"a"</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>"b"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>}</span></code> will be treated as equal.</li> </ul> <h3 id=key-order>Key order<a class=headerlink href=#key-order title="Permanent link">¶</a></h3> <p>The order in which name/value pairs are added to the object is <em>not</em> preserved by the library. Therefore, iterating an object may return name/value pairs in a different order than they were originally stored. In fact, keys will be traversed in alphabetical order as <code>std::map</code> with <code>std::less</code> is used by default. Please note this behavior conforms to <a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a>, because any order implements the specified "unordered" nature of <abbr title="JavaScript Object Notation">JSON</abbr> objects.</p> <h3 id=limits>Limits<a class=headerlink href=#limits title="Permanent link">¶</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 object's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the <code>max_size</code> function of a <abbr title="JavaScript Object Notation">JSON</abbr> object.</p> <h3 id=storage_1>Storage<a class=headerlink href=#storage_1 title="Permanent link">¶</a></h3> <p>Objects are stored as pointers in a <code>basic_json</code> type. That is, for any access to obLine 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>m</span><span class=p>;</span>
|
||||
<span class=c1>// j is [[1,"one"],[2,"two"]], not {"1":"one","2":"two"}</span>
|
||||
<span class=c1>// j is [[1,"one"],[2,"two"]], not {"1":"one","2":"two"}</span>
|
||||
</code></pre></div> <h2 id=arrays>Arrays<a class=headerlink href=#arrays title="Permanent link">¶</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> arrays as follows:</p> <blockquote> <p>An array is an ordered sequence of zero or more values.</p> </blockquote> <h3 id=default-type_1>Default type<a class=headerlink href=#default-type_1 title="Permanent link">¶</a></h3> <p>With the default values for <em>ArrayType</em> (<code>std::vector</code>) and <em>AllocatorType</em> (<code>std::allocator</code>), the default value for <code>array_t</code> is:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span>
|
||||
<span class=w> </span><span class=n>basic_json</span><span class=p>,</span><span class=w> </span><span class=c1>// value_type</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>allocator</span><span class=o><</span><span class=n>basic_json</span><span class=o>></span><span class=w> </span><span class=c1>// allocator_type</span>
|
||||
<span class=o>></span>
|
||||
</code></pre></div> <h3 id=limits_1>Limits<a class=headerlink href=#limits_1 title="Permanent link">¶</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">¶</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">¶</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">¶</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">¶</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">¶</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">¶</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">¶</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">¶</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">¶</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">¶</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
|
||||
</code></pre></div> <h3 id=limits_1>Limits<a class=headerlink href=#limits_1 title="Permanent link">¶</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">¶</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">¶</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">¶</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">¶</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">¶</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">¶</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">¶</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">¶</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">¶</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">¶</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
|
||||
@@ -1,15 +1,15 @@
|
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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-9.7.7"><title>Number Handling - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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<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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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 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 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5Line 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><<</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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>"%.*g</span><span class=se>\n</span><span class=s>"</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><</span><span class=kt>double</span><span class=o>>::</span><span class=n>max_digits10</span><span class=p>,</span><span class=w> </span><span class=n>double_value</span><span class=p>);</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</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>"%.*g</span><span class=se>\n</span><span class=s>"</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><</span><span class=kt>double</span><span class=o>>::</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">¶</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><</span><span class=kt>double</span><span class=o>>::</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><<</span><span class=w> </span><span class=s>"val="</span><span class=w> </span><span class=o><<</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>endl</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><<</span><span class=w> </span><span class=s>"val="</span><span class=w> </span><span class=o><<</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>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><<</span><span class=w> </span><span class=s>"j="</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>dump</span><span class=p>()</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>endl</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><<</span><span class=w> </span><span class=s>"j="</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>dump</span><span class=p>()</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>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><<</span><span class=w> </span><span class=s>"val="</span><span class=w> </span><span class=o><<</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>endl</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><<</span><span class=w> </span><span class=s>"val="</span><span class=w> </span><span class=o><<</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>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
|
||||
@@ -52,6 +52,6 @@ val=nan
|
||||
<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">¶</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>"number"</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>"number"</span></code></td> <td><code class=highlight><span class=s>"number"</span></code></td> <td><code class=highlight><span class=s>"number"</span></code></td> <td><code class=highlight><span class=s>"string"</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">¶</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
|
||||
</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">¶</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>"number"</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>"number"</span></code></td> <td><code class=highlight><span class=s>"number"</span></code></td> <td><code class=highlight><span class=s>"number"</span></code></td> <td><code class=highlight><span class=s>"string"</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">¶</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=highlight><span class=n>std</span><span class=o>::</spanLine 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>></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 7, 2026 17:29:58 UTC">August 7, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</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 19, 2026 18:19:47 UTC">August 19, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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