deploy: 78ddd95794

2026-10-29 12:40:52 +00:00 · 2026-10-04 09:47:40 +00:00
1 parent ba443db2af
commit 19f538472d
@@ -79,6 +79,7 @@ Some important things:
 * When using `get<your_type>()`, `your_type` **MUST** be [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). (There is a way to bypass this requirement described later.)
 * In function `from_json`, use function [`at()`](../api/basic_json/at.md) to access the object values rather than `operator[]`. In case a key does not exist, `at` throws an exception that you can handle, whereas `operator[]` exhibits undefined behavior.
 * You do not need to add serializers or deserializers for STL types like `std::vector`: the library already implements these.
+* If you control the type, consider defining `to_json`/`from_json` as `friend` functions inside the class ("hidden friends"). Argument-dependent lookup then only finds them for your type, which also avoids a [GCC < 11 compilation error](../home/faq.md#incomplete-detector-type-with-gcc-11).

 ??? example "Example: serialize a `person` to JSON with `to_json`"

@@ -50,7 +50,7 @@
 <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>
 <span class=w>    </span><span class=p>}</span>
 <span class=p>}</span><span class=w> </span><span class=c1>// namespace ns</span>
-</code></pre></div> <p>That's all! When calling the <code>json</code> constructor with your type, your custom <code>to_json</code> method will be automatically called. Likewise, when calling <code>get&lt;your_type&gt;()</code> or <code>get_to(your_type&amp;)</code>, the <code>from_json</code> method will be called.</p> <p>Some important things:</p> <ul> <li>Those methods <strong>MUST</strong> be in your type's namespace (which can be the global namespace), or the library will not be able to locate them (in this example, they are in namespace <code>ns</code>, where <code>person</code> is defined).</li> <li>Those methods <strong>MUST</strong> be available (e.g., proper headers must be included) everywhere you use these conversions. Look at <a href="https://github.com/nlohmann/json/issues/1108">#1108</a> for errors that may occur otherwise.</li> <li>When using <code>get&lt;your_type&gt;()</code>, <code>your_type</code> <strong>MUST</strong> be <a href="https://en.cppreference.com/w/cpp/named_req/DefaultConstructible">DefaultConstructible</a>. (There is a way to bypass this requirement described later.)</li> <li>In function <code>from_json</code>, use function <a href=../../api/basic_json/at/ ><code>at()</code></a> to access the object values rather than <code>operator[]</code>. In case a key does not exist, <code>at</code> throws an exception that you can handle, whereas <code>operator[]</code> exhibits undefined behavior.</li> <li>You do not need to add serializers or deserializers for <abbr title="Standard Template Library">STL</abbr> types like <code>std::vector</code>: the library already implements these.</li> </ul> <details class=example> <summary>Example: serialize a <code>person</code> to <abbr title="JavaScript Object Notation">JSON</abbr> with <code>to_json</code></summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
+</code></pre></div> <p>That's all! When calling the <code>json</code> constructor with your type, your custom <code>to_json</code> method will be automatically called. Likewise, when calling <code>get&lt;your_type&gt;()</code> or <code>get_to(your_type&amp;)</code>, the <code>from_json</code> method will be called.</p> <p>Some important things:</p> <ul> <li>Those methods <strong>MUST</strong> be in your type's namespace (which can be the global namespace), or the library will not be able to locate them (in this example, they are in namespace <code>ns</code>, where <code>person</code> is defined).</li> <li>Those methods <strong>MUST</strong> be available (e.g., proper headers must be included) everywhere you use these conversions. Look at <a href="https://github.com/nlohmann/json/issues/1108">#1108</a> for errors that may occur otherwise.</li> <li>When using <code>get&lt;your_type&gt;()</code>, <code>your_type</code> <strong>MUST</strong> be <a href="https://en.cppreference.com/w/cpp/named_req/DefaultConstructible">DefaultConstructible</a>. (There is a way to bypass this requirement described later.)</li> <li>In function <code>from_json</code>, use function <a href=../../api/basic_json/at/ ><code>at()</code></a> to access the object values rather than <code>operator[]</code>. In case a key does not exist, <code>at</code> throws an exception that you can handle, whereas <code>operator[]</code> exhibits undefined behavior.</li> <li>You do not need to add serializers or deserializers for <abbr title="Standard Template Library">STL</abbr> types like <code>std::vector</code>: the library already implements these.</li> <li>If you control the type, consider defining <code>to_json</code>/<code>from_json</code> as <code>friend</code> functions inside the class ("hidden friends"). Argument-dependent lookup then only finds them for your type, which also avoids a <a href=../../home/faq/#incomplete-detector-type-with-gcc-11><abbr title="GNU Compiler Collection">GCC</abbr> &lt; 11 compilation error</a>.</li> </ul> <details class=example> <summary>Example: serialize a <code>person</code> to <abbr title="JavaScript Object Notation">JSON</abbr> with <code>to_json</code></summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
 <span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>

 <span class=k>using</span><span class=w> </span><span class=n>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>
@@ -357,4 +357,4 @@
 <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>&lt;</span><span class=n>T</span><span class=o>&gt;</span><span class=p>();</span><span class=w> </span><span class=c1>// oops!</span>
 <span class=w>    </span><span class=p>}</span>
 <span class=p>};</span>
-</code></pre></div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../conversions/ >Converting values</a> - the general overview of <code>get</code>/<code>get_to</code> and implicit conversions</li> <li><a href=../enum_conversion/ >Specializing enum conversion</a> - map enums to <abbr title="JavaScript Object Notation">JSON</abbr> strings instead of integers</li> <li><a href=../macros/ >Supported macros</a> - reference for <code>NLOHMANN_DEFINE_TYPE_*</code> and related macros</li> <li><a href=../../api/adl_serializer/ ><code>adl_serializer</code></a> - the default <code>JSONSerializer</code> used in the conversion dispatch</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 2, 2026 09:32:15 UTC">October 2, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+</code></pre></div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../conversions/ >Converting values</a> - the general overview of <code>get</code>/<code>get_to</code> and implicit conversions</li> <li><a href=../enum_conversion/ >Specializing enum conversion</a> - map enums to <abbr title="JavaScript Object Notation">JSON</abbr> strings instead of integers</li> <li><a href=../macros/ >Supported macros</a> - reference for <code>NLOHMANN_DEFINE_TYPE_*</code> and related macros</li> <li><a href=../../api/adl_serializer/ ><code>adl_serializer</code></a> - the default <code>JSONSerializer</code> used in the conversion dispatch</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 4, 2026 09:46:13 UTC">October 4, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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@@ -78,6 +78,7 @@ Some important things:
 - When using `get<your_type>()`, `your_type` **MUST** be [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). (There is a way to bypass this requirement described later.)
 - In function `from_json`, use function [`at()`](https://json.nlohmann.me/api/basic_json/at/index.md) to access the object values rather than `operator[]`. In case a key does not exist, `at` throws an exception that you can handle, whereas `operator[]` exhibits undefined behavior.
 - You do not need to add serializers or deserializers for STL types like `std::vector`: the library already implements these.
+- If you control the type, consider defining `to_json`/`from_json` as `friend` functions inside the class ("hidden friends"). Argument-dependent lookup then only finds them for your type, which also avoids a [GCC < 11 compilation error](https://json.nlohmann.me/home/faq/#incomplete-detector-type-with-gcc-11).

 Example: serialize a `person` to JSON with `to_json`