deploy: 2e91641de2

api/macros/json_brace_init_copy_semantics/index.html

@@ -2,7 +2,10 @@
 </code></pre></div> <p>When defined to <code>1</code>, single-element brace initialization of a <code>basic_json</code> value is treated as a copy/move of the element rather than wrapping it in a single-element array.</p> <h2 id=default-definition>Default definition<a class=headerlink href=#default-definition title="Permanent link">&para;</a></h2> <p>The default value is <code>0</code> (disabled — existing behavior is preserved).</p> <div class=highlight><pre><span></span><code><span class=cp>#define JSON_BRACE_INIT_COPY_SEMANTICS 0</span>
 </code></pre></div> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <div class="admonition note"> <p class=admonition-title>Background</p> <p>C++ always prefers the <code>initializer_list</code> constructor over the copy/move constructor for brace initialization. This means that code like</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>obj</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=n>json</span><span class=w> </span><span class=n>j</span><span class=p>{</span><span class=n>obj</span><span class=p>};</span>
-</code></pre></div> <p>creates a single-element <strong>array</strong> <code>[{"key":"value"}]</code> instead of a copy of <code>obj</code>. This behavior is compiler-dependent for older compilers (<abbr title="GNU Compiler Collection">GCC</abbr> wrapped, Clang did not), but starting from Clang 20, both compilers behave the same way.</p> <p>Enabling this macro opts into copy/move semantics for this case (see <a href="https://github.com/nlohmann/json/issues/5074">#5074</a>).</p> </div> <div class="admonition warning"> <p class=admonition-title>Opt-in only</p> <p>This macro must be defined <strong>before</strong> including <code>&lt;nlohmann/json.hpp&gt;</code>. Defining it after the include has no effect.</p> </div> <div class="admonition tip"> <p class=admonition-title>Workaround without the macro</p> <p>To explicitly create a single-element array without enabling this macro, use <code>json::array()</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>array</span><span class=p>({</span><span class=n>obj</span><span class=p>});</span><span class=w>  </span><span class=c1>// always creates [obj]</span>
+</code></pre></div> <p>creates a single-element <strong>array</strong> <code>[{"key":"value"}]</code> instead of a copy of <code>obj</code>. This behavior is compiler-dependent for older compilers (<abbr title="GNU Compiler Collection">GCC</abbr> wrapped, Clang did not), but starting from Clang 20, both compilers behave the same way.</p> <p>Enabling this macro opts into copy/move semantics for this case (see <a href="https://github.com/nlohmann/json/issues/5074">#5074</a>).</p> </div> <div class="admonition warning"> <p class=admonition-title>Opt-in only</p> <p>This macro must be defined <strong>before</strong> including <code>&lt;nlohmann/json.hpp&gt;</code>. Defining it after the include has no effect.</p> </div> <div class="admonition warning"> <p class=admonition-title>Applies to every single-element list</p> <p>The macro does not only affect a single <abbr title="JavaScript Object Notation">JSON</abbr> value in braces. <strong>Any</strong> single-element braced list is treated as its element, so it no longer creates a one-element array:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j1</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=c1>// 1, not [1]</span>
+<span class=n>json</span><span class=w> </span><span class=n>j2</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=s>"text"</span><span class=p>};</span><span class=w>  </span><span class=c1>// "text", not ["text"]</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=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=c1>// [1,2], not [[1,2]]</span>
+</code></pre></div> <p>Code that relies on these producing arrays must use <code>json::array()</code> instead (see below). Lists with more than one element, and a single <code>[string, value]</code> pair such as <code>{{"key", "value"}}</code>, which still creates an object, are not affected. The library's own conversions are not affected either: for example, <code>std::tuple&lt;int&gt;{5}</code> still becomes <code>[5]</code>.</p> </div> <div class="admonition note"> <p class=admonition-title>ABI compatibility</p> <p>The value of this macro is encoded in the <a href=../../../features/namespace/ >namespace</a> (tag <code>_bics</code>), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.</p> </div> <div class="admonition tip"> <p class=admonition-title>Workaround without the macro</p> <p>To explicitly create a single-element array without enabling this macro, use <code>json::array()</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>array</span><span class=p>({</span><span class=n>obj</span><span class=p>});</span><span class=w>  </span><span class=c1>// always creates [obj]</span>
 </code></pre></div> </div> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">&para;</a></h2> <details class=example> <summary>Default behavior (macro not defined)</summary> <p>Without the macro, single-element brace initialization wraps the value in an array:</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
 
 <span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
@@ -26,4 +29,4 @@
 <span class=w>    </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>{</span><span class=n>obj</span><span class=p>};</span>
 <span class=w>    </span><span class=c1>// j is {"key":"value"}  -- copy of obj</span>
 <span class=p>}</span>
-</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../../home/faq/#brace-initialization-yields-arrays><abbr title="Frequently Asked Questions">FAQ</abbr>: Brace initialization yields arrays</a></li> <li><a href=../../basic_json/basic_json/ ><strong>basic_json(initializer_list_t)</strong></a> - the affected constructor</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Added in version 3.13.0.</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="September 24, 2026 15:01:19 UTC">September 24, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../../home/faq/#brace-initialization-yields-arrays><abbr title="Frequently Asked Questions">FAQ</abbr>: Brace initialization yields arrays</a></li> <li><a href=../../basic_json/basic_json/ ><strong>basic_json(initializer_list_t)</strong></a> - the affected constructor</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Added in version 3.13.0.</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="September 24, 2026 15:05:37 UTC">September 24, 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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api/macros/json_brace_init_copy_semantics/index.md

@@ -33,6 +33,22 @@ Opt-in only
 
 This macro must be defined **before** including `<nlohmann/json.hpp>`. Defining it after the include has no effect.
 
+Applies to every single-element list
+
+The macro does not only affect a single JSON value in braces. **Any** single-element braced list is treated as its element, so it no longer creates a one-element array:
+
+```
+json j1 = {1};       // 1, not [1]
+json j2 = {"text"};  // "text", not ["text"]
+json j3 = {{1, 2}};  // [1,2], not [[1,2]]
+```
+
+Code that relies on these producing arrays must use `json::array()` instead (see below). Lists with more than one element, and a single `[string, value]` pair such as `{{"key", "value"}}`, which still creates an object, are not affected. The library's own conversions are not affected either: for example, `std::tuple<int>{5}` still becomes `[5]`.
+
+ABI compatibility
+
+The value of this macro is encoded in the [namespace](https://json.nlohmann.me/features/namespace/index.md) (tag `_bics`), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.
+
 Workaround without the macro
 
 To explicitly create a single-element array without enabling this macro, use `json::array()`: