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authorCraig Jennings <c@cjennings.net>2025-08-14 22:58:58 -0500
committerCraig Jennings <c@cjennings.net>2025-08-14 22:58:58 -0500
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- <h1 id="firstHeading" class="firstHeading">C attribute: nodiscard <span class="t-mark-rev t-since-c23">(since C23)</span>
-</h1> <p>If a function declared <code>nodiscard</code> or a function returning a struct/union/enum declared <code>nodiscard</code> by value is called from a <a href="../expressions" title="c/language/expressions">discarded-value expression</a> other than a cast to <code>void</code>, the compiler is encouraged to issue a warning.</p>
-<h3 id="Syntax"> Syntax</h3> <table class="t-sdsc-begin"> <tr class="t-sdsc"> <td> <code>[[</code> <code>nodiscard</code> <code>]]</code><br><code>[[</code> <code>__nodiscard__</code> <code>]]</code> </td> <td> (1) </td> <td class="t-sdsc-nopad"> </td>
-</tr> <tr class="t-sdsc"> <td> <code>[[</code> <code>nodiscard</code> <code>(</code> <span class="t-spar">string-literal</span> <code>)</code> <code>]]</code><br><code>[[</code> <code>__nodiscard__</code> <code>(</code> <span class="t-spar">string-literal</span> <code>)</code> <code>]]</code> </td> <td> (2) </td> <td class="t-sdsc-nopad"> </td>
-</tr>
-</table> <table class="t-par-begin"> <tr class="t-par"> <td> <span class="t-spar">string-literal</span> </td> <td> - </td> <td> text that could be used to explain the rationale for why the result should not be discarded </td>
-</tr>
-</table> <h3 id="Explanation"> Explanation</h3> <p>Appears in a function declaration, enumeration declaration, or struct/union declaration.</p>
-<p>If, from a <a href="../expressions" title="c/language/expressions">discarded-value expression</a> other than a cast to <code>void</code>,</p>
-<ul>
-<li> a function declared <code>nodiscard</code> is called, or </li>
-<li> a function returning a struct/union/enum declared <code>nodiscard</code> is called, </li>
-</ul> <p>the compiler is encouraged to issue a warning.</p>
-<p>The <span class="t-spar">string-literal</span>, if specified, is usually included in the warnings.</p>
-<h3 id="Example"> Example</h3> <div class="t-example"> <div class="c source-c"><pre data-language="c">struct [[nodiscard]] error_info { int status; /*...*/ };
-struct error_info enable_missile_safety_mode() { /*...*/ return (struct error_info){0}; }
-void launch_missiles() { /*...*/ }
-void test_missiles() {
- enable_missile_safety_mode(); // compiler may warn on discarding a nodiscard value
- launch_missiles();
-}
-struct error_info* foo() { static struct error_info e; /*...*/ return &amp;e; }
-void f1() {
- foo(); // nodiscard type itself is not returned, no warning
-}
-// nodiscard( string-literal ):
-[[nodiscard("PURE FUN")]] int strategic_value(int x, int y) { return x ^ y; }
-
-int main()
-{
- strategic_value(4,2); // compiler may warn on discarding a nodiscard value
- int z = strategic_value(0,0); // OK: return value is not discarded
- return z;
-}</pre></div> <p>Possible output:</p>
-<div class="text source-text"><pre data-language="c">game.cpp:5:4: warning: ignoring return value of function declared with 'nodiscard' attribute
-game.cpp:17:5: warning: ignoring return value of function declared with 'nodiscard' attribute: PURE FUN</pre></div> </div> <h3 id="See_also"> See also</h3> <table class="t-dsc-begin"> <tr class="t-dsc"> <td colspan="2"> <span><a href="https://en.cppreference.com/w/cpp/language/attributes/nodiscard" title="cpp/language/attributes/nodiscard">C++ documentation</a></span> for <code>nodiscard</code> </td>
-</tr> </table> <div class="_attribution">
- <p class="_attribution-p">
- &copy; cppreference.com<br>Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.<br>
- <a href="https://en.cppreference.com/w/c/language/attributes/nodiscard" class="_attribution-link">https://en.cppreference.com/w/c/language/attributes/nodiscard</a>
- </p>
-</div>