99 lines
8.4 KiB
HTML
99 lines
8.4 KiB
HTML
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<title>C 属性： nodiscard (C23 起)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_language_attributes_nodiscard skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">C 属性：</span> nodiscard <span class="t-mark-rev t-since-c23">(C23 起)</span></h1>
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<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-language.html">language</a>‎ | <a href="c-language-attributes.html">attributes</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<p>若从转型到 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">void</span></span></span> 以外的<a href="c-language-expressions.html">弃值表达式</a>，调用声明为 <code>nodiscard</code> 的函数，或调用返回声明 <code>nodiscard</code> 的结构体/联合体/枚举的函数，则鼓励编译器发出警告。</p>
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<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<table class="t-sdsc-begin">
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<td><code><b>[[</b></code> <code><b>nodiscard</b></code> <code><b>]]</b></code><br />
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<code><b>[[</b></code> <code><b>__nodiscard__</b></code> <code><b>]]</b></code></td>
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<td>(1)</td>
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<td><code><b>[[</b></code> <code><b>nodiscard</b></code> <code><b>(</b></code> <span class="t-spar">字符串字面量</span> <code><b>)</b></code> <code><b>]]</b></code><br />
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<code><b>[[</b></code> <code><b>__nodiscard__</b></code> <code><b>(</b></code> <span class="t-spar">字符串字面量</span> <code><b>)</b></code> <code><b>]]</b></code></td>
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<td>(2)</td>
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<td class="t-sdsc-nopad"></td>
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<td><span class="t-spar">字符串字面量</span></td>
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<td>-</td>
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<td>能用于解释为何不应舍弃结果的原理的文本</td>
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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<p>出现于函数声明、枚举声明或结构体/联合体声明。</p>
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<p>若从转型到 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">void</span></span></span> 以外的<a href="c-language-expressions.html">弃值表达式</a>，</p>
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<ul>
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<li>调用声明为 <code>nodiscard</code> 的函数，或</li>
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<li>调用返回声明 <code>nodiscard</code> 的结构体/联合体/枚举的函数，</li>
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</ul>
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<p>则鼓励编译器发出警告。</p>
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<p>若指定 <span class="t-spar">字符串字面量</span> 则通常包含它于警告中。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="c source-c">
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<pre class="de1"><span class="kw1">struct</span> <span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span> error_info <span class="br0">{</span> <span class="kw4">int</span> status<span class="sy4">;</span> <span class="coMULTI">/*...*/</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> error_info enable_missile_safety_mode<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/*...*/</span> <span class="kw1">return</span> <span class="br0">(</span><span class="kw1">struct</span> error_info<span class="br0">)</span><span class="br0">{</span><span class="nu0">0</span><span class="br0">}</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">void</span> launch_missiles<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/*...*/</span> <span class="br0">}</span>
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<span class="kw4">void</span> test_missiles<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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enable_missile_safety_mode<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 编译器可能在舍弃 nodiscard 值时警告</span>
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launch_missiles<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">struct</span> error_info<span class="sy2">*</span> foo<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw4">static</span> <span class="kw1">struct</span> error_info e<span class="sy4">;</span> <span class="coMULTI">/*...*/</span> <span class="kw1">return</span> <span class="sy3">&</span>e<span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">void</span> f1<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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foo<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 不以值返回 nodiscard 类型，无警告</span>
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<span class="br0">}</span>
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<span class="co1">// nodiscard( string-literal ):</span>
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<span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">(</span><span class="st0">"PURE FUN"</span><span class="br0">)</span><span class="br0">]</span><span class="br0">]</span> <span class="kw4">int</span> strategic_value<span class="br0">(</span><span class="kw4">int</span> x, <span class="kw4">int</span> y<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> x <span class="sy3">^</span> y<span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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strategic_value<span class="br0">(</span><span class="nu0">4</span>,<span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// compiler may warn on discarding a nodiscard value</span>
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<span class="kw4">int</span> z <span class="sy1">=</span> strategic_value<span class="br0">(</span><span class="nu0">0</span>,<span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: return value is not discarded</span>
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<span class="kw1">return</span> z<span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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<p>可能的输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">game.cpp:5:4: warning: ignoring return value of function declared with 'nodiscard' attribute
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game.cpp:17:5: warning: ignoring return value of function declared with 'nodiscard' attribute: PURE FUN</pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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