87 lines
7.3 KiB
HTML
87 lines
7.3 KiB
HTML
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<title>C++ 属性： likely, unlikely (C++20 起)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_attributes_likely 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> likely, unlikely <span class="t-mark-rev t-since-cxx20">(C++20 起)</span></h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-language.html">language</a>‎ | <a href="cpp-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>允许编译器为包含该语句的执行路径，比任何其他不包含该语句的执行路径，更可能或更不可能的情况进行优化。</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>[[likely]]</b></code></td>
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<td>(1)</td>
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<td class="t-sdsc-nopad"></td>
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<td colspan="10" class="t-sdsc-sep"></td>
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<td><code><b>[[unlikely]]</b></code></td>
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<td>(2)</td>
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<td class="t-sdsc-nopad"></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>这些属性可应用于标号或语句（除了声明语句）。它们不可同时应用到同一标号或语句。</p>
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<div class="t-li1"><span class="t-li">1)</span> 应用到语句，允许编译器为包含该语句的执行路径，比任何其他不包含该语句的执行路径更可能的情况进行优化。</div>
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<div class="t-li1"><span class="t-li">2)</span> 应用到语句，允许编译器为包含该语句的执行路径，比任何其他不包含该语句的执行路径更不可能的情况进行优化。</div>
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<p>当且仅当执行路径中含有到某个标号的跳转时，才认为该执行路径包含该标号：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">switch</span><span class="br0">(</span>i<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">case</span> <span class="nu0">1</span><span class="sy4">:</span> <span class="br0">[</span><span class="br0">[</span>fallthrough<span class="br0">]</span><span class="br0">]</span><span class="sy4">;</span>
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<span class="br0">[</span><span class="br0">[</span>likely<span class="br0">]</span><span class="br0">]</span> <span class="kw1">case</span> <span class="nu0">2</span><span class="sy4">:</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> <span class="nu0">2</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp">i <span class="sy1">==</span> <span class="nu0">2</span></span></span> 被认为比 <code>i</code> 的任何其他值更可能，但 <code>[[<strong class="selflink">likely</strong>]]</code> 在 <span class="t-c"><span class="mw-geshi cpp source-cpp">i <span class="sy1">==</span> <span class="nu0">1</span></span></span> 的情况无效果，尽管它直落到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">case</span> <span class="nu0">2</span><span class="sy4">:</span></span></span> 标号。</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="t-example">
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="br0">(</span>i <span class="sy1"><</span> <span class="nu0">0</span><span class="br0">)</span> <span class="br0">[</span><span class="br0">[</span>unlikely<span class="br0">]</span><span class="br0">]</span> <span class="br0">{</span>
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<span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span>
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<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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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> f<span class="br0">(</span><span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span> <span class="sy1"><<</span> <a href="cpp-io-manip-endl.html"><span class="kw1791">std::<span class="me2">endl</span></span></a><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> f<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span> <span class="sy1"><<</span> <a href="cpp-io-manip-endl.html"><span class="kw1791">std::<span class="me2">endl</span></span></a><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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