258 lines
20 KiB
HTML
258 lines
20 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>return 语句</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_return skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading">return 语句</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-language.html">language</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>终止当前函数并返回指定值（如果存在）给调用方函数。</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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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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<tr class="t-sdsc">
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<td><span class="t-spar">属性</span><span class="t-mark">(可选)</span> <code><b>return</b></code> <span class="t-spar">表达式</span><span class="t-mark">(可选)</span> <code><b>;</b></code></td>
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<td>(1)</td>
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<td class="t-sdsc-nopad"></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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<tr class="t-sdsc">
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<td><span class="t-spar">属性</span><span class="t-mark">(可选)</span> <code><b>return</b></code> <span class="t-spar">花括号初始化器列表</span> <code><b>;</b></code></td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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<tr class="t-sdsc">
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<td><span class="t-spar">属性</span><span class="t-mark">(可选)</span> <code><b>co_return</b></code> <span class="t-spar">表达式</span><span class="t-mark">(可选)</span> <code><b>;</b></code></td>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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<tr class="t-sdsc">
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<td><span class="t-spar">属性</span><span class="t-mark">(可选)</span> <code><b>co_return</b></code> <span class="t-spar">花括号初始化器列表</span> <code><b>;</b></code></td>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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</table>
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<table class="t-par-begin">
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<tr class="t-par">
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<td><span class="t-spar">属性</span></td>
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<td>-</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span> 任意个<a href="cpp-language-attributes.html">属性</a>组成的可选序列</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">表达式</span></td>
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<td>-</td>
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<td>可以转换到函数返回类型的<a href="cpp-language-expressions.html">表达式</a></td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">花括号初始化器列表</span></td>
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<td>-</td>
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<td>初始化器的花括号环绕列表，及其他花括号初始化器列表</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<div class="t-li1"><span class="t-li">1)</span> 对 <span class="t-spar">表达式</span> 求值，终止当前函数，并将 <span class="t-spar">表达式</span> <a href="cpp-language-implicit_conversion.html">隐式转换</a>到函数返回类型后的结果返回给调用方。<span class="t-spar">表达式</span> 在返回类型是（可有 cv 限定的）<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 的函数中可以不出现，在构造函数中和析构函数中不能出现。</div>
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<div class="t-li1"><span class="t-li">2)</span> 使用<a href="cpp-language-list_initialization.html">复制列表初始化</a>对函数的返回值进行构造。</div>
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<div class="t-li1"><span class="t-li">3,4)</span> 在协程中，对于最终暂停点，必须用关键字 <code>co_return</code> 取代 <code>return</code>（细节见<a href="cpp-language-coroutines.html">协程</a>）。</div>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>如果控制流抵达返回（可有 cv 限定的） <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 的函数结尾、构造函数结尾、析构函数结尾或返回（可有 cv 限定的） <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 的函数的<a href="cpp-language-function-try-block.html">函数 try 块</a>结尾，且没有 return 语句，那么执行 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span><span class="sy4">;</span></span></span>。</p>
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<p>如果控制流抵达 <a href="cpp-language-main_function.html">main 函数</a>的结尾，那么执行 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span></span></span>。</p>
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<p>如果控制流抵达有返回值的函数（除了 <span class="t-c"><span class="mw-geshi cpp source-cpp">main</span></span>）的结尾却没有 return 语句是未定义行为。</p>
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<p>在返回（可有 cv 限定的）<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 的函数中，当表达式类型是（可有 cv 限定的）<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 时，可以使用带 <span class="t-spar">表达式</span> 的 return 语句。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx14">
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<td>
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<p>函数返回类型如果是 <a href="cpp-language-auto.html">占位类型</a>，那么会根据返回值自动<a href="cpp-language-function.html#Return_type_deduction">推导</a>它的具体类型。</p>
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<p>对函数调用结果的复制初始化<a href="cpp-language-eval_order.html">按顺序早于</a>在 <span class="t-spar">表达式</span> 结尾处对所有临时量的销毁，后者<i>按顺序早于</i>该 return 的外围块的局部变量的销毁。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td>
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</tr>
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</table>
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<p>以值返回可能会涉及临时对象的构造和复制/移动，除非使用<a href="cpp-language-copy_elision.html">复制消除</a>。具体而言，复制/移动的条件如下：</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx11">
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<td>
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<h4><span class="mw-headline" id=".E8.87.AA.E5.8A.A8.E4.BB.8E.E5.B1.80.E9.83.A8.E5.8F.98.E9.87.8F.E5.92.8C.E5.BD.A2.E5.8F.82.E7.A7.BB.E5.8A.A8">自动从局部变量和形参移动</span></h4>
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<p>如果 <span class="t-spar">表达式</span> 是（可有括号的）<a href="cpp-language-identifiers.html">标识表达式</a>并且指名类型如下的变量：</p>
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<dl>
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<dd>
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<ul>
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<li>非 volatile 对象类型</li>
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</ul>
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</dd>
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</dl>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx20">
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<td>
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<dl>
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<dd>
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<ul>
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<li>或到对象类型的非 volatile 右值引用类型</li>
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</ul>
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</dd>
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</dl>
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</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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</table>
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<p>，而该变量声明于最内层外围函数或 lambda 表达式</p>
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<dl>
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<dd>
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<ul>
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<li>的体内，或</li>
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<li>为其形参</li>
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</ul>
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</dd>
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</dl>
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<p>那么进行<i>两次</i><a href="cpp-language-overload_resolution.html">重载决议</a>，以选择用于初始化返回值的构造函数<span class="t-rev-inl t-since-cxx20"><span>，或对于 <span class="t-c"><span class="mw-geshi cpp source-cpp">co_return</span></span> 为选择 <span class="t-c"><span class="mw-geshi cpp source-cpp">promise.<span class="me1">return_value</span><span class="br0">(</span><span class="br0">)</span></span></span> 的重载</span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span>：</p>
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<ul>
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<li>首先如同假设 <span class="t-spar">表达式</span> 是右值表达式（从而它可以选择<a href="cpp-language-move_constructor.html">移动构造函数</a>），然后</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>如果第一次重载决议失败</li>
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</ul>
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</dd>
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</dl>
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<table class="t-rev-begin">
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<tr class="t-rev t-until-cxx20">
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<td>
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<dl>
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<dd>
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<ul>
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<li>或如果它成功，但不选择<a href="cpp-language-move_constructor.html">移动构造函数</a>（正式而言，选择的构造函数的首参数不是到 <span class="t-spar">表达式</span> 类型的（可有 cv 限定）的右值引用，</li>
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</ul>
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</dd>
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</dl>
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</td>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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</tr>
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</table>
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<ul>
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<li>那么照常进行重载决议，把 <span class="t-spar">表达式</span> 当做左值（从而它可以选择<a href="cpp-language-copy_constructor.html">复制构造函数</a>）。</li>
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</ul>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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</table>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx17">
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<td>
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<h4><span class="mw-headline" id=".E5.8F.97.E4.BF.9D.E8.AF.81.E7.9A.84.E5.A4.8D.E5.88.B6.E6.B6.88.E9.99.A4">受保证的复制消除</span></h4>
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<p>如果 <span class="t-spar">表达式</span> 是纯右值，那么直接以该表达式初始化结果对象。其中在类型相匹配时并不涉及复制或移动构造函数（见<a href="cpp-language-copy_elision.html">复制消除</a>）。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
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<p><a href="cpp-keyword-return.html"><tt>return</tt></a>, <a href="cpp-keyword-co_return.html"><tt>co_return</tt></a></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 class="t-example-live-link"></div>
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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="co2">#include <string></span>
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<span class="co2">#include <utility></span>
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<span class="kw4">void</span> fa<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">2</span><span class="br0">)</span>
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<span class="kw1">return</span><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> i <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span> <span class="co1">// 隐含的 return;</span>
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<span class="kw4">int</span> fb<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">4</span><span class="br0">)</span>
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<span class="kw1">return</span> <span class="nu0">4</span><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> i <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</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>
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<a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a>, <span class="kw4">int</span><span class="sy1">></span> fc<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> p, <span class="kw4">int</span> x<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> <span class="br0">{</span>p, x<span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> fd<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> fa<span class="br0">(</span><span class="nu0">10</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// fa(10) 是 void 表达式</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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fa<span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 返回，当 i==2 时无动作</span>
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fa<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印其实参，然后返回</span>
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<span class="kw4">int</span> i <span class="sy1">=</span> fb<span class="br0">(</span><span class="nu0">5</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 返回 4</span>
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i <span class="sy1">=</span> fb<span class="br0">(</span>i<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印其实参，返回 2</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> i <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> fc<span class="br0">(</span><span class="st0">"Hello"</span>, <span class="nu0">7</span><span class="br0">)</span>.<span class="me1">second</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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fd<span class="br0">(</span><span class="br0">)</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>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">1
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4
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2
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7
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10</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue1579">CWG 1579</a></td>
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<td>C++11</td>
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<td>不允许经由转换移动构造函数返回</td>
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<td>启用对转换移动构造函数的查找</td>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue1885">CWG 1885</a></td>
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<td>C++14</td>
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<td>自动变量的销毁顺序不明确</td>
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<td>添加了定序规则</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<ul>
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