215 lines
28 KiB
HTML
215 lines
28 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>占位类型说明符 (C++11 起)</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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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_auto skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading">占位类型说明符 <span class="t-mark-rev t-since-cxx11">(C++11 起)</span></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>对于变量，指定要从其初始化器自动推导出其类型。</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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<p>对于函数，指定要从其 return 语句推导出其返回类型。</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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<table class="t-rev-begin">
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<p>对于非类型模板形参，指定要从实参推导出其类型。</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=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<table class="t-sdsc-begin">
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<td colspan="10" class="t-sdsc-sep"></td>
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<td><span class="t-spar">类型约束</span><span class="t-mark">(可选)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span></td>
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<td>(1)</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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<td colspan="10" class="t-sdsc-sep"></td>
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<td><span class="t-spar">类型约束</span><span class="t-mark">(可选)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span></span></span></td>
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<td>(2)</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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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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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">类型约束</span></td>
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<td>-</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span> <a href="cpp-language-constraints.html#.E6.A6.82.E5.BF.B5">概念</a>名，可以有限定或后随被 <> 包围的模板实参列表</td>
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<div class="t-li1"><span class="t-li">1)</span> 用<a href="cpp-language-template_argument_deduction.html#.E5.85.B6.E4.BB.96.E8.AF.AD.E5.A2.83">模板实参推导</a>的规则推导类型。</div>
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<div class="t-li1"><span class="t-li">2)</span> 类型为 <a href="cpp-language-decltype.html"><code>decltype(expr)</code></a>，其中 <code>expr</code> 是初始化器。</div>
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<p>占位符 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> 可伴随如 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&</span></span></span> 这样的修饰符，它们参与类型推导。<span class="t-rev-inl t-since-cxx14"><span>占位符 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span></span></span> 必须是被声明类型的唯一组分。</span> <span><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></span></span></p>
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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>占位类型说明符可出现于下列语境：</p>
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<ul>
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<li>变量的类型说明符中：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span> x <span class="sy1">=</span> expr<span class="sy4">;</span></span></span>。从初始化器推导类型。<br />
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如果占位类型说明符是 <code>auto</code> <span class="t-rev-inl t-since-cxx20"><span>或 <span class="t-spar">类型约束</span> <code>auto</code></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span>，那么采用从函数调用进行<a href="cpp-language-template_argument_deduction.html">模板实参推导</a>的规则，从初始化器推导变量的类型（细节见<a href="cpp-language-template_argument_deduction.html#.E5.85.B6.E4.BB.96.E8.AF.AD.E5.A2.83">其他语境</a>）。<br />
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例如，给定 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&</span> i <span class="sy1">=</span> expr<span class="sy4">;</span></span></span>，则 <code>i</code> 的类型恰是某个虚构模板 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">const</span> U<span class="sy3">&</span> u<span class="br0">)</span></span></span> 中参数 <code>u</code> 的类型（假如函数调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">f<span class="br0">(</span>expr<span class="br0">)</span></span></span> 通过编译）。因此，取决于初始化器，<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span><span class="sy3">&&</span></span></span> 可被推导成左值引用或右值引用类型，这被用于基于范围的 for 循环。<br />
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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>如果占位类型说明符是 <code>decltype(auto)</code> <span class="t-rev-inl t-since-cxx20"><span>或 <span class="t-spar">类型约束</span> <code>decltype(auto)</code></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span>，那么推导出的类型是 <code>decltype(expr)</code>，其中 <code>expr</code> 是初始化器。</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>如果用占位类型说明符声明多个变量，那么推导出的类型必须互相匹配。例如，声明 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span> i <span class="sy1">=</span> <span class="nu0">0</span>, d <span class="sy1">=</span> <span class="nu16">0.0</span><span class="sy4">;</span></span></span> 非良构，而声明 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span> i <span class="sy1">=</span> <span class="nu0">0</span>, <span class="sy2">*</span>p <span class="sy1">=</span> <span class="sy3">&</span>i<span class="sy4">;</span></span></span> 良构并将 <code>auto</code> 推导为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span></span></span>。</p>
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</li>
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<li><a href="cpp-language-new.html">new 表达式</a>中的类型标识。从初始化器推导类型。对于 <code>new <i>T</i> <i>init</i></code>（其中 <i>T</i> 含占位符类型，而 <i>init</i> 是带括号的初始化器或带花括号的初始化器列表），如同在虚设的声明 <span class="t-c"><span class="mw-geshi cpp source-cpp">T x init<span class="sy4">;</span></span></span> 中对变量 <i>x</i> 一般推导 <i>T</i> 的类型。</li>
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<li><span class="t-mark-rev t-since-cxx14">(C++14 起)</span> <a href="cpp-language-function.html">函数</a>或 lambda 表达式的返回类型中：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span><span class="sy3">&</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span>。从其<span class="t-rev-inl t-since-cxx17"><span>未<a href="cpp-language-if.html#constexpr_if">舍弃</a>的</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span> <a href="cpp-language-return.html">return 语句</a>的操作数推导返回类型。<br />
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见<a href="cpp-language-function.html#.E8.BF.94.E5.9B.9E.E7.B1.BB.E5.9E.8B.E6.8E.A8.E5.AF.BC">返回类型推导</a>。</li>
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<li><span class="t-mark-rev t-since-cxx17">(C++17 起)</span> <a href="cpp-language-template_parameters.html#.E9.9D.9E.E7.B1.BB.E5.9E.8B.E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82">非类型模板形参</a>的形参声明中：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span><span class="kw4">auto</span> I<span class="sy1">></span> <span class="kw1">struct</span> A<span class="sy4">;</span></span></span>。从对应的实参推导其类型。</li>
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</ul>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx23">
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<td>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> 说明符亦可出现在<a href="cpp-language-explicit_cast.html">显式类型转换</a>的简单类型说明符中： <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span><span class="br0">(</span>expr<span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span><span class="br0">{</span>expr<span class="br0">}</span></span></span> 。从表达式推导其类型。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 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-cxx14">
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<td>
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<p>此外，<code>auto</code> <span class="t-rev-inl t-since-cxx20"><span>与 <span class="t-spar">类型约束</span> <code>auto</code></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span> 还可出现于：</p>
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<ul>
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<li><a href="cpp-language-lambda.html">lambda 表达式</a>的形参声明：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&&</span><span class="br0">)</span><span class="br0">{</span><span class="br0">}</span></span></span>。这种 lambda 表达式是<i>泛型 lambda</i>。</li>
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<li><span class="t-mark-rev t-since-cxx20">(C++20 起)</span> <a href="cpp-language-function.html#.E5.BD.A2.E5.8F.82.E5.88.97.E8.A1.A8">函数形参声明</a>：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span><span class="sy4">;</span></span></span>。这种函数声明引入<a href="cpp-language-function_template.html#.E7.AE.80.E5.86.99.E5.87.BD.E6.95.B0.E6.A8.A1.E6.9D.BF">简写函数模板</a>。</li>
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</ul>
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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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<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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<h4><span class="mw-headline" id=".E7.B1.BB.E5.9E.8B.E7.BA.A6.E6.9D.9F">类型约束</span></h4>
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<p>如果 <span class="t-spar">类型约束</span> 存在，令 <code>T</code> 为该占位符的被推导类型，那么 <code>T</code> 必须满足 <span class="t-spar">类型约束</span> 的立即声明的约束。即，</p>
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<ul>
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<li>如果 <span class="t-spar">类型约束</span> 是 <code>Concept<A<sub>1</sub>, ..., A<sub>n</sub>></code>，那么约束表达式 <code>Concept<T, A<sub>1</sub>, ..., A<sub>n</sub>></code> 必须合法并返回 <code>true</code>；</li>
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<li>否则（<span class="t-spar">类型约束</span> 是无实参列表的 <code>Concept</code>），约束表达式 <code>Concept<T></code> 必须合法并返回 <code>true</code>。</li>
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</ul>
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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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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>C++11 之前，<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> 具有<a href="cpp-language-storage_duration.html">存储期说明符</a>的语义。</p>
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<p>不允许在一个声明中混合 <code>auto</code> 的变量和函数，如 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span> f<span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> <span class="kw4">int</span>, i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span></span></span>。</p>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> 说明符也可以用于后随尾随返回类型的函数声明符，此时返回类型为其尾随返回类型（它也可以是占位符类型）。</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">auto</span> <span class="br0">(</span><span class="sy2">*</span>p<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> <span class="kw4">int</span><span class="sy4">;</span> <span class="co1">// 声明指向【返回 int 的函数】的指针</span>
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<span class="kw4">auto</span> <span class="br0">(</span><span class="sy2">*</span>q<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> <span class="kw4">auto</span> <span class="sy1">=</span> p<span class="sy4">;</span> <span class="co1">// 声明 q 为指向【返回 T 的函数】的指针</span>
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<span class="co1">// 其中 T 从 p 的类型推导</span></pre></div>
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</div>
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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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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> 说明符也可以用于<a href="cpp-language-structured_binding.html">结构化绑定</a>声明。</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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<table class="t-rev-begin">
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<tr class="t-rev t-since-concepts_ts">
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<td>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> 关键词也可以用于嵌套名说明符。形如 <span class="t-c"><span class="mw-geshi cpp source-cpp">auto<span class="sy4">::</span></span></span> 的嵌套名说明符是一个占位符，它会遵循<a href="cpp-experimental-constraints.html">受约束类型</a>占位符的推导规则被替换为某个类或枚举类型。</p>
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</td>
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<td><span class="t-mark-rev t-since-concepts-ts t-mark-ts">(概念 TS)</span></td>
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</tr>
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</table>
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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 <utility></span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">auto</span> add<span class="br0">(</span>T t, U u<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> t <span class="sy2">+</span> u<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// 返回类型是 operator+(T, U) 的类型</span>
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<span class="co1">// 在其所调用的函数返回引用的情况下</span>
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<span class="co1">// 函数调用的完美转发必须用 decltype(auto)</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> F, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span>
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decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> PerfectForward<span class="br0">(</span>F fun, Args<span class="sy3">&&</span>... <span class="me1">args</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> fun<span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span><span class="br0">(</span>args<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">template</span><span class="sy1"><</span><span class="kw4">auto</span> n<span class="sy1">></span> <span class="co1">// C++17 auto 形参声明</span>
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<span class="kw4">auto</span> f<span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>n<span class="br0">)</span>, decltype<span class="br0">(</span>n<span class="br0">)</span><span class="sy1">></span> <span class="co1">// auto 不能从花括号初始化器列表推导</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> <span class="br0">{</span>n, n<span class="br0">}</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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<span class="kw4">auto</span> a <span class="sy1">=</span> <span class="nu0">1</span> <span class="sy2">+</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="co1">// a 的类型是 int</span>
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<span class="kw4">auto</span> b <span class="sy1">=</span> add<span class="br0">(</span><span class="nu0">1</span>, <span class="nu16">1.2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// b 的类型是 double</span>
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static_assert<span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>a<span class="br0">)</span>, <span class="kw4">int</span><span class="sy1">></span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>b<span class="br0">)</span>, <span class="kw4">double</span><span class="sy1">></span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> c0 <span class="sy1">=</span> a<span class="sy4">;</span> <span class="co1">// c0 的类型是 int，保有 a 的副本</span>
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decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> c1 <span class="sy1">=</span> a<span class="sy4">;</span> <span class="co1">// c1 的类型是 int，保有 a 的副本</span>
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decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> c2 <span class="sy1">=</span> <span class="br0">(</span>a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// c2 的类型是 int&，它是 a 的别名</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> <span class="st0">"通过 c2 修改前，a = "</span> <span class="sy1"><<</span> a <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="sy2">++</span>c2<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> <span class="st0">"通过 c2 修改后，a = "</span> <span class="sy1"><<</span> a <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw4">auto</span> <span class="br0">[</span>v, w<span class="br0">]</span> <span class="sy1">=</span> f<span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 结构化绑定声明</span>
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<span class="kw4">auto</span> d <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK：d 的类型是 std::initializer_list<int></span>
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<span class="kw4">auto</span> n <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK：n 的类型是 std::initializer_list<int></span>
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<span class="co1">// auto e{1, 2}; // C++17 起错误，之前是 std::initializer_list<int></span>
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<span class="kw4">auto</span> m<span class="br0">{</span><span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK：DR N3922 起 m 的类型是 int，之前是 initializer_list<int></span>
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<span class="co1">// decltype(auto) z = { 1, 2 } // 错误：{1, 2} 不是表达式</span>
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<span class="co1">// auto 常用于无名类型，例如 lambda 表达式的类型</span>
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<span class="kw4">auto</span> lambda <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> x <span class="sy2">+</span> <span class="nu0">3</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// auto int x; // 于 C++98 合法，C++11 起错误</span>
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<span class="co1">// auto x; // 于 C 合法，于 C++ 错误</span>
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<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span>...<span class="br0">)</span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>c0, c1, v, w, d, n, m, lambda<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 阻止“变量未使用”警告</span>
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<span class="br0">}</span></pre></div>
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</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">通过 c2 修改前，a = 3
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通过 c2 修改后，a = 4</pre></div>
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