340 lines
33 KiB
HTML
340 lines
33 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>类声明</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_class 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">类声明</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>类（class）与结构体（struct）是用户定义类型，以 <span class="t-spar">类说明符</span> 定义，它在<a href="cpp-language-declarations.html">声明</a>语法的 <span class="t-spar">声明说明符序列</span> 中出现。类说明符拥有下列语法：</p>
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<tr class="t-sdsc">
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<td class="t-sdsc-nopad"><span class="t-spar">类关键词</span> <span class="t-spar">属性</span> <span class="t-spar">类头名</span> <span class="t-spar">基类子句</span> <code><b>{</b></code> <span class="t-spar">成员说明</span> <code><b>}</b></code></td>
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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">类关键词</span></td>
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<td>-</td>
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<td><a href="cpp-keyword-class.html"><tt>class</tt></a> 或 <a href="cpp-keyword-struct.html"><tt>struct</tt></a> 之一。除了默认<a href="cpp-language-access.html">成员访问</a>和默认<a href="cpp-language-derived_class.html">基类访问</a>之外，两个关键词是等同的。</td>
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</tr>
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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> 可选的任任意数量<a href="cpp-language-attributes.html">属性</a>序列，可以包含 <a href="cpp-language-alignas.html">alignas 指定符</a></td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">类头名</span></td>
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<td>-</td>
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<td>所定义的类的名字。可以<a href="cpp-language-identifiers.html#.E6.9C.89.E9.99.90.E5.AE.9A.E7.9A.84.E6.A0.87.E8.AF.86.E7.AC.A6">有限定</a>，也可以后随关键词 <a href="cpp-keyword-final.html">final</a>。名字可以省略，此时类是无名的（注意无名类不能是 <a href="cpp-language-final.html">final</a> 的）</td>
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<tr class="t-par">
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<td><span class="t-spar">基类子句</span></td>
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<td>-</td>
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<td>一或多个基类以及各自所用的继承模型的可选列表（见<a href="cpp-language-derived_class.html">派生类</a>）</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">成员说明</span></td>
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<td>-</td>
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<td>访问说明符、成员对象及成员函数的声明和定义的列表（见下文）</td>
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<p>语法综述见<a href="cpp-language-classes.html">类</a>。如果 <span class="t-spar">类关键词</span> 是 <a href="cpp-keyword-union.html"><tt>union</tt></a>，那么声明引入一个<a href="cpp-language-union.html">联合体类型</a>。</p>
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<h3><span class="mw-headline" id=".E5.89.8D.E7.BD.AE.E5.A3.B0.E6.98.8E">前置声明</span></h3>
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<p>下列形式的声明</p>
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<td class="t-sdsc-nopad"><span class="t-spar">类关键词</span> <span class="t-spar">属性</span> <span class="t-spar">标识符</span> <code><b>;</b></code></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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<p>声明一个将稍后在此作用域定义的类类型。直到定义出现前，此类名具有<a href="cpp-language-type.html#.E4.B8.8D.E5.AE.8C.E6.95.B4.E7.B1.BB.E5.9E.8B">不完整类型</a>。这允许类之间彼此引用：</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="kw1">class</span> Vector<span class="sy4">;</span> <span class="co1">// 前置声明</span>
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<span class="kw1">class</span> Matrix <span class="br0">{</span>
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<span class="co1">// ...</span>
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<span class="kw1">friend</span> Vector operator<span class="sy2">*</span><span class="br0">(</span><span class="kw4">const</span> Matrix<span class="sy3">&</span>, <span class="kw4">const</span> Vector<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">class</span> Vector <span class="br0">{</span>
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<span class="co1">// ...</span>
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<span class="kw1">friend</span> Vector operator<span class="sy2">*</span><span class="br0">(</span><span class="kw4">const</span> Matrix<span class="sy3">&</span>, <span class="kw4">const</span> Vector<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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</div>
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<p>而且如果特定的源文件只用到该类的指针和引用，使用前置声明也可以减少 #include 依赖：</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="co1">// 在 MyStruct.h 中</span>
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<span class="co2">#include <iosfwd> // 含有 std::ostream 的前置声明</span>
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<span class="kw1">struct</span> MyStruct <span class="br0">{</span>
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<span class="kw4">int</span> value<span class="sy4">;</span>
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<span class="kw1">friend</span> <a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> os, <span class="kw4">const</span> S<span class="sy3">&</span> s<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 其定义在 MyStruct.cpp 文件中提供，该文件使用 #include <ostream></span>
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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</div>
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<p>如果前置声明在局部作用域出现，那么它会<i>隐藏</i>它的外围作用域中可能出现的先前声明的相同名字的类、变量、函数，以及所有其他声明：</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="kw1">struct</span> s <span class="br0">{</span> <span class="kw4">int</span> a<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> s<span class="sy4">;</span> <span class="co1">// 不做任何事（s 已经在此作用域定义）</span>
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<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">struct</span> s<span class="sy4">;</span> <span class="co1">// 新的局部结构体“s”的前置声明</span>
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<span class="co1">// 它隐藏全局的结构体 s 直至此块结尾</span>
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s<span class="sy2">*</span> p<span class="sy4">;</span> <span class="co1">// 指向局部结构体 s 的指针</span>
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<span class="kw1">struct</span> s <span class="br0">{</span> <span class="kw4">char</span><span class="sy2">*</span> p<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 局部结构体 s 的定义</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>注意，通过作为其他声明一部分的<a href="cpp-language-elaborated_type_specifier.html">详述类型说明符</a>也可以引入新的类名，但只有在<a href="cpp-language-lookup.html">名字查找</a>无法找到先前声明的有此名的类时才可以。</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="kw1">class</span> U<span class="sy4">;</span>
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<span class="kw1">namespace</span> ns<span class="br0">{</span>
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<span class="kw1">class</span> Y f<span class="br0">(</span><span class="kw1">class</span> T p<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 声明函数 ns::f 并声明 ns::T 与 ns::Y</span>
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<span class="kw1">class</span> U f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// U 指代 ::U</span>
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Y<span class="sy2">*</span> p<span class="sy4">;</span> T<span class="sy2">*</span> q<span class="sy4">;</span> <span class="co1">// 可以使用到 T 和 Y 的指针及引用</span>
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<span class="br0">}</span></pre></div>
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</div>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E8.AF.B4.E6.98.8E">成员说明</span></h3>
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<p>成员说明，或类定义的<i>体</i>，是花括号环绕的任何数量下列各项的序列：</p>
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<div class="t-li1"><span class="t-li">1)</span> 具有下列形式的成员声明
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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 class="t-sdsc-nopad"><span class="t-spar">属性</span><span class="t-mark">(可选)</span> <span class="t-spar">声明说明符序列</span><span class="t-mark">(可选)</span> <span class="t-spar">成员声明符列表</span><span class="t-mark">(可选)</span> <code><b>;</b></code></td>
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<td class="t-sdsc-nopad"></td>
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<td class="t-sdsc-nopad"></td>
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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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<td><span class="t-spar">attr</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> 可选的任意数量的<a href="cpp-language-attributes.html">属性</a>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">声明说明符序列</span></td>
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<td>-</td>
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<td><a href="cpp-language-declarations.html#.E8.AF.B4.E6.98.8E.E7.AC.A6">说明符</a>的序列。它只在构造函数，析构函数，以及用户定义<a href="cpp-language-cast_operator.html">转换函数</a>中可以省略。</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">成员声明符列表</span></td>
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<td>-</td>
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<td>与 <a href="cpp-language-declarations.html">初始化声明符列表</a>相同，但额外允许<a href="cpp-language-bit_field.html">位域定义</a>、<a href="cpp-language-abstract_class.html">纯说明符</a><span class="t-rev-inl t-since-cxx11"><span>和虚说明符（ <code><a href="cpp-language-override.html">override</a></code> 或 <code><a href="cpp-language-final.html">final</a></code>）</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>，并且不允许<a href="cpp-language-direct_initialization.html">直接非列表初始化语法</a>。</td>
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</tr>
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</table>
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<p>这种声明可以声明<a href="cpp-language-static.html">静态</a>及非静态的<a href="cpp-language-data_members.html">数据成员</a>与<a href="cpp-language-member_functions.html">成员函数</a>、成员 <a href="cpp-language-typedef.html">typedef</a>、成员<a href="cpp-language-enum.html">枚举</a>以及<a href="cpp-language-nested_types.html">嵌套类</a>。它也可以是<a href="cpp-language-friend.html">友元声明</a>。</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="kw1">class</span> S <span class="br0">{</span>
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<span class="kw4">int</span> d1<span class="sy4">;</span> <span class="co1">// 非静态数据成员</span>
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<span class="kw4">int</span> a<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span> <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">// 带初始化器的非静态数据成员 (C++11)</span>
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<span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> d2 <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// 带初始化器的静态数据成员</span>
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<span class="kw1">virtual</span> <span class="kw4">void</span> f1<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// 纯虚成员函数</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> d3, <span class="sy2">*</span>d4, f2<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 两个数据成员和一个成员函数</span>
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<span class="kw2">enum</span> <span class="br0">{</span>NORTH, SOUTH, EAST, WEST<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> NestedS <span class="br0">{</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s<span class="sy4">;</span>
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<span class="br0">}</span> d5, <span class="sy2">*</span>d6<span class="sy4">;</span>
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<span class="kw1">typedef</span> NestedS value_type, <span class="sy2">*</span>pointer_type<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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</div>
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</div>
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<div class="t-li1"><span class="t-li">2)</span> 函数定义，同时声明并定义<a href="cpp-language-member_functions.html">成员函数</a>或者<a href="cpp-language-friend.html">友元函数</a>。成员函数定义之后的分号是可选的。所有定义于类体之内的函数自动为<a href="cpp-language-inline.html">内联</a>的<span class="t-rev-inl t-since-cxx20"><span>，除非它们附着到<a href="cpp-language-modules.html">具名模块</a></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span>。
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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="kw1">class</span> M <span class="br0">{</span>
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<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> C<span class="sy4">;</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> data<span class="sy4">;</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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M<span class="br0">(</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> R, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> C<span class="br0">)</span> <span class="sy4">:</span> C<span class="br0">(</span>C<span class="br0">)</span>, data<span class="br0">(</span>R<span class="sy2">*</span>C<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// 构造函数定义</span>
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<span class="kw4">int</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>size_t r, size_t c<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="co1">// 成员函数定义</span>
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<span class="kw1">return</span> data<span class="br0">[</span>r<span class="sy2">*</span>C<span class="sy2">+</span>c<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><span class="sy3">&</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>size_t r, size_t c<span class="br0">)</span> <span class="br0">{</span> <span class="co1">// 另一个成员函数定义</span>
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<span class="kw1">return</span> data<span class="br0">[</span>r<span class="sy2">*</span>C<span class="sy2">+</span>c<span class="br0">]</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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</div>
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</div>
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<div class="t-li1"><span class="t-li">3)</span> <a href="cpp-language-access.html">访问说明符</a> <code>public:</code>、<code>protected:</code> 和 <code>private:</code>
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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="kw1">class</span> S <span class="br0">{</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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S<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 公开的构造函数</span>
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S<span class="br0">(</span><span class="kw4">const</span> S<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 公开的复制构造函数</span>
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<span class="kw1">virtual</span> ~S<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 公开的虚析构函数</span>
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<span class="kw1">private</span><span class="sy4">:</span>
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<span class="kw4">int</span><span class="sy2">*</span> ptr<span class="sy4">;</span> <span class="co1">// 私有的数据成员</span>
|
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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</div>
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</div>
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<div class="t-li1"><span class="t-li">4)</span> <a href="cpp-language-using_declaration.html">using 声明</a>
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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="kw1">class</span> Base <span class="br0">{</span>
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<span class="kw1">protected</span><span class="sy4">:</span>
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<span class="kw4">int</span> d<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">class</span> Derived <span class="sy4">:</span> <span class="kw1">public</span> Base <span class="br0">{</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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<span class="kw1">using</span> Base<span class="sy4">::</span><span class="me2">d</span><span class="sy4">;</span> <span class="co1">// 令 Base 的受保护成员 d 为 Derived 的公开成员</span>
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<span class="kw1">using</span> Base<span class="sy4">::</span><span class="me2">Base</span><span class="sy4">;</span> <span class="co1">// 继承基类的所有构造函数 (C++11)</span>
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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</div>
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</div>
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<div class="t-li1"><span class="t-li">5)</span> <a href="cpp-language-static_assert.html">static_assert</a> 声明
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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="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
|
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<span class="kw1">struct</span> Foo <span class="br0">{</span>
|
|
static_assert<span class="br0">(</span><a href="cpp-types-is_floating_point.html"><span class="kw458">std::<span class="me2">is_floating_point</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span>, <span class="st0">"Foo<T>: T 必须是浮点数"</span><span class="br0">)</span><span class="sy4">;</span>
|
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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</div>
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|
</div>
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|
<div class="t-li1"><span class="t-li">6)</span> <a href="cpp-language-member_template.html">成员模板声明</a>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
|
|
<span class="kw4">void</span> f<span class="br0">(</span>T<span class="sy3">&&</span> n<span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> CharT<span class="sy1">></span>
|
|
<span class="kw1">struct</span> NestedS <span class="br0">{</span>
|
|
<a href="cpp-string-basic_string.html"><span class="kw1228">std::<span class="me2">basic_string</span></span></a><span class="sy1"><</span>CharT<span class="sy1">></span> s<span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
</div>
|
|
<div class="t-li1"><span class="t-li">7)</span> <a href="cpp-language-type_alias.html">别名声明</a>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
|
|
<span class="kw1">struct</span> identity
|
|
<span class="br0">{</span>
|
|
<span class="kw1">using</span> type <span class="sy1">=</span> T<span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
</div>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx17">
|
|
<td>
|
|
<div class="t-li1"><span class="t-li">8)</span> 成员类模板的<a href="cpp-language-class_template_argument_deduction.html">推导指引</a>：
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> CharT<span class="sy1">></span>
|
|
<span class="kw1">struct</span> NestedS <span class="br0">{</span>
|
|
<a href="cpp-string-basic_string.html"><span class="kw1228">std::<span class="me2">basic_string</span></span></a><span class="sy1"><</span>CharT<span class="sy1">></span> s<span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> CharT<span class="sy1">></span>
|
|
NestedS<span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1228">std::<span class="me2">basic_string</span></span></a><span class="sy1"><</span>CharT<span class="sy1">></span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> NestedS<span class="sy1"><</span>CharT<span class="sy1">></span><span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx20">
|
|
<td>
|
|
<div class="t-li1"><span class="t-li">9)</span> <a href="cpp-language-enum.html#using_enum_.E5.A3.B0.E6.98.8E">using enum 声明</a>：
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw2">enum</span> <span class="kw1">class</span> color <span class="br0">{</span> red, orange, yellow <span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">struct</span> highlight <span class="br0">{</span>
|
|
<span class="kw1">using</span> <span class="kw2">enum</span> color<span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E5.B1.80.E9.83.A8.E7.B1.BB">局部类</span></h3>
|
|
<p>类声明可以在函数体内出现，此时它定义<i>局部类</i>。这种类的名字只存在于函数作用域中，且无法在函数外访问。</p>
|
|
<ul>
|
|
<li>局部类不能拥有静态数据成员</li>
|
|
<li>局部类的成员函数无链接</li>
|
|
<li>局部类的成员函数必须完全在类体内定义</li>
|
|
<li><span class="t-rev-inl t-since-cxx14"><span>除<a href="cpp-language-lambda.html">闭包类型</a>以外的</span> <span><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></span></span>局部类不能拥有成员模板</li>
|
|
<li>局部类不能拥有<a href="cpp-language-friend.html#.E6.A8.A1.E6.9D.BF.E5.8F.8B.E5.85.83">友元模板</a></li>
|
|
<li>局部类不能在类定义内定义<a href="cpp-language-friend.html">友元函数</a></li>
|
|
<li>函数（包括成员函数）内的局部类的外围函数能访问的名字也可以被该局部类访问。</li>
|
|
</ul>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-until-cxx11">
|
|
<td>
|
|
<ul>
|
|
<li>局部类不能用作模板实参</li>
|
|
</ul>
|
|
</td>
|
|
<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td>
|
|
</tr>
|
|
</table>
|
|
<div class="t-example">
|
|
<div class="t-example-live-link"></div>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="co2">#include <vector></span>
|
|
<span class="co2">#include <algorithm></span>
|
|
<span class="co2">#include <iostream></span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="br0">{</span><span class="nu0">1</span>,<span class="nu0">2</span>,<span class="nu0">3</span><span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="kw1">struct</span> Local <span class="br0">{</span>
|
|
<span class="kw4">bool</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span> n, <span class="kw4">int</span> m<span class="br0">)</span> <span class="br0">{</span>
|
|
<span class="kw1">return</span> n <span class="sy1">></span> m<span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
<a href="cpp-algorithm-sort.html"><span class="kw1618">std::<span class="me2">sort</span></span></a><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, Local<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// C++11 起</span>
|
|
<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n<span class="sy4">:</span> v<span class="br0">)</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> n <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">3 2 1</pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<table class="t-dsc-begin"></table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
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</html>
|