278 lines
36 KiB
HTML
278 lines
36 KiB
HTML
<!DOCTYPE html>
|
|
<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
|
|
<head><meta http-equiv="x-ua-compatible" content="ie=edge">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
|
<title>复制赋值运算符</title>
|
|
<link rel="stylesheet" href="ext.css" />
|
|
<link rel="stylesheet" href="site_modules.css" />
|
|
</head>
|
|
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_copy_assignment skin-cppreference2 action-view cpp-navbar">
|
|
<div id="cpp-content-base">
|
|
<div id="content"><a id="top"></a>
|
|
<h1 id="firstHeading" class="firstHeading">复制赋值运算符</h1>
|
|
<div id="bodyContent">
|
|
<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-language.html">language</a></span></div>
|
|
<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
|
|
<p>类 <code>T</code> 的复制赋值运算符是名为 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1">=</span></span></span> 的非模板<a href="cpp-language-member_functions.html">非静态成员函数</a>，它接受恰好一个 <span class="t-c"><span class="mw-geshi cpp source-cpp">T</span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> T<span class="sy3">&</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">volatile</span> T<span class="sy3">&</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">volatile</span> T<span class="sy3">&</span></span></span> 类型的形参。<a href="cpp-named_req-CopyAssignable.html"><span style="font-style:italic">可复制赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span></a> 类型必须有公开的复制赋值运算符。</p>
|
|
<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
|
|
<table class="t-sdsc-begin">
|
|
<tr>
|
|
<td colspan="10" class="t-sdsc-sep"></td>
|
|
</tr>
|
|
<tr class="t-sdsc">
|
|
<td><span class="t-spar">类名</span> <code><b>&</b></code> <span class="t-spar">类名</span> <code><b>::</b></code> <code><b>operator=</b></code> ( <span class="t-spar">类名</span> )</td>
|
|
<td>(1)</td>
|
|
<td class="t-sdsc-nopad"></td>
|
|
</tr>
|
|
<tr>
|
|
<td colspan="10" class="t-sdsc-sep"></td>
|
|
</tr>
|
|
<tr class="t-sdsc">
|
|
<td><span class="t-spar">类名</span> <code><b>&</b></code> <span class="t-spar">类名</span> <code><b>::</b></code> <code><b>operator=</b></code> ( <code><b>const</b></code> <span class="t-spar">类名</span> <code>&</code> )</td>
|
|
<td>(2)</td>
|
|
<td class="t-sdsc-nopad"></td>
|
|
</tr>
|
|
<tr>
|
|
<td colspan="10" class="t-sdsc-sep"></td>
|
|
</tr>
|
|
<tr class="t-sdsc">
|
|
<td><span class="t-spar">类名</span> <code><b>&</b></code> <span class="t-spar">类名</span> <code><b>::</b></code> <code><b>operator=</b></code> ( <code><b>const</b></code> <span class="t-spar">类名</span> <code>&</code> ) = <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">default</span><span class="sy4">;</span></span></span></td>
|
|
<td>(3)</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
<tr>
|
|
<td colspan="10" class="t-sdsc-sep"></td>
|
|
</tr>
|
|
<tr class="t-sdsc">
|
|
<td><span class="t-spar">类名</span> <code><b>&</b></code> <span class="t-spar">类名</span> <code><b>::</b></code> <code><b>operator=</b></code> ( <code><b>const</b></code> <span class="t-spar">类名</span> <code>&</code> ) = <span class="t-c"><span class="mw-geshi cpp source-cpp">delete<span class="sy4">;</span></span></span></td>
|
|
<td>(4)</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
<tr>
|
|
<td colspan="10" class="t-sdsc-sep"></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
|
|
<div class="t-li1"><span class="t-li">1)</span> 复制赋值运算符在采用<a href="cpp-language-operators.html#.E8.B5.8B.E5.80.BC.E8.BF.90.E7.AE.97.E7.AC.A6">复制交换法</a>时的典型声明。</div>
|
|
<div class="t-li1"><span class="t-li">2)</span> 复制赋值运算符在不采用复制交换法时的典型声明。</div>
|
|
<div class="t-li1"><span class="t-li">3)</span> 强制编译器生成复制赋值运算符。</div>
|
|
<div class="t-li1"><span class="t-li">4)</span> 避免隐式复制赋值。</div>
|
|
<p>每当<a href="cpp-language-overload_resolution.html">重载决议</a>选择复制赋值运算符时，它都会被调用，例如对象出现在赋值表达式左侧时。</p>
|
|
<h3><span class="mw-headline" id=".E9.9A.90.E5.BC.8F.E5.A3.B0.E6.98.8E.E7.9A.84.E5.A4.8D.E5.88.B6.E8.B5.8B.E5.80.BC.E8.BF.90.E7.AE.97.E7.AC.A6">隐式声明的复制赋值运算符</span></h3>
|
|
<p>如果没有对类类型（<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">class</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">union</span></span></span>）提供任何用户定义的复制赋值运算符，那么编译器将始终声明一个，作为类的 inline public 成员。如果满足下列所有条件，那么这个隐式声明的复制赋值运算符拥有形式 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span> T<span class="sy4">::</span><span class="me2">operator</span><span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span><span class="br0">)</span></span></span>：</p>
|
|
<ul>
|
|
<li><code>T</code> 的每个直接基类 <code>B</code> 均拥有形参为 <span class="t-c"><span class="mw-geshi cpp source-cpp">B</span></span>，<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> B<span class="sy3">&</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">volatile</span> B<span class="sy3">&</span></span></span> 的复制赋值运算符；</li>
|
|
<li><code>T</code> 的每个类类型或类数组类型的非静态数据成员 <code>M</code> 均拥有形参为 <span class="t-c"><span class="mw-geshi cpp source-cpp">M</span></span>，<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> M<span class="sy3">&</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">volatile</span> M<span class="sy3">&</span></span></span> 的复制赋值运算符。</li>
|
|
</ul>
|
|
<p>否则，隐式声明的复制赋值运算符会被声明为 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span> T<span class="sy4">::</span><span class="me2">operator</span><span class="sy1">=</span><span class="br0">(</span>T<span class="sy3">&</span><span class="br0">)</span></span></span>。（注意因为这些规则，隐式声明的复制赋值运算符不能绑定到 volatile 左值实参。）</p>
|
|
<p>类可以拥有多个复制赋值运算符，如 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span> T<span class="sy4">::</span><span class="me2">operator</span><span class="sy1">=</span><span class="br0">(</span>T<span class="sy3">&</span><span class="br0">)</span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span> T<span class="sy4">::</span><span class="me2">operator</span><span class="sy1">=</span><span class="br0">(</span>T<span class="br0">)</span></span></span>。<span class="t-rev-inl t-since-cxx11"><span>当存在用户定义的复制赋值运算符时，用户仍然可以通过关键词 <code>default</code> 强迫编译器生成隐式声明的复制赋值运算符。</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span></p>
|
|
<p>隐式声明（或在它的首个声明被预置）的复制赋值运算符具有<span class="t-rev-inl t-until-cxx17"><span><a href="cpp-language-except_spec.html">动态异常说明</a></span> <span><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span><span class="t-rev-inl t-since-cxx17"><span><a href="cpp-language-noexcept_spec.html">noexcept 说明</a></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>中所描述的异常说明。</p>
|
|
<p>因为每个类总是会声明复制赋值运算符，所以基类的赋值运算符始终会被隐藏。当使用 <a href="cpp-language-using_declaration.html#.E5.9C.A8.E7.B1.BB.E5.AE.9A.E4.B9.89.E4.B8.AD">using 声明</a>从基类带入复制赋值运算符，且它的实参类型与派生类的隐式复制赋值运算符的实参类型相同时，该 using 声明也会被隐式声明隐藏。</p>
|
|
<h3><span class="mw-headline" id=".E5.BC.83.E7.BD.AE.E7.9A.84.E9.9A.90.E5.BC.8F.E5.A3.B0.E6.98.8E.E7.9A.84.E5.A4.8D.E5.88.B6.E8.B5.8B.E5.80.BC.E8.BF.90.E7.AE.97.E7.AC.A6">弃置的隐式声明的复制赋值运算符</span></h3>
|
|
<p>如果满足下列任一条件，那么类 <code>T</code> 的隐式声明的复制赋值运算符被定义为<i>弃置的</i>：</p>
|
|
<ul>
|
|
<li><code>T</code> 拥有用户声明的移动构造函数；</li>
|
|
<li><code>T</code> 拥有用户声明的移动赋值运算符。</li>
|
|
</ul>
|
|
<p>否则，它被定义为预置的。</p>
|
|
<p>如果满足下列任一条件，那么类 <code>T</code> 的预置的复制赋值运算符被定义为<i>弃置的</i>：</p>
|
|
<ul>
|
|
<li><code>T</code> 拥有具有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> 限定的非类类型（或其数组）的非静态数据成员；</li>
|
|
<li><code>T</code> 拥有引用类型的非静态数据成员；</li>
|
|
<li><code>T</code> 拥有无法复制赋值的非静态数据成员，直接基类或虚基类（对复制赋值的重载决议失败，或选择弃置或不可访问的函数）；</li>
|
|
<li><code>T</code> 是<a href="cpp-language-union.html#.E8.81.94.E5.90.88.E4.BD.93.E5.BC.8F.E7.9A.84.E7.B1.BB">联合体式的类</a>，且拥有的某个变体成员对应的复制赋值运算符是非平凡的。</li>
|
|
</ul>
|
|
<h3><span class="mw-headline" id=".E5.B9.B3.E5.87.A1.E7.9A.84.E5.A4.8D.E5.88.B6.E8.B5.8B.E5.80.BC.E8.BF.90.E7.AE.97.E7.AC.A6">平凡的复制赋值运算符</span></h3>
|
|
<p>如果满足下列所有条件，那么类 <code>T</code> 的复制赋值运算符是平凡的：</p>
|
|
<ul>
|
|
<li>它不是用户提供的（即它是隐式定义或预置的）；</li>
|
|
<li><code>T</code> 没有虚成员函数；</li>
|
|
<li><code>T</code> 没有虚基类；</li>
|
|
<li>为 <code>T</code> 的每个直接基类选择的复制赋值运算符都是平凡的；</li>
|
|
<li>为 <code>T</code> 的每个类类型（或类类型的数组）的非静态数据成员选择的复制赋值运算符都是平凡的。</li>
|
|
</ul>
|
|
<p>平凡复制赋值运算符如同用 <span class="t-lc"><a href="cpp-string-byte-memmove.html">std::memmove</a></span> 进行对象表示的复制。所有与 C 语言兼容的数据类型（POD 类型）都可以平凡复制。</p>
|
|
<h3><span class="mw-headline" id=".E5.90.88.E6.A0.BC.E7.9A.84.E5.A4.8D.E5.88.B6.E8.B5.8B.E5.80.BC.E8.BF.90.E7.AE.97.E7.AC.A6">合格的复制赋值运算符</span></h3>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-until-cxx11">
|
|
<td>
|
|
<p>被用户声明或者同时被隐式声明且可定义的复制赋值运算符是合格的。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td>
|
|
</tr>
|
|
<tr class="t-rev t-since-cxx11 t-until-cxx20">
|
|
<td>
|
|
<p>没有被弃置的复制赋值运算符是合格的。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br />
|
|
<span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
|
|
</tr>
|
|
<tr class="t-rev t-since-cxx20">
|
|
<td>
|
|
<p>满足下列所有条件的复制赋值运算符是合格的：</p>
|
|
<ul>
|
|
<li>它没有被弃置，且</li>
|
|
<li>满足它的所有<a href="cpp-language-constraints.html">关联约束</a>（如果存在），且</li>
|
|
<li>没有比它<a href="cpp-language-constraints.html#.E7.BA.A6.E6.9D.9F.E7.9A.84.E9.83.A8.E5.88.86.E6.8E.92.E5.BA.8F">更受约束</a>且拥有相同的第一形参类型和相同的 cv 或引用限定符（如果存在）的复制赋值运算符。</li>
|
|
</ul>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<p>合格复制赋值运算符的平凡性确定该类是否为<a href="cpp-named_req-TriviallyCopyable.html">可平凡复制类型</a>。</p>
|
|
<h3><span class="mw-headline" id=".E9.9A.90.E5.BC.8F.E5.AE.9A.E4.B9.89.E7.9A.84.E5.A4.8D.E5.88.B6.E8.B5.8B.E5.80.BC.E8.BF.90.E7.AE.97.E7.AC.A6">隐式定义的复制赋值运算符</span></h3>
|
|
<p>如果隐式声明的复制赋值运算符既没有被弃置也不平凡，那么当它被 <a href="cpp-language-definition.html#ODR_.E5.BC.8F.E4.BD.BF.E7.94.A8">ODR 式使用</a><span class="t-rev-inl t-since-cxx14"><span>或用于<a href="cpp-language-constant_expression.html#.E5.B8.B8.E9.87.8F.E6.B1.82.E5.80.BC.E6.89.80.E9.9C.80.E8.A6.81.E7.9A.84.E5.87.BD.E6.95.B0.E4.B8.8E.E5.8F.98.E9.87.8F">常量求值</a></span> <span><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></span></span>时，它会被编译器定义（即生成并编译函数体）。对于联合体类型，隐式定义的复制赋值运算符（如同以 <span class="t-lc"><a href="cpp-string-byte-memmove.html">std::memmove</a></span>）复制其对象表示。对于非联合类类型（<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">class</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span></span></span>），编译器按照声明顺序对对象的各基类和非静态成员进行逐成员复制赋值，其中对标量进行内建赋值，而对类类型使用复制赋值运算符，</p>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx14">
|
|
<td>
|
|
<p>如果满足下列所有条件，那么类 <code>T</code> 的隐式定义的复制赋值运算符是 <a href="cpp-language-constexpr.html"><code>constexpr</code></a> 的：</p>
|
|
<ul>
|
|
<li><code>T</code> 是<a href="cpp-named_req-LiteralType.html">字面类型</a>，且</li>
|
|
<li>复制每个直接基类子对象时选中的赋值运算符都是 constexpr 函数，且</li>
|
|
<li>复制 <code>T</code> 的每个类（或其数组）类型的数据成员时选中的赋值运算符都是 constexpr 函数。</li>
|
|
</ul>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx11">
|
|
<td>
|
|
<p>当 <code>T</code> 拥有用户定义的析构函数或用户定义的复制赋值运算符时，隐式定义的复制赋值运算符的生成被弃用。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
|
<p>如果复制和移动赋值运算符都有提供，那么重载决议会在实参是<a href="cpp-language-value_category.html"><i>右值</i></a>（例如无名临时量的<a href="cpp-language-value_category.html"><i>纯右值</i></a> 或 <span class="t-lc">std::move</span> 的结果的<a href="cpp-language-value_category.html"><i>亡值</i></a>）时选择移动赋值，而在实参是<a href="cpp-language-value_category.html"><i>左值</i></a>（具名对象或返回左值引用的函数或运算符）时选择复制赋值。如果只提供了复制赋值，那么重载决议对于所有值类别都会选择它（只要它按值或按到 const 的引用接收其实参），从而当移动赋值不可用时，复制赋值将会成为它的后备。</p>
|
|
<p>隐式定义的复制赋值运算符是否会多次对在继承网格中可通过多于一条路径访问的虚基类子对象赋值是未指明的（同样适用于<a href="cpp-language-move_assignment.html">移动赋值</a>）。</p>
|
|
<p>有关用户定义的复制赋值运算符应当有哪些行为，见<a href="cpp-language-operators.html#.E8.B5.8B.E5.80.BC.E8.BF.90.E7.AE.97.E7.AC.A6">赋值运算符重载</a>。</p>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<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 <iostream></span>
|
|
<span class="co2">#include <memory></span>
|
|
<span class="co2">#include <string></span>
|
|
<span class="co2">#include <algorithm></span>
|
|
|
|
<span class="kw1">struct</span> A
|
|
<span class="br0">{</span>
|
|
<span class="kw4">int</span> n<span class="sy4">;</span>
|
|
<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s1<span class="sy4">;</span>
|
|
<span class="co1">// 用户定义的复制赋值（复制交换法）</span>
|
|
A<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span>A other<span class="br0">)</span>
|
|
<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> <span class="st0">"A 的复制赋值<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<a href="cpp-algorithm-swap.html"><span class="kw1599">std::<span class="me2">swap</span></span></a><span class="br0">(</span>n, other.<span class="me1">n</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-algorithm-swap.html"><span class="kw1599">std::<span class="me2">swap</span></span></a><span class="br0">(</span>s1, other.<span class="me1">s1</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">struct</span> B <span class="sy4">:</span> A
|
|
<span class="br0">{</span>
|
|
<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s2<span class="sy4">;</span>
|
|
<span class="co1">// 隐式定义的复制赋值</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">struct</span> C
|
|
<span class="br0">{</span>
|
|
<a href="cpp-memory-unique_ptr.html"><span class="kw733">std::<span class="me2">unique_ptr</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="br0">[</span><span class="br0">]</span><span class="sy1">></span> data<span class="sy4">;</span>
|
|
<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> size<span class="sy4">;</span>
|
|
<span class="co1">// 用户定义的复制赋值（非复制交换法）</span>
|
|
<span class="co1">// 注意：复制交换法总是会重新分配资源</span>
|
|
C<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> C<span class="sy3">&</span> other<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">if</span> <span class="br0">(</span>this <span class="sy3">!</span><span class="sy1">=</span> <span class="sy3">&</span>other<span class="br0">)</span> <span class="co1">// 非自赋值</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">if</span> <span class="br0">(</span>size <span class="sy3">!</span><span class="sy1">=</span> other.<span class="me1">size</span><span class="br0">)</span> <span class="co1">// 资源无法复用</span>
|
|
<span class="br0">{</span>
|
|
data.<span class="me1">reset</span><span class="br0">(</span>new <span class="kw4">int</span><span class="br0">[</span>other.<span class="me1">size</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
|
|
size <span class="sy1">=</span> other.<span class="me1">size</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<a href="cpp-algorithm-copy.html"><span class="kw1582">std::<span class="me2">copy</span></span></a><span class="br0">(</span><span class="sy3">&</span>other.<span class="me1">data</span><span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span>, <span class="sy3">&</span>other.<span class="me1">data</span><span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span> <span class="sy2">+</span> size, <span class="sy3">&</span>data<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
A a1, a2<span class="sy4">;</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"a1 = a2 调用 "</span><span class="sy4">;</span>
|
|
a1 <span class="sy1">=</span> a2<span class="sy4">;</span> <span class="co1">// 用户定义的复制赋值</span>
|
|
|
|
B b1, b2<span class="sy4">;</span>
|
|
b2.<span class="me1">s1</span> <span class="sy1">=</span> <span class="st0">"foo"</span><span class="sy4">;</span>
|
|
b2.<span class="me1">s2</span> <span class="sy1">=</span> <span class="st0">"bar"</span><span class="sy4">;</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"b1 = b2 调用 "</span><span class="sy4">;</span>
|
|
b1 <span class="sy1">=</span> b2<span class="sy4">;</span> <span class="co1">// 隐式定义的复制赋值</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"b1.s1 = "</span> <span class="sy1"><<</span> b1.<span class="me1">s1</span> <span class="sy1"><<</span> <span class="st0">" b1.s2 = "</span> <span class="sy1"><<</span> b1.<span class="me1">s2</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</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">a1 = a2 调用 A 的复制赋值
|
|
b1 = b2 调用 A 的复制赋值
|
|
b1.s1 = foo b1.s2 = bar</pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
|
|
<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>
|
|
<table class="dsctable" style="font-size:0.8em">
|
|
<tr>
|
|
<th>DR</th>
|
|
<th>应用于</th>
|
|
<th>出版时的行为</th>
|
|
<th>正确行为</th>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue2094">CWG 2094</a></td>
|
|
<td>C++11</td>
|
|
<td>volatile 子对象使得预置的默认复制赋值运算符非平凡（CWG 496）</td>
|
|
<td>平凡性不受影响</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue2171">CWG 2171</a></td>
|
|
<td>C++11</td>
|
|
<td><span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1">=</span><span class="br0">(</span>X<span class="sy3">&</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span></span></span> 非平凡</td>
|
|
<td>令它平凡</td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<ul>
|
|
<li><a href="cpp-language-converting_constructor.html">转换构造函数</a></li>
|
|
<li><a href="cpp-language-copy_constructor.html">复制构造函数</a></li>
|
|
<li><a href="cpp-language-copy_elision.html">复制消除</a></li>
|
|
<li><a href="cpp-language-default_constructor.html">默认构造函数</a></li>
|
|
<li><a href="cpp-language-destructor.html">析构函数</a></li>
|
|
<li><a href="cpp-language-explicit.html">explicit</a></li>
|
|
<li><a href="cpp-language-initialization.html">初始化</a>
|
|
<ul>
|
|
<li><a href="cpp-language-aggregate_initialization.html">聚合初始化</a></li>
|
|
<li><a href="cpp-language-constant_initialization.html">常量初始化</a></li>
|
|
<li><a href="cpp-language-copy_initialization.html">复制初始化</a></li>
|
|
<li><a href="cpp-language-default_initialization.html">默认初始化</a></li>
|
|
<li><a href="cpp-language-direct_initialization.html">直接初始化</a></li>
|
|
<li><a href="cpp-language-initializer_list.html" class="mw-redirect">初始化列表</a></li>
|
|
<li><a href="cpp-language-list_initialization.html">列表初始化</a></li>
|
|
<li><a href="cpp-language-reference_initialization.html">引用初始化</a></li>
|
|
<li><a href="cpp-language-value_initialization.html">值初始化</a></li>
|
|
<li><a href="cpp-language-zero_initialization.html">零初始化</a></li>
|
|
</ul>
|
|
</li>
|
|
<li><a href="cpp-language-move_assignment.html">移动赋值</a></li>
|
|
<li><a href="cpp-language-move_constructor.html">移动构造函数</a></li>
|
|
<li><a href="cpp-language-new.html">new</a></li>
|
|
</ul>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|