106 lines
11 KiB
HTML
106 lines
11 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>C++ 具名要求：可交换 (Swappable)</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_named_req_Swappable skin-cppreference2 action-view cpp-navbar">
|
|
<div id="cpp-content-base">
|
|
<div id="content"><a id="top"></a>
|
|
<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">C++ 具名要求：</span><span style="font-style:italic">可交换</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Swappable)</span></h1>
|
|
<div id="bodyContent">
|
|
<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-named_req.html">named req</a></span></div>
|
|
<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
|
|
<p>在 <span class="t-lc"><a href="cpp-algorithm-swap.html">std::swap</a></span> 与用户定义 <span class="t-c"><span class="mw-geshi cpp source-cpp">swap<span class="br0">(</span><span class="br0">)</span></span></span> 均可见的语境中，此类型的任何左值或右值能与某个其他类型的任何左值或右值，可以用非限定函数调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">swap<span class="br0">(</span><span class="br0">)</span></span></span> 来进行交换。</p>
|
|
<h3><span class="mw-headline" id=".E8.A6.81.E6.B1.82">要求</span></h3>
|
|
<p>若对于任意 U 类型的对象 u 和任意 T 类型的对象 t，满足以下情况，则类型 U 与类型 T 可交换：</p>
|
|
<table class="wikitable">
|
|
<tr>
|
|
<th>表达式</th>
|
|
<th>要求</th>
|
|
<th>语义</th>
|
|
</tr>
|
|
<tr>
|
|
<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="co2">#include <utility></span><br /></span></span>
|
|
<p><span class="kw1">using</span> <a href="cpp-algorithm-swap.html"><span class="kw1599">std::<span class="me2">swap</span></span></a><span class="sy4">;</span><br />
|
|
swap<span class="br0">(</span>u, t<span class="br0">)</span><span class="sy4">;</span></p>
|
|
</td>
|
|
<td>调用后，<code>t</code> 的值是 <code>u</code> 在调用前保有的值，而 <code>u</code> 的值是 <code>t</code> 在调用前保有的值。</td>
|
|
<td>调用名为 <span class="t-c"><span class="mw-geshi cpp source-cpp">swap<span class="br0">(</span><span class="br0">)</span></span></span> 的函数，它是在由<a href="cpp-language-adl.html">实参依赖查找</a>所找到的所有具有这个名字的函数，和定义于头文件 <a href="cpp-header-utility.html"><tt><utility></tt></a> 中的两个 <span class="t-lc"><a href="cpp-algorithm-swap.html">std::swap</a></span> 模板中，由重载决议所找到的函数。</td>
|
|
</tr>
|
|
<tr>
|
|
<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="co2">#include <utility></span><br /></span></span>
|
|
<p><span class="kw1">using</span> <a href="cpp-algorithm-swap.html"><span class="kw1599">std::<span class="me2">swap</span></span></a><span class="sy4">;</span><br />
|
|
swap<span class="br0">(</span>t, u<span class="br0">)</span><span class="sy4">;</span></p>
|
|
</td>
|
|
<td>同上</td>
|
|
<td>同上</td>
|
|
</tr>
|
|
</table>
|
|
<p>许多标准库函数（例如许多算法）期待其实参满足<span style="font-style:italic">可交换</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Swappable)</span> ，这意味着每当标准库进行交换时，都使用等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">using</span> <a href="cpp-algorithm-swap.html"><span class="kw1599">std::<span class="me2">swap</span></span></a><span class="sy4">;</span> swap<span class="br0">(</span>t, u<span class="br0">)</span><span class="sy4">;</span></span></span> 的写法。</p>
|
|
<p>典型的实现进行二者之一</p>
|
|
<div class="t-li1"><span class="t-li">1)</span> 在外围命名空间中定义非成员 swap，若要求访问非公开数据成员，则它可转发到成员 <code>swap</code></div>
|
|
<div class="t-li1"><span class="t-li">2)</span> 于类中定义<a href="cpp-language-friend.html">友元函数</a>（此方法对于除 ADL 之外的名字查找隐藏了类特有的 <code>swap</code>）</div>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
|
<p>标准库进行交换时是否实际包含 <a href="cpp-header-utility.html"><tt><utility></tt></a> 是未指明的，因此用户提供的 <span class="t-c"><span class="mw-geshi cpp source-cpp">swap<span class="br0">(</span><span class="br0">)</span></span></span> 不应当期待它已被包含。</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 <vector></span>
|
|
|
|
<span class="kw1">class</span> IntVector <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="sy4">;</span>
|
|
IntVector<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span>IntVector<span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span> <span class="co1">// 不可赋值</span>
|
|
<span class="kw1">public</span><span class="sy4">:</span>
|
|
<span class="kw4">void</span> swap<span class="br0">(</span>IntVector<span class="sy3">&</span> other<span class="br0">)</span> <span class="br0">{</span>
|
|
v.<span class="me1">swap</span><span class="br0">(</span>other.<span class="me1">v</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="kw4">void</span> swap<span class="br0">(</span>IntVector<span class="sy3">&</span> v1, IntVector<span class="sy3">&</span> v2<span class="br0">)</span> <span class="br0">{</span>
|
|
v1.<span class="me1">swap</span><span class="br0">(</span>v2<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
IntVector v1, v2<span class="sy4">;</span>
|
|
<span class="co1">// std::swap(v1, v2); // 编译器错误！std::swap 要求可移动赋值 (MoveAssignable)</span>
|
|
<a href="cpp-algorithm-iter_swap.html"><span class="kw1601">std::<span class="me2">iter_swap</span></span></a><span class="br0">(</span><span class="sy3">&</span>v1, <span class="sy3">&</span>v2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：标准库调用无限定的 swap()</span>
|
|
<span class="br0">}</span></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">
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-types-is_swappable.html"><span class="t-lines"><span>is_swappable_with</span><span>is_swappable</span><span>is_nothrow_swappable_with</span><span>is_nothrow_swappable</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>检查一个类型的对象是否能与同类型或不同类型的对象交换<br />
|
|
<span class="t-mark">(类模板)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-concepts-swappable.html"><span class="t-lines"><span>swappable</span><span>swappable_with</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>指定一个类型能进行交换，或两个类型能彼此交换<br />
|
|
<span class="t-mark">(概念)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|