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<title>C++ 具名要求：值可交换 (ValueSwappable)</title>
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<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">值可交换</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(ValueSwappable)</span></h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-named_req.html">named req</a></span></div>
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<p>此类型的两个对象可以解引用，且在 <span class="t-lc"><a href="cpp-algorithm-swap.html">std::swap</a></span> 和用户定义 <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，其结果值可以用无限定的函数调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">swap<span class="br0">(</span><span class="br0">)</span></span></span> 进行交换。</p>
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<h3><span class="mw-headline" id=".E8.A6.81.E6.B1.82">要求</span></h3>
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<p>以下情况下，类型 T 为<span style="font-style:italic">值可交换</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(ValueSwappable)</span> :</p>
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<ol>
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<li>类型 <code>T</code> 满足<a href="cpp-named_req-Iterator.html"><span style="font-style:italic">老式迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyIterator)</span></a> 要求</li>
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<li>对任何 <code>T</code> 类型的可解引用对象 <code>x</code>（即并非尾迭代器的值），<code>*x</code> 满足<a href="cpp-named_req-Swappable.html"><span style="font-style:italic">可交换</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Swappable)</span></a> 要求。</li>
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<p>许多标准库函数均期待其实参满足<span style="font-style:italic">值可交换</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(ValueSwappable)</span> ，这意味着凡在标准库进行交换时，它都使用 <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><span class="sy2">*</span>iter1, <span class="sy2">*</span>iter2<span class="br0">)</span><span class="sy4">;</span></span></span> 的等价物。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <vector></span>
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<span class="kw1">class</span> IntVector <span class="br0">{</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> v<span class="sy4">;</span>
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IntVector<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span>IntVector<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 不可赋值</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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<span class="kw4">void</span> swap<span class="br0">(</span>IntVector<span class="sy3">&</span> other<span class="br0">)</span> <span class="br0">{</span>
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v.<span class="me1">swap</span><span class="br0">(</span>other.<span class="me1">v</span><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>
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<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>
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v1.<span class="me1">swap</span><span class="br0">(</span>v2<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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IntVector v1, v2<span class="sy4">;</span> <span class="co1">// IntVector 为可交换 (Swappable)，但非可移动赋值 (MoveAssignable)</span>
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IntVector<span class="sy2">*</span> p1 <span class="sy1">=</span> <span class="sy3">&</span>v1<span class="sy4">;</span>
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IntVector<span class="sy2">*</span> p2 <span class="sy1">=</span> <span class="sy3">&</span>v2<span class="sy4">;</span> <span class="co1">// IntVector* 为值可交换 (ValueSwappable)</span>
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<a href="cpp-algorithm-iter_swap.html"><span class="kw1601">std::<span class="me2">iter_swap</span></span></a><span class="br0">(</span>p1, p2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：iter_swap 要求值可交换 (ValueSwappable)</span>
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<span class="co1">// std::swap(v1, v2); // 编译器错误！std::swap 要求可移动赋值 (MoveAssignable)</span>
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<span class="br0">}</span></pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<td>
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<div><a href="cpp-iterator-indirectly_swappable.html"><span class="t-lines"><span>indirectly_swappable</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>指定能交换两个 <a href="cpp-iterator-indirectly_readable.html"><code>indirectly_readable</code></a> 类型所引用的值<br />
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<span class="t-mark">(概念)</span></td>
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