127 lines
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127 lines
11 KiB
HTML
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<title>C++ 具名要求：老式双向迭代器 (LegacyBidirectionalIterator)</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">老式双向迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyBidirectionalIterator)</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-named_req.html">named req</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><strong class="selflink"><span style="font-style:italic">老式双向迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyBidirectionalIterator)</span></strong> 是能双向移动（即自增与自减）的<a href="cpp-named_req-ForwardIterator.html"><span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</span></a> 。</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 <code>It</code> 满足 <span style="font-style:italic">老式双向迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyBidirectionalIterator)</span></p>
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<ul>
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<li>类型 <code>It</code> 满足<a href="cpp-named_req-ForwardIterator.html"><span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</span></a></li>
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</ul>
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<p>且，给定</p>
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<ul>
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<li><code>a</code> 与 <code>b</code>，为类型 <code>It</code> 的迭代器</li>
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<li><code>reference</code>，为 <code>std::iterator_traits<It>::reference</code> 所指代的类型</li>
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</ul>
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<p>则下列表达式必须合法并拥有其效果</p>
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<table class="wikitable">
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<tr>
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<th>表达式</th>
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<th>返回</th>
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<th>等价表达式</th>
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<th>注意</th>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span>a</span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">It<span class="sy3">&</span></span></span></td>
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<td></td>
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<td>前提条件：
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<ul>
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<li><code>a</code> 可自减（存在满足 <span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1">==</span> <span class="sy2">++</span>b</span></span> 的 <code>b</code>）</li>
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</ul>
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<p>后条件：</p>
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<ul>
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<li>a 可解引用</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span><span class="br0">(</span><span class="sy2">++</span>a<span class="br0">)</span> <span class="sy1">==</span> a</span></span></li>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span>a <span class="sy1">==</span> <span class="sy2">--</span>b</span></span> 则 <span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1">==</span> b</span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">a</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span>a</span></span> 指代同一迭代器对象</li>
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</ul>
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</td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">a<span class="sy2">--</span></span></span></td>
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<td>可转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> It<span class="sy3">&</span></span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">It temp <span class="sy1">=</span> a<span class="sy4">;</span><br /></span></span>
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<p><span class="sy2">--</span>a<span class="sy4">;</span><br /></p>
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<span class="kw1">return</span> temp<span class="sy4">;</span></td>
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<td></td>
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</tr>
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<tr>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>a<span class="sy2">--</span></span></span></td>
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<td><code>reference</code></td>
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<td></td>
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<td></td>
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</tr>
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</table>
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<p><i>可变（mutable）</i><strong class="selflink"><span style="font-style:italic">老式双向迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyBidirectionalIterator)</span></strong> 是还额外满足<a href="cpp-named_req-OutputIterator.html"><span style="font-style:italic">老式输出迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyOutputIterator)</span></a> 要求的<strong class="selflink"><span style="font-style:italic">老式双向迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyBidirectionalIterator)</span></strong> 。</p>
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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>起始迭代器不可自减。且若求值 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span>container.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span></span></span> 则行为未定义。</p>
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<p>双向迭代器不必可解引用、可自减（尤其是尾迭代器不可解引用但可自减）。</p>
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<h3><span class="mw-headline" id=".E6.A6.82.E5.BF.B5">概念</span></h3>
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<p>为了定义 <span class="t-lc"><a href="cpp-iterator-iterator_traits.html">std::iterator_traits</a></span>，定义了以下仅用于阐释的概念：</p>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> I<span class="sy1">></span><br /></span>
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<p>concept __LegacyBidirectionalIterator <span class="sy1">=</span><br />
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__LegacyForwardIterator<span class="sy1"><</span>I<span class="sy1">></span> <span class="sy3">&&</span> requires<span class="br0">(</span>I i<span class="br0">)</span> <span class="br0">{</span><br />
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<span class="br0">{</span> <span class="sy2">--</span>i <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span>I<span class="sy3">&</span><span class="sy1">></span><span class="sy4">;</span><br />
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<span class="br0">{</span> i<span class="sy2">--</span> <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> std<span class="sy4">::</span><span class="me2">convertible_to</span><span class="sy1"><</span><span class="kw4">const</span> I<span class="sy3">&</span><span class="sy1">></span><span class="sy4">;</span><br />
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<span class="br0">{</span> <span class="sy2">*</span>i<span class="sy2">--</span> <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1"><</span>I<span class="sy1">>></span><span class="sy4">;</span><br /></p>
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<span class="br0">}</span><span class="sy4">;</span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td></td>
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<td></td>
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<p>其中仅用于阐释的概念 <code>__LegacyForwardIterator</code> 描述于 <a href="cpp-named_req-ForwardIterator.html#.E6.A6.82.E5.BF.B5">LegacyForwardIterator#概念</a>。</p>
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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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<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 class="t-dsc-member-div">
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<div><a href="cpp-iterator-bidirectional_iterator.html"><span class="t-lines"><span>bidirectional_iterator</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 <a href="cpp-iterator-forward_iterator.html"><code>forward_iterator</code></a> 为双向迭代器，支持向后移动<br />
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<span class="t-mark">(概念)</span></td>
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</tr>
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<tr class="t-dsc">
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<td><a href="cpp-iterator.html"><b>迭代器库</b></a></td>
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<td>提供迭代器的定义、迭代器表征、适配器及工具函数</td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</div>
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