129 lines
13 KiB
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129 lines
13 KiB
HTML
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<title>C++ 具名要求：老式迭代器 (LegacyIterator)</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">老式迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyIterator)</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><strong class="selflink"><span style="font-style:italic">老式迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyIterator)</span></strong> 要求描述可以用来标识和遍历容器中的元素的类型。</p>
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<p><span style="font-style:italic">老式迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyIterator)</span> 是用于其他迭代器类型的基础集合：<a href="cpp-named_req-InputIterator.html"><span style="font-style:italic">老式输入迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyInputIterator)</span></a> 、<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> 、<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> 、<a href="cpp-named_req-BidirectionalIterator.html"><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></a> 及<a href="cpp-named_req-RandomAccessIterator.html"><span style="font-style:italic">老式随机访问迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyRandomAccessIterator)</span></a> 。可以把迭代器想象成指针的抽象。</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>若下列条件成立，则类型 <code>It</code> 满足<span style="font-style:italic">老式迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyIterator)</span></p>
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<ul>
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<li>类型 <code>It</code> 满足<a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> ，且</li>
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<li>类型 <code>It</code> 满足<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> ，且</li>
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<li>类型 <code>It</code> 满足<a href="cpp-named_req-Destructible.html"><span style="font-style:italic">可析构</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Destructible)</span></a></li>
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<li>类型 <code>It</code> 的<a href="cpp-language-value_category.html">左值</a>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> ，且</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>It<span class="sy1">></span></span></span> 拥有成员 typedef <code>value_type</code>、<code>difference_type</code>、<code>reference</code>、<code>pointer</code> 和 <code>iterator_category</code> 且</li>
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</ul>
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<p>给定</p>
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<ul>
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<li>类型 <code>It</code> 的左值 <code>r</code>，</li>
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</ul>
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<p>下列表达式必须合法，且拥有其指定的效果：</p>
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<table class="wikitable">
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<th>表达式</th>
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<th>返回类型</th>
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<th>前条件</th>
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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>r</span></span></td>
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<td>未指明</td>
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<td><code><b>r</b></code> <i>可解引用</i>（见下文）</td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">++</span>r</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><code><b>r</b></code> <i>可增</i>（表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">++</span>r</span></span> 的行为有定义）</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.A7.A3.E5.BC.95.E7.94.A8.E7.9A.84.E8.BF.AD.E4.BB.A3.E5.99.A8">可解引用的迭代器</span></h3>
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<p>表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>i</span></span> 的行为得到定义的迭代器被称作<i>可解引用（dereferenceable）</i>。</p>
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<p>下列情况下的迭代器不可解引用</p>
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<ul>
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<li>它们是<a href="cpp-container-vector-end.html">末尾后迭代器</a>（包括数组末尾后面一个位置的指针）或<a href="cpp-container-forward_list-before_begin.html">起始前迭代器</a>。在特定实现中这些迭代器可能可解引用，但库从不假设它们能。</li>
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<li>它们是<i>孤立</i>迭代器，即是说，没有关联到任何序列的迭代器。空指针，以及默认构造的指针（保有不确定的值）是孤立的。</li>
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<li>在它们所指代的序列上，它们已经因使迭代器失效的操作之一而<a href="cpp-container.html#.E8.BF.AD.E4.BB.A3.E5.99.A8.E5.A4.B1.E6.95.88">失效</a>了。</li>
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</ul>
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<table class="t-rev-begin">
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<td>
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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 __LegacyIterator <span class="sy1">=</span><br />
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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> __Referenceable<span class="sy4">;</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> <span class="sy2">*</span>i<span class="sy2">++</span> <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> __Referenceable<span class="sy4">;</span><br /></p>
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<span class="br0">}</span> <span class="sy3">&&</span> std<span class="sy4">::</span><span class="me2">copyable</span><span class="sy1"><</span>I<span class="sy1">></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 class="t-dcl-nopad"></td>
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<tr class="t-dcl-sep">
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<td></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>__Referenceable<T></code> 当且仅当 <code>T&</code> 是合法类型时得到满足（特别是 <code>T</code> 必须不是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span>）。</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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</table>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<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>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3420">LWG 3420</a></td>
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<td>C++20</td>
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<td>仅用于阐释的概念首先检查 <code>copyable</code></td>
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<td>仅若 requires 表达式返回 <code>true</code> 才检查 <code>copyable</code></td>
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</tr>
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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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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-input_or_output_iterator.html"><span class="t-lines"><span>input_or_output_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>指定该类型对象可以自增且可以解引用<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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