205 lines
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205 lines
19 KiB
HTML
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<title>C++ 具名要求：老式随机访问迭代器 (LegacyRandomAccessIterator)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_named_req_RandomAccessIterator skin-cppreference2 action-view cpp-navbar">
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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">老式随机访问迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyRandomAccessIterator)</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">老式随机访问迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyRandomAccessIterator)</span></strong> 是能在常数时间内移动到指向任何元素的<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> 。</p>
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<p>指向数组元素的指针满足所有<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> 的要求。</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>类型 <code>It</code> 满足<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> ，若</p>
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<ul>
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<li>类型 <code>It</code> 满足<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></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>value_type</code>，为 <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 class="sy4">::</span><span class="me2">value_type</span></span></span> 所指代的类型</li>
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<li><code>difference_type</code>，为 <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 class="sy4">::</span><span class="me2">difference_type</span></span></span> 所指代的类型</li>
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<li><code>reference</code>，为 <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 class="sy4">::</span><span class="me2">reference</span></span></span> 所指代的类型</li>
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<li><code>i</code>、<code>a</code>、<code>b</code>，为 <code>It</code> 或 <code>const It</code> 类型的对象</li>
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<li><code>r</code>，为 <code>It&</code> 类型的值</li>
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<li><code>n</code>，为 <code>difference_type</code> 类型的整数</li>
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</ul>
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<p>则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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<tr>
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<th>表达式</th>
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<th>返回类型</th>
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<th>操作语义</th>
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<th>注意</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">r <span class="sy2">+</span><span class="sy1">=</span> n</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><span class="t-c"><span class="mw-geshi cpp source-cpp">difference_type m <span class="sy1">=</span> n<span class="sy4">;</span><br /></span></span>
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<p><span class="kw1">if</span> <span class="br0">(</span>m <span class="sy1">>=</span> <span class="nu0">0</span><span class="br0">)</span> <span class="kw1">while</span> <span class="br0">(</span>m<span class="sy2">--</span><span class="br0">)</span> <span class="sy2">++</span>r<span class="sy4">;</span><br />
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<span class="kw1">else</span> <span class="kw1">while</span> <span class="br0">(</span>m<span class="sy2">++</span><span class="br0">)</span> <span class="sy2">--</span>r<span class="sy4">;</span><br />
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<span class="kw1">return</span> r<span class="sy4">;</span></p>
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</td>
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<td>
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<ul>
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<li><code><b>n</b></code> 可为正或负</li>
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<li>复杂度为常数（即实现不能按操作语义所示实际执行循环）</li>
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</ul>
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</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">a <span class="sy2">+</span> n</span></span>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp">n <span class="sy2">+</span> a</span></span></p>
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</td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">It</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="kw1">return</span> temp <span class="sy2">+</span><span class="sy1">=</span> n<span class="sy4">;</span></p>
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</td>
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<td>
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<ul>
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<li><code><b>n</b></code> 可为正或负</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy2">+</span> n <span class="sy1">==</span> n <span class="sy2">+</span> a</span></span></li>
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</ul>
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</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">r <span class="sy2">-</span><span class="sy1">=</span> n</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><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> r <span class="sy2">+</span><span class="sy1">=</span> <span class="sy2">-</span>n<span class="sy4">;</span></span></span></td>
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<td><code>n</code> 的绝对值必须在 <code>difference_type</code> 的可表示值范围内。</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">i <span class="sy2">-</span> n</span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">It</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> i<span class="sy4">;</span><br /></span></span>
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<p><span class="kw1">return</span> temp <span class="sy2">-</span><span class="sy1">=</span> n<span class="sy4">;</span></p>
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</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">b <span class="sy2">-</span> a</span></span></td>
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<td><code>difference_type</code></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> n<span class="sy4">;</span></span></span></td>
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<td>
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<p>前条件：</p>
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<ul>
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<li>存在 <code>difference_type</code> 类型的值 <code>n</code> 满足 <span class="t-c"><span class="mw-geshi cpp source-cpp">a<span class="sy2">+</span>n<span class="sy1">==</span>b</span></span></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><span class="t-c"><span class="mw-geshi cpp source-cpp">b <span class="sy1">==</span> a <span class="sy2">+</span> <span class="br0">(</span>b <span class="sy2">-</span> a<span class="br0">)</span></span></span></li>
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</ul>
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</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">i<span class="br0">[</span>n<span class="br0">]</span></span></span></td>
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<td>可转换到 <code>reference</code></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span><span class="br0">(</span>i <span class="sy2">+</span> n<span class="br0">)</span></span></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">a <span class="sy1"><</span> b</span></span></td>
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<td>可按语境转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span></td>
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<td>等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> b <span class="sy2">-</span> a <span class="sy1">></span> <span class="nu0">0</span><span class="sy4">;</span></span></span></td>
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<td>前条件：
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<ul>
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<li>同 <span class="t-c"><span class="mw-geshi cpp source-cpp">b <span class="sy2">-</span> a</span></span></li>
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</ul>
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<p>严格全序关系：</p>
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<ul>
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<li><code><b>!(a < a)</b></code></li>
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<li>若 <code><b>a < b</b></code> 则 <code><b>!(b < a)</b></code></li>
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<li>若 <code><b>a < b</b></code> 且 <code><b>b < c</b></code> 则 <code><b>a < c</b></code></li>
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<li><code><b>a < b</b></code> 或 <code><b>b < a</b></code> 或 <code><b>a == b</b></code><br />
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（表达式恰有一者为 true）</li>
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</ul>
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</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">a <span class="sy1">></span> b</span></span></td>
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<td>可按语境转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">b <span class="sy1"><</span> a</span></span></td>
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<td>与 <span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1"><</span> b</span></span> 相反的全序关系</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">a <span class="sy1">>=</span> b</span></span></td>
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<td>可按语境转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="br0">(</span>a <span class="sy1"><</span> b<span class="br0">)</span></span></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">a <span class="sy1"><=</span> b</span></span></td>
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<td>可按语境转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="br0">(</span>a <span class="sy1">></span> b<span class="br0">)</span></span></span></td>
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<td></td>
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</tr>
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</table>
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<p>上述规则隐含了<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> 亦实现<a href="cpp-named_req-LessThanComparable.html"><span style="font-style:italic">可小于比较</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LessThanComparable)</span></a> 。</p>
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<p><i>可变（mutable）</i><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> 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<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> 。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx20">
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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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<table class="t-dcl-begin">
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<tbody>
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<tr class="t-dcl">
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<td class="t-dcl-nopad">
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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 __LegacyRandomAccessIterator <span class="sy1">=</span><br />
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__LegacyBidirectionalIterator<span class="sy1"><</span>I<span class="sy1">></span> <span class="sy3">&&</span> std<span class="sy4">::</span><span class="me2">totally_ordered</span><span class="sy1"><</span>I<span class="sy1">></span> <span class="sy3">&&</span><br />
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requires<span class="br0">(</span>I i, <span class="kw1">typename</span> std<span class="sy4">::</span><span class="me2">incrementable_traits</span><span class="sy1"><</span>I<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span> n<span class="br0">)</span> <span class="br0">{</span><br />
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<span class="br0">{</span> i <span class="sy2">+</span><span class="sy1">=</span> n <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="sy1">=</span> n <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> n <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="sy1">></span><span class="sy4">;</span><br />
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<span class="br0">{</span> n <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="sy1">></span><span class="sy4">;</span><br />
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<span class="br0">{</span> i <span class="sy2">-</span> n <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="sy1">></span><span class="sy4">;</span><br />
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<span class="br0">{</span> i <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>decltype<span class="br0">(</span>n<span class="br0">)</span><span class="sy1">></span><span class="sy4">;</span><br />
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<span class="br0">{</span> i<span class="br0">[</span>n<span class="br0">]</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>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 class="t-dcl-nopad"></td>
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</tr>
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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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</tr>
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</tbody>
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</table>
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<p>其中仅用于阐释的概念 <code>__LegacyBidirectionalIterator</code> 描述于 <a href="cpp-named_req-BidirectionalIterator.html#.E6.A6.82.E5.BF.B5">LegacyBidirectionalIterator#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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</tr>
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</table>
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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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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-random_access_iterator.html"><span class="t-lines"><span>random_access_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-bidirectional_iterator.html"><code>bidirectional_iterator</code></a> 为随机访问迭代器，支持常数时间内的前进和下标访问<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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</div>
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</body>
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</html>
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