147 lines
17 KiB
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147 lines
17 KiB
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<title>C++ 具名要求：老式输入迭代器 (LegacyInputIterator)</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">老式输入迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyInputIterator)</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">老式输入迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyInputIterator)</span></strong> 是能从所指向元素进行读取的<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> 。<strong class="selflink"><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></strong> 仅保证单趟算法的有效性：一旦自增<strong class="selflink"><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></strong> <span class="t-c"><span class="mw-geshi cpp source-cpp">i</span></span>，则所有其先前值的副本都可能失效。</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>若满足下列条件，则类型 <code>It</code> 满足<strong class="selflink"><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></strong></p>
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<ul>
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<li>类型 <code>It</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></li>
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<li>类型 <code>It</code> 满足<a href="cpp-named_req-EqualityComparable.html"><span style="font-style:italic">可相等比较</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(EqualityComparable)</span></a></li>
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</ul>
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<p>并且，给定</p>
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<ul>
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<li><code>i</code> 和 <code>j</code>，为 <span class="t-c"><span class="mw-geshi cpp source-cpp">It</span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> It</span></span> 类型的值</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> 所代表的类型</li>
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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> 所代表的类型</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</th>
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<th>等价表达式</th>
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<th>注解</th>
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</tr>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">i <span class="sy3">!</span><span class="sy1">=</span> j</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>i <span class="sy1">==</span> j<span class="br0">)</span></span></span></td>
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<td><b>前条件</b>：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>i, j<span class="br0">)</span></span></span> 在 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">==</span></span></span> 的定义域中。</td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>i</span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">reference</span></span>，可转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp">value_type</span></span></td>
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<td>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">i <span class="sy1">==</span> j</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>i, j<span class="br0">)</span></span></span> 在 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">==</span></span></span> 的定义域中，则这等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>j</span></span>。</td>
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<td>
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<p><b>前条件</b>：<span class="t-c"><span class="mw-geshi cpp source-cpp">i</span></span> 可解引用。</p>
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<p>表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="sy2">*</span>i, <span class="sy2">*</span>i</span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>i</span></span>。</p>
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</td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">i<span class="sy2">-</span><span class="sy1">></span>m</span></span></td>
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<td></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="sy2">*</span>i<span class="br0">)</span>.<span class="me1">m</span></span></span></td>
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<td><b>前条件</b>：<span class="t-c"><span class="mw-geshi cpp source-cpp">i</span></span> 可解引用。</td>
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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>i</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>
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<p><b>前条件</b>：<span class="t-c"><span class="mw-geshi cpp source-cpp">i</span></span> 可解引用</p>
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<p><b>后条件</b>：<span class="t-c"><span class="mw-geshi cpp source-cpp">i</span></span> 可解引用或 <span class="t-c"><span class="mw-geshi cpp source-cpp">i</span></span> 为末尾后迭代器。</p>
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<p><b>后条件</b>：不再要求任何 <span class="t-c"><span class="mw-geshi cpp source-cpp">i</span></span> 的先前值的副本是可解引用的，或在 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">==</span></span></span> 的定义域内。</p>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>i<span class="sy2">++</span></span></span></td>
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<td></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="sy2">++</span>i</span></span></td>
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<td></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>i<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">value_type</span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">value_type x <span class="sy1">=</span> <span class="sy2">*</span>i<span class="sy4">;</span><br /></span></span>
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<p><span class="sy2">++</span>i<span class="sy4">;</span><br /></p>
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<span class="kw1">return</span> x<span class="sy4">;</span></td>
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<td></td>
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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>“在 <code>==</code> 的定义域内”的意思是，两个迭代器值之间定义了相等比较。对于输入迭代器，相等比较不需要对所有值定义，且 <code>==</code> 的定义域中的值的集合可能随时间而改变。</p>
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<p>对于并非同时为<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> 的输入迭代器，其 <span class="t-c"><span class="mw-geshi cpp source-cpp">reference</span></span> 类型不必是引用类型：解引用输入迭代器可以返回一个代理对象，或以值返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">value_type</span></span> 本身（如 <a href="cpp-iterator-istreambuf_iterator.html"><tt>std::istreambuf_iterator</tt></a> 的情况）。</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 __LegacyInputIterator <span class="sy1">=</span><br />
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__LegacyIterator<span class="sy1"><</span>I<span class="sy1">></span> <span class="sy3">&&</span> std<span class="sy4">::</span><span class="me2">equality_comparable</span><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="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><span class="sy4">;</span><br />
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<span class="kw1">typename</span> std<span class="sy4">::</span><span class="me2">readable_traits</span><span class="sy1"><</span>I<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span><span class="sy4">;</span><br />
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<span class="kw1">typename</span> std<span class="sy4">::</span><span class="me2">common_reference_t</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="sy3">&&</span>,<br />
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<span class="kw1">typename</span> std<span class="sy4">::</span><span class="me2">readable_traits</span><span class="sy1"><</span>I<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span><span class="sy3">&</span><span class="sy1">></span><span class="sy4">;</span><br />
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<span class="sy2">*</span>i<span class="sy2">++</span><span class="sy4">;</span><br />
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<span class="kw1">typename</span> std<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1"><</span>decltype<span class="br0">(</span><span class="sy2">*</span>i<span class="sy2">++</span><span class="br0">)</span><span class="sy3">&&</span>,<br />
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<span class="kw1">typename</span> std<span class="sy4">::</span><span class="me2">readable_traits</span><span class="sy1"><</span>I<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span><span class="sy3">&</span><span class="sy1">></span><span class="sy4">;</span><br />
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requires std<span class="sy4">::</span><span class="me2">signed_integral</span><span class="sy1"><</span><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><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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<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>__LegacyIterator</code> 描述于 <a href="cpp-named_req-Iterator.html#.E6.A6.82.E5.BF.B5">LegacyIterator#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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-input_iterator.html"><span class="t-lines"><span>input_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>指定类型为输入迭代器，即可读取其所引用的值，且可前/后自增<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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