84 lines
8.2 KiB
HTML
84 lines
8.2 KiB
HTML
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<title>std::output_iterator</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_output_iterator skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>output_iterator</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-iterator.html">iterator</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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<table class="t-dcl-begin">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-iterator.html"><iterator></a></code></div>
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<td></td>
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<td></td>
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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="kw1">class</span> T<span class="sy1">></span><br /></span>
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<p> concept output_iterator <span class="sy1">=</span><br />
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std<span class="sy4">::</span><span class="me2">input_or_output_iterator</span><span class="sy1"><</span>I<span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">indirectly_writable</span><span class="sy1"><</span>I, T<span class="sy1">></span> <span class="sy3">&&</span><br />
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requires<span class="br0">(</span>I i, T<span class="sy3">&&</span> t<span class="br0">)</span> <span class="br0">{</span><br />
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<span class="sy2">*</span>i<span class="sy2">++</span> <span class="sy1">=</span> <a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 不要求保持相等性</span><br /></p>
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<span class="br0">}</span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p><code>output_iterator</code> 概念是 <a href="cpp-iterator-input_or_output_iterator.html"><code>input_or_output_iterator</code></a> 的细化，添加了能用于写入值类型和值类别为 <code>T</code> 所编码的值的要求（经由 <a href="cpp-iterator-indirectly_writable.html"><code>indirectly_writable</code></a> ）。不要求 <a href="cpp-concepts-equality_comparable.html"><code>equality_comparable</code></a> 。</p>
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<h3><span class="mw-headline" id=".E8.AF.AD.E4.B9.89.E8.A6.81.E6.B1.82">语义要求</span></h3>
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<p>令 <code>E</code> 为满足 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>E<span class="br0">)</span><span class="br0">)</span></span></span> 为 <code>T</code> 的表达式，而 <code>i</code> 为 <code>I</code> 类型的可解引用对象。 <code>output_iterator<I, T></code> 仅若其所蕴含的概念均被实现，且 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>i<span class="sy2">++</span> <span class="sy1">=</span> E<span class="sy4">;</span></span></span> 拥有等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>i <span class="sy1">=</span> E<span class="sy4">;</span> <span class="sy2">++</span>i<span class="sy4">;</span></span></span> 的效果才得以实现。</p>
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<h3><span class="mw-headline" id=".E7.9B.B8.E7.AD.89.E6.80.A7.E4.BF.9D.E6.8C.81">相等性保持</span></h3>
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<p>若表达式对给定的相等输入产生相等输出，则它<i>保持相等性</i>。</p>
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<ul>
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<li>表达式的输入由其操作数组成。</li>
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<li>表达式的输出由其结果和表达式所修改的所有操作数（若存在）组成。</li>
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</ul>
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<p>在标准概念的规范中，操作数定义为仅包含下列内容的最大子表达式：</p>
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<ul>
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<li><a href="cpp-language-expressions.html#.E5.88.9D.E7.AD.89.E8.A1.A8.E8.BE.BE.E5.BC.8F">标识表达式</a>，及</li>
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<li>对 <a href="cpp-utility-move.html"><tt>std::move</tt></a> 、 <span class="t-lc"><a href="cpp-utility-forward.html">std::forward</a></span> 与 <span class="t-lc"><a href="cpp-utility-declval.html">std::declval</a></span> 的调用。</li>
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</ul>
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<p>每个操作数的 cv 限定与值类别，是通过假设每个模板类型形参代表一个 cv 无限定的非数组对象类型确定的。</p>
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<p>进一步要求每个要求保持相等性的表达式都<i>稳定</i>：这种表达式带相同输入对象的二次求值必须拥有相等的输出，而无任何对这些输入对象的显式中间修改。</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>不同于<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， <code>output_iterator</code> 概念不要求定义迭代器类别标签。</p>
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<p>输出迭代器上的算法应该为单趟。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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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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</table>
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</div>
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