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78 lines
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<title>std::indirectly_writable</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_indirectly_writable 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>indirectly_writable</h1>
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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>定义于头文件 <code><a href="cpp-header-iterator.html"><iterator></a></code></div>
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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> Out, <span class="kw1">class</span> T<span class="sy1">></span><br /></span>
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<p> concept indirectly_writable <span class="sy1">=</span><br />
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requires<span class="br0">(</span>Out<span class="sy3">&&</span> o, T<span class="sy3">&&</span> t<span class="br0">)</span> <span class="br0">{</span><br />
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<span class="sy2">*</span>o <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><br />
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<span class="sy2">*</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>Out<span class="sy1">></span><span class="br0">(</span>o<span class="br0">)</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><br />
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<span class="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1"><</span>Out<span class="sy1">></span><span class="sy3">&&</span><span class="sy1">></span><span class="br0">(</span><span class="sy2">*</span>o<span class="br0">)</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><br />
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<span class="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1"><</span>Out<span class="sy1">></span><span class="sy3">&&</span><span class="sy1">></span><span class="br0">(</span><span class="sy2">*</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>Out<span class="sy1">></span><span class="br0">(</span>o<span class="br0">)</span><span class="br0">)</span> <span class="sy1">=</span><br />
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<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><br />
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<span class="br0">}</span><span class="sy4">;</span><br /></p>
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<span class="co1">// 上述四个表达式都不要求保持相等性</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>概念 <code>indirectly_writable<Out, T></code> 指定写入类型和值类别为 <code>T</code> 所编码的值到迭代器 <code>Out</code> 所引用对象的要求。</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>o</code> 为 <code>Out</code> 类型的可解引用对象，则 <code>indirectly_writable<Out, T></code> 仅若符合下列条件才被实现：</p>
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">indirectly_readable</span><span class="sy1"><</span>Out<span class="sy1">></span></span></span> 得到实现且 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">iter_value_t</span><span class="sy1"><</span>Out<span class="sy1">></span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 为同一类型，则在任何上述赋值后 <code>*o</code> 等于 <code>e</code> 在赋值前的值。</li>
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<p>不要求 <code>o</code> 在求值任何上述赋值表达式后可解引用。若 <code>e</code> 为亡值，则其代表的对象的结果状态合法但未指定。</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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<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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<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><code>operator*</code> 仅有的合法用法是在赋值表达式左侧。通过相同的间接可写类型值的赋值只可以发生一次。</p>
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<p>带 <code>const_cast</code> 的要求表达式阻止有纯右值 <code>reference</code> 类型的 <a href="cpp-iterator-indirectly_readable.html"><code>indirectly_readable</code></a> 对象意外满足 <code>indirectly_writable</code> 的语法要求，同时容许代理引用继续工作，只要其常性是浅的。见<a rel="nofollow" class="external text" href="https://github.com/ericniebler/stl2/issues/381">范围 TS 问题 381</a> 。</p>
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