72 lines
9.4 KiB
HTML
72 lines
9.4 KiB
HTML
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::experimental::ranges::</span>Writable</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-experimental.html">experimental</a>‎ | <a href="cpp-experimental-ranges.html">ranges</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-experimental-ranges-iterator.html"><experimental/ranges/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 <span class="kw4">bool</span> 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> ranges<span class="sy4">::</span><span class="me2">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><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="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">const</span> ranges<span class="sy4">::</span><span class="me2">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="coMULTI">/* 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-ranges-ts t-mark-ts">(范围 TS)</span></td>
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<p>概念 <code>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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<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>Writable<Out, T></code> 仅若符合下列条件才得到满足：</p>
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-experimental-ranges-iterator-Readable.html"><span class="kw2562">ranges::<span class="me2">Readable</span></span></a><span class="sy1"><</span>Out<span class="sy1">></span> <span class="sy3">&&</span> <a href="cpp-experimental-ranges-concepts-Same.html"><span class="kw2530">ranges::<span class="me2">Same</span></span></a><span class="sy1"><</span>ranges<span class="sy4">::</span><span class="me2">value_type_t</span><span class="sy1"><</span>Out<span class="sy1">></span>, <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> 得到满足，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>表达式的输入由其运算数组成。</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运算数（若存在）组成。</li>
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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。通过可写类型相同值的赋值可以只发生一次。</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-experimental-ranges-iterator-Readable.html"><code>Readable</code></a> 对象意外地满足 <code>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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