89 lines
9.5 KiB
HTML
89 lines
9.5 KiB
HTML
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<title>std::indirectly_readable</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_indirectly_readable 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_readable</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> In<span class="sy1">></span><br /></span>
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<p> concept __IndirectlyReadableImpl <span class="sy1">=</span> <span class="co1">// exposition only</span><br />
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requires<span class="br0">(</span><span class="kw4">const</span> In in<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">iter_value_t</span><span class="sy1"><</span>In<span class="sy1">></span><span class="sy4">;</span><br />
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<span class="kw1">typename</span> std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1"><</span>In<span class="sy1">></span><span class="sy4">;</span><br />
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<span class="kw1">typename</span> std<span class="sy4">::</span><span class="me2">iter_rvalue_reference_t</span><span class="sy1"><</span>In<span class="sy1">></span><span class="sy4">;</span><br />
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<span class="br0">{</span> <span class="sy2">*</span>in <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>std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1"><</span>In<span class="sy1">>></span><span class="sy4">;</span><br />
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<span class="br0">{</span> ranges<span class="sy4">::</span><span class="me2">iter_move</span><span class="br0">(</span>in<span class="br0">)</span> <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>std<span class="sy4">::</span><span class="me2">iter_rvalue_reference_t</span><span class="sy1"><</span>In<span class="sy1">>></span><span class="sy4">;</span><br />
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<span class="br0">}</span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">common_reference_with</span><span class="sy1"><</span><br />
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std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1"><</span>In<span class="sy1">></span><span class="sy3">&&</span>, std<span class="sy4">::</span><span class="me2">iter_value_t</span><span class="sy1"><</span>In<span class="sy1">></span><span class="sy3">&</span><br />
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<span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">common_reference_with</span><span class="sy1"><</span><br />
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std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1"><</span>In<span class="sy1">></span><span class="sy3">&&</span>, std<span class="sy4">::</span><span class="me2">iter_rvalue_reference_t</span><span class="sy1"><</span>In<span class="sy1">></span><span class="sy3">&&</span><br />
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<span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">common_reference_with</span><span class="sy1"><</span><br />
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std<span class="sy4">::</span><span class="me2">iter_rvalue_reference_t</span><span class="sy1"><</span>In<span class="sy1">></span><span class="sy3">&&</span>, <span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">iter_value_t</span><span class="sy1"><</span>In<span class="sy1">></span><span class="sy3">&</span><br /></p>
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<span class="sy1">></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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> In<span class="sy1">></span><br /></span>
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<p> concept indirectly_readable <span class="sy1">=</span><br /></p>
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__IndirectlyReadableImpl<span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>In<span class="sy1">>></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>概念 <code>indirectly_readable</code> 由通过应用 <code>operator*</code> 可读的类型实现，例如指针、智能指针及输入迭代器。</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>给定 <code>I</code> 类型值 <code>i</code> ，仅若被蕴含的所有概念均得到实现且表达式 <code>*i</code> 保持相等性， <code>I</code> 才实现 <code>indirectly_readable</code> 。</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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<p>除非另外提醒，每个用于 <i>requires 表达式</i>中的表达式都要求保持相等性且稳定，而表达式的求值必须只修改其非常操作数。必须不修改常操作数。</p>
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