104 lines
9.6 KiB
HTML
104 lines
9.6 KiB
HTML
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<title>std::common_reference_with</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_concepts_common_reference_with 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>common_reference_with</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-concepts.html">concepts</a></span></div>
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<table class="t-dcl-begin">
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<div>定义于头文件 <code><a href="cpp-header-concepts.html"><concepts></a></code></div>
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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> T, <span class="kw1">class</span> U <span class="sy1">></span><br /></span>
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<p>concept common_reference_with <span class="sy1">=</span><br />
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std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1"><</span>T, U<span class="sy1">></span>, std<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1"><</span>U, T<span class="sy1">>></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">convertible_to</span><span class="sy1"><</span>T, std<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1"><</span>T, U<span class="sy1">>></span> <span class="sy3">&&</span><br /></p>
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std<span class="sy4">::</span><span class="me2">convertible_to</span><span class="sy1"><</span>U, std<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1"><</span>T, U<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>common_reference_with<T, U></code> 指定二个类型 <code>T</code> 与 <code>U</code> 共享均能转换到的<i>共用引用类型</i>（以 <span class="t-lc">std::common_reference_t</span> 计算）。</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>T 与 U 实现 <code>common_reference_with<T, U></code> ，仅若给定保持相等性的表达式 <code>t1</code> 、 <code>t2</code> 、 <code>u1</code> 及 <code>u2</code> ，且它们满足 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>t1<span class="br0">)</span><span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>t2<span class="br0">)</span><span class="br0">)</span></span></span> 均为 <code>T</code> 而 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>u1<span class="br0">)</span><span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>u2<span class="br0">)</span><span class="br0">)</span></span></span> 均为 <code>U</code> ，且</p>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1"><</span>T, U<span class="sy1">></span><span class="br0">(</span>t1<span class="br0">)</span></span></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1"><</span>T, U<span class="sy1">></span><span class="br0">(</span>t2<span class="br0">)</span></span></span> 当且仅当 <code>t1</code> 等于 <code>t2</code> ；而</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1"><</span>T, U<span class="sy1">></span><span class="br0">(</span>u1<span class="br0">)</span></span></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1"><</span>T, U<span class="sy1">></span><span class="br0">(</span>u2<span class="br0">)</span></span></span> 当且仅当 <code>u1</code> 等于 <code>u2</code> 。</li>
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</ul>
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<p>换言之，转换到共用引用类型必须保持相等性。</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=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<div class="t-dsc-member-div">
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<div><a href="cpp-types-common_reference.html"><span class="t-lines"><span>common_reference</span><span>basic_common_reference</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>确定类型组的共用引用类型<br />
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<span class="t-mark">(类模板)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-concepts-common_with.html"><span class="t-lines"><span>common_with</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>指定两个类型共有一个公共类型<br />
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<span class="t-mark">(概念)</span></td>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-types-common_type.html"><span class="t-lines"><span>common_type</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>确定一组类型的公共类型<br />
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<span class="t-mark">(类模板)</span></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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