106 lines
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106 lines
11 KiB
HTML
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<title>std::assignable_from</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_concepts_assignable_from 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>assignable_from</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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<div>定义于头文件 <code><a href="cpp-header-concepts.html"><concepts></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> LHS, <span class="kw1">class</span> RHS <span class="sy1">></span><br /></span>
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<p>concept assignable_from <span class="sy1">=</span><br />
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<a href="cpp-types-is_lvalue_reference.html"><span class="kw465">std::<span class="me2">is_lvalue_reference_v</span></span></a><span class="sy1"><</span>LHS<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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<span class="kw4">const</span> <a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>LHS<span class="sy1">></span><span class="sy3">&</span>,<br />
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<span class="kw4">const</span> <a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>RHS<span class="sy1">></span><span class="sy3">&</span><span class="sy1">></span> <span class="sy3">&&</span><br />
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requires<span class="br0">(</span>LHS lhs, RHS<span class="sy3">&&</span> rhs<span class="br0">)</span> <span class="br0">{</span><br />
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<span class="br0">{</span> lhs <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>RHS<span class="sy1">></span><span class="br0">(</span>rhs<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>LHS<span class="sy1">></span><span class="sy4">;</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>概念 <code>assignable_from<LHS, RHS></code> 指定能赋值类型和值类别由 <code>RHS</code> 编码的表达式给类型为 <code>LHS</code> 所指定的左值表达式。</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</p>
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<li><code>lhs</code> ，指代对象 <code>lcopy</code> 的左值，使得 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>lhs<span class="br0">)</span><span class="br0">)</span></span></span> 为 <code>LHS</code> ，</li>
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<li><code>rhs</code> ，使得 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>rhs<span class="br0">)</span><span class="br0">)</span></span></span> 为 <code>RHS</code> 的表达式，</li>
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<li><code>rcopy</code> ，等于 <code>rhs</code> 的单独对象，</li>
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</ul>
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<p><code>assignable_from<LHS, RHS></code> 仅若符合下列条件才得到实现</p>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a><span class="br0">(</span>lhs <span class="sy1">=</span> rhs<span class="br0">)</span> <span class="sy1">==</span> <a href="cpp-memory-addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a><span class="br0">(</span>lcopy<span class="br0">)</span></span></span> （即赋值表达式生成指代左操作数的左值）；</li>
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<li>求值 <span class="t-c"><span class="mw-geshi cpp source-cpp">lhs <span class="sy1">=</span> rhs</span></span> 后：
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<ul>
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<li><code>lhs</code> 等于 <code>rcopy</code> ，除非 <code>rhs</code> 是指代 <code>lcopy</code> 的非 const 亡值（即赋值为自移动赋值），</li>
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<li>若 <code>rhs</code> 是泛左值；
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<ul>
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<li>若它是非 const 亡值，则其所指代的对象在合法但未指定的状态；</li>
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<li>否则，不修改其所指代的对象；</li>
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</ul>
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</ul>
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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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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>赋值不必为全函数。具体来说，若赋值给某对象 <code>x</code> 可能导致修改某其他对象 <code>y</code> ，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">x <span class="sy1">=</span> y</span></span> 可能不在 <code>=</code> 的定义域中。若左操作数直接或间接占有右操作数（例如用指向基于结点的数据结构中的结点的智能指针，或用类似 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><a href="cpp-utility-any.html"><span class="kw1071">std::<span class="me2">any</span></span></a><span class="sy1">></span></span></span> 的构造），则这常发生。</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><a href="cpp-types-is_assignable.html"><span class="t-lines"><span>is_assignable</span><span>is_trivially_assignable</span><span>is_nothrow_assignable</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><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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<td>检查类型是否拥有针对特定实参的赋值运算符<br />
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<span class="t-mark">(类模板)</span></td>
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<div class="visualClear"></div>
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