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88 lines
11 KiB
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<title>std::experimental::ranges::Assignable</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_experimental_ranges_concepts_Assignable 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::experimental::ranges::</span>Assignable</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-concepts.html"><experimental/ranges/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> T, <span class="kw1">class</span> U <span class="sy1">></span><br /></span>
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<p>concept <span class="kw4">bool</span> Assignable <span class="sy1">=</span><br />
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<a href="cpp-types-is_lvalue_reference.html"><span class="kw464">std::<span class="me2">is_lvalue_reference</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span> <span class="sy3">&&</span><br />
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<a href="cpp-experimental-ranges-concepts-CommonReference.html"><span class="kw2533">ranges::<span class="me2">CommonReference</span></span></a><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>T<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>U<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>T t, U<span class="sy3">&&</span> u<span class="br0">)</span> <span class="br0">{</span><br />
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<span class="br0">{</span> t <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>U<span class="sy1">></span><span class="br0">(</span>u<span class="br0">)</span> <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> <a href="cpp-experimental-ranges-concepts-Same.html"><span class="kw2530">ranges::<span class="me2">Same</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&&</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-ranges-ts t-mark-ts">(范围 TS)</span></td>
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<p>概念 <code>Assignable<T, U></code> 指定类型和值类别为 <code>U</code> 所指定的表达式能被赋值给类型为 <code>T</code> 所指定的左值表达式。</p>
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<p>给定</p>
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<ul>
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<li><code>t</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 类型的指代对象 <code>o</code> 的左值，</li>
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<li><code>u</code> ，使得 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>u<span class="br0">)</span><span class="br0">)</span></span></span> 为 <code>U</code> 的表达式，</li>
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<li><code>u2</code> ，等于 <code>u</code> 的另一对象。</li>
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<p><code>Assignable<T, U></code> 得到满足，仅若</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>t <span class="sy1">=</span> u<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>o<span class="br0">)</span></span></span> （即赋值表达式产生指代左运算数的左值）；</li>
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<li>求值 <span class="t-c"><span class="mw-geshi cpp source-cpp">t <span class="sy1">=</span> u</span></span> 后：
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<ul>
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<li><code>t</code> 等于 <code>u2</code> ，除非 <code>u</code> 是指代 <code>o</code> 的非 const 亡值（即赋值是自移动赋值），</li>
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<li>若 <code>u</code> 为泛左值：
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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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<p><code>Assignable<T, U></code> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_lvalue_reference.html"><span class="kw464">std::<span class="me2">is_lvalue_reference</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span></span></span> 间不必有任何蕴含关系。</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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</ul>
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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运算数。必须不修改常运算数。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">{</span> expression <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> Same<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&&</span></span></span> 形式的推导制约等效地要求 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>expression<span class="br0">)</span><span class="br0">)</span><span class="sy3">&&</span></span></span> 为与 <code>T&&</code> 准确相同的类型。这会制约表达式的类型与其值类别。</p>
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<p>赋值不需要是全函数。特别是，若赋值给某对象 <code>x</code> 能导致修改某个其他对象 <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。若右运算数直接或间接地为左运算数所占有（例如用基于结点的数据结构中的指向结点的智能指针，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），则这通常会发生。</p>
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