92 lines
8.9 KiB
HTML
92 lines
8.9 KiB
HTML
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<title>std::experimental::ranges::ConvertibleTo</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_experimental_ranges_concepts_ConvertibleTo 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>ConvertibleTo</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> From, <span class="kw1">class</span> To<span class="sy1">></span><br /></span>
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<p>concept <span class="kw4">bool</span> ConvertibleTo <span class="sy1">=</span><br />
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<a href="cpp-types-is_convertible.html"><span class="kw592">std::<span class="me2">is_convertible</span></span></a><span class="sy1"><</span>From, To<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span> <span class="sy3">&&</span><br />
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requires<span class="br0">(</span>From <span class="br0">(</span><span class="sy3">&</span>f<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span><br />
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<span class="kw1">static_cast</span><span class="sy1"><</span>To<span class="sy1">></span><span class="br0">(</span>f<span class="br0">(</span><span class="br0">)</span><span class="br0">)</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 <span class="t-c"><span class="mw-geshi cpp source-cpp">ConvertibleTo<span class="sy1"><</span>From, To<span class="sy1">></span></span></span> 指定拥有 <code>From</code> 所指定的类型和值类别的表达式能隐式和显式转换为类型 <code>To</code> ，且二种转换形式等价。</p>
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<p>具体而言， <span class="t-c"><span class="mw-geshi cpp source-cpp">ConvertibleTo<span class="sy1"><</span>From, To<span class="sy1">></span></span></span> 仅若满足以下条件才得到满足。给定虚设函数 <span class="t-c"><span class="mw-geshi cpp source-cpp">To test<span class="br0">(</span>From <span class="br0">(</span><span class="sy3">&</span>f<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp">From <span class="br0">(</span><span class="br0">)</span></span></span> 类型函数 <code>F</code> ，使得表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">F<span class="br0">(</span><span class="br0">)</span></span></span> 保持相等性（见后述），而</p>
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<ul>
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<li>以下之一
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<ul>
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<li><code>To</code> 既非对象类型亦非到对象引用类型，或</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1"><</span>To<span class="sy1">></span><span class="br0">(</span>f<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 等于 <code>test(f)</code> ，且</li>
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</li>
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</ul>
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<ul>
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<li>下列之一为真：
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<li><code>From</code> 不是到对象引用类型，或</li>
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<li><code>From</code> 为到非 const 限定的右值引用类型，且 <span class="t-c"><span class="mw-geshi cpp source-cpp">f<span class="br0">(</span><span class="br0">)</span></span></span> 所引用对象的结果状态合法，但在任一上述表达式后未指定；或</li>
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<li><code>f()</code> 所指代的对象不为任一上述表达式所修改。</li>
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</ul>
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</li>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp">ConvertibleTo<span class="sy1"><</span>From, To<span class="sy1">></span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw592">std::<span class="me2">is_convertible</span></span></a><span class="sy1"><</span>From, To<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=".E5.8F.82.E9.98.85">参阅</span></h3>
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<div><a href="cpp-types-is_convertible.html"><span class="t-lines"><span>is_convertible</span><span>is_nothrow_convertible</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-cxx20">(C++20)</span></span></span></div>
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</td>
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<td>检查是否能转换一个类型为另一类型<br />
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<span class="t-mark">(类模板)</span></td>
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