98 lines
10 KiB
HTML
98 lines
10 KiB
HTML
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<title>std::convertible_to</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_concepts_convertible_to 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>convertible_to</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> From, <span class="kw1">class</span> To<span class="sy1">></span><br /></span>
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<p>concept convertible_to <span class="sy1">=</span><br />
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<a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span>From, To<span class="sy1">></span> <span class="sy3">&&</span><br />
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requires <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><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>From<span class="sy1">></span><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-cxx20">(C++20 起)</span></td>
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<p>概念 <span class="t-c"><span class="mw-geshi cpp source-cpp">convertible_to<span class="sy1"><</span>From, To<span class="sy1">></span></span></span> 指定类型和值类别均与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>From<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span></span></span> 相同的表达式能隐式和显式转换成 <code>To</code> 类型，而二种转换形式等价。</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><span class="t-c"><span class="mw-geshi cpp source-cpp">convertible_to<span class="sy1"><</span>From, To<span class="sy1">></span></span></span> 仅若符合下列条件才得以实现。给定 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-add_reference.html"><span class="kw611">std::<span class="me2">add_rvalue_reference_t</span></span></a><span class="sy1"><</span>From<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span></span></span> 类型函数 <code>fun</code> ，满足表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">fun<span class="br0">(</span><span class="br0">)</span></span></span> 保持相等性（见下方），</p>
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<li>以下任一成立
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<li><code>To</code> 既非对象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>fun<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> To <span class="br0">{</span> <span class="kw1">return</span> fun<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">(</span><span class="br0">)</span></span></span> ，且</li>
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<li>下列之一为真：
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-add_reference.html"><span class="kw611">std::<span class="me2">add_rvalue_reference_t</span></span></a><span class="sy1"><</span>From<span class="sy1">></span></span></span> 不是到对象引用类型，或</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-add_reference.html"><span class="kw611">std::<span class="me2">add_rvalue_reference_t</span></span></a><span class="sy1"><</span>From<span class="sy1">></span></span></span> 是到非 const 限定类型的右值引用，而 <span class="t-c"><span class="mw-geshi cpp source-cpp">fun<span class="br0">(</span><span class="br0">)</span></span></span> 所引用的结果状态在上述任一表达式后合法但未指定；或</li>
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<li>上述任一表达式不修改 <span class="t-c"><span class="mw-geshi cpp source-cpp">fun<span class="br0">(</span><span class="br0">)</span></span></span> 所引用的对象。</li>
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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=".E5.8F.82.E9.98.85">参阅</span></h3>
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<div class="t-dsc-member-div">
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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>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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