277 lines
41 KiB
HTML
277 lines
41 KiB
HTML
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<title>结构化绑定声明 (C++17 起)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_structured_binding skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">结构化绑定声明 <span class="t-mark-rev t-since-cxx17">(C++17 起)</span></h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-language.html">language</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<p>绑定指定名称到初始化器的子对象或元素。</p>
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<p>类似引用，结构化绑定是既存对象的别名。不同于引用的是，结构化绑定的类型不必为引用类型。</p>
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<table class="t-sdsc-begin">
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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td><span class="t-spar">属性</span><span class="t-mark">(可选)</span> <span class="t-spar">cv-auto</span> <span class="t-spar">引用运算符</span><span class="t-mark">(可选)</span> <code><b>[</b></code> <span class="t-spar">标识符列表</span> <code><b>]</b></code> <code><b>=</b></code> <span class="t-spar">表达式</span> <code><b>;</b></code></td>
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<td>(1)</td>
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<td class="t-sdsc-nopad"></td>
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</tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td><span class="t-spar">属性</span><span class="t-mark">(可选)</span> <span class="t-spar">cv-auto</span> <span class="t-spar">引用运算符</span><span class="t-mark">(可选)</span> <code><b>[</b></code> <span class="t-spar">标识符列表</span> <code><b>]</b></code> <code><b>{</b></code> <span class="t-spar">表达式</span> <code><b>}</b></code> <code><b>;</b></code></td>
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<td>(2)</td>
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<td class="t-sdsc-nopad"></td>
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<td colspan="10" class="t-sdsc-sep"></td>
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<td><span class="t-spar">属性</span><span class="t-mark">(可选)</span> <span class="t-spar">cv-auto</span> <span class="t-spar">引用运算符</span><span class="t-mark">(可选)</span> <code><b>[</b></code> <span class="t-spar">标识符列表</span> <code><b>]</b></code> <code><b>(</b></code> <span class="t-spar">表达式</span> <code><b>)</b></code> <code><b>;</b></code></td>
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<td>(3)</td>
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<td class="t-sdsc-nopad"></td>
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<td colspan="10" class="t-sdsc-sep"></td>
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</table>
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<table class="t-par-begin">
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<tr class="t-par">
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<td><span class="t-spar">属性</span></td>
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<td>-</td>
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<td>任意数量的<a href="cpp-language-attributes.html">属性</a>的序列</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">cv-auto</span></td>
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<td>-</td>
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<td>可有 cv 限定的 <code><b>auto</b></code> 类型说明符<span class="t-rev-inl t-since-cxx20"><span>，也可以包含<a href="cpp-language-storage_duration.html">存储类说明符</a> <code><b>static</b></code> 或 <code><b>thread_local</b></code> ；在 cv 限定符中包含 <code><b>volatile</b></code> 是被弃用的</span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span></td>
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<tr class="t-par">
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<td><span class="t-spar">引用运算符</span></td>
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<td>-</td>
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<td><code><b>&</b></code> 或 <code><b>&&</b></code> 之一</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">标识符列表</span></td>
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<td>-</td>
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<td>此声明所引入的各标识符的逗号分隔的列表</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">表达式</span></td>
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<td>-</td>
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<td>顶层没有逗号运算符的表达式（文法上为<i>赋值表达式</i>），且具有数组或非联合类之一的类型。如果 <span class="t-spar">表达式</span> 涉及任何来自 <span class="t-spar">标识符列表</span> 的名字，则声明非良构。</td>
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</tr>
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</table>
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<p>结构化绑定声明将 <span class="t-spar">标识符列表</span> 中的所有标识符引入作为其外围作用域中的名字，并将它们绑定到 <span class="t-spar">表达式</span> 所指代的对象的各个子对象或元素。以此方式引入的绑定被称作<i>结构化绑定</i>。</p>
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<p>结构化绑定声明首先引入一个唯一命名的变量（此处以 <code><b><i>e</i></b></code> 指代）来保有其初始化器的值，方式如下：</p>
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<ul>
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<li>如果 <span class="t-spar">表达式</span> 具有数组类型 <code>A</code> 且不存在 <span class="t-spar">引用运算符</span>，则 <code><b><i>e</i></b></code> 具有类型 <i>cv</i> <code>A</code>，其中 <i>cv</i> 是 <span class="t-spar">cv-auto</span> 序列中的 cv 限定符，且 <code><b><i>e</i></b></code> 中的各个元素从 <span class="t-spar">表达式</span> 的对应元素进行复制（对于 <span class="t-v">(1)</span>）或直接（对于 <span class="t-v">(2,3)</span>）初始化。</li>
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<li>否则 <code><b><i>e</i></b></code> 如同于声明中以其名取代 <code><b>[</b></code> <span class="t-spar">标识符列表</span> <code><b>]</b></code> 一般定义。</li>
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</ul>
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<p>我们用 <code><b><i>E</i></b></code> 代表表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 的类型。（换言之，<code><b><i>E</i></b></code> 等价于 <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>decltype<span class="br0">(</span><span class="br0">(</span>e<span class="br0">)</span><span class="br0">)</span><span class="sy1">></span></span></span>）。</p>
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<p>然后，结构化绑定可以以下三种方式之一进行绑定，取决于 <code><b><i>E</i></b></code>：</p>
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<ul>
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<li>情况 1：如果 <code><b><i>E</i></b></code> 是数组类型，则绑定各个名字到各个数组元素。</li>
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<li>情况 2：如果 <code><b><i>E</i></b></code> 是非联合类类型且 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple-tuple_size.html"><span class="kw1101">std::<span class="me2">tuple_size</span></span></a><span class="sy1"><</span>E<span class="sy1">></span></span></span> 是完整类型并拥有名为 <code>value</code> 的成员（无关乎这种成员的类型或可访问性），则使用“元组式”绑定协议。</li>
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<li>情况 3：如果 <code><b><i>E</i></b></code> 是非联合类类型但 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple-tuple_size.html"><span class="kw1101">std::<span class="me2">tuple_size</span></span></a><span class="sy1"><</span>E<span class="sy1">></span></span></span> 不是完整类型，则绑定各个名字到 <code><b><i>E</i></b></code> 的各个可访问数据成员。</li>
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</ul>
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<p>下文对每种情况都有更详细的描述。</p>
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<p>每个结构化绑定都有一个<i>被引用类型</i>，它会在后文的描述中被定义。此类型是对无括号的结构化绑定应用 <a href="cpp-language-decltype.html"><code>decltype</code></a> 所返回的类型。</p>
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<h4><span class="mw-headline" id=".E6.83.85.E5.86.B5_1.EF.BC.9A.E7.BB.91.E5.AE.9A.E6.95.B0.E7.BB.84">情况 1：绑定数组</span></h4>
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<p><span class="t-spar">标识符列表</span> 中的每个标识符均成为指代数组的对应元素的左值。标识符的数量必须等于数组的元素数量。</p>
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<p>每个标识符的<i>被引用类型</i>都是数组的元素类型。数组类型 <code><b><i>E</i></b></code> 的 cv 限定性与其元素的相同。</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw4">int</span> a<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>,<span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> <span class="br0">[</span>x,y<span class="br0">]</span> <span class="sy1">=</span> a<span class="sy4">;</span> <span class="co1">// 创建 e[2]，复制 a 到 e，然后 x 指代 e[0]，y 指代 e[1]</span>
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<span class="kw4">auto</span><span class="sy3">&</span> <span class="br0">[</span>xr, yr<span class="br0">]</span> <span class="sy1">=</span> a<span class="sy4">;</span> <span class="co1">// xr 指代 a[0]，yr 指代 a[1]</span></pre></div>
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</div>
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<h4><span class="mw-headline" id=".E6.83.85.E5.86.B5_2.EF.BC.9A.E7.BB.91.E5.AE.9A.E5.85.83.E7.BB.84.E5.BC.8F.E7.B1.BB.E5.9E.8B">情况 2：绑定元组式类型</span></h4>
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<p>表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple-tuple_size.html"><span class="kw1101">std::<span class="me2">tuple_size</span></span></a><span class="sy1"><</span>E<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span></span></span> 必须是良构的整数常量表达式，且标识符的数量必须等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple-tuple_size.html"><span class="kw1101">std::<span class="me2">tuple_size</span></span></a><span class="sy1"><</span>E<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span></span></span>。</p>
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<p>对于每个标识符，引入一个类型为“<span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple-tuple_element.html"><span class="kw1099">std::<span class="me2">tuple_element</span></span></a><span class="sy1"><</span>i, E<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span></span></span> 的引用”的变量：如果它对应的初始化器是左值，那么它是左值引用，否则它是右值引用。第 i 个变量的初始化器</p>
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<ul>
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<li>如果在 <code><b><i>E</i></b></code> 的作用域中对标识符 <code>get</code> 按类成员访问进行的查找中，至少找到一个声明是首个模板形参为非类型形参的函数模板，则为 <span class="t-c"><span class="mw-geshi cpp source-cpp">e.<span class="me1">get</span><span class="sy1"><</span>i<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span></span></span></li>
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<li>否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp">get<span class="sy1"><</span>i<span class="sy1">></span><span class="br0">(</span>e<span class="br0">)</span></span></span>，其中 <span class="t-c"><span class="mw-geshi cpp source-cpp">get</span></span> 只进行<a href="cpp-language-adl.html">实参依赖查找</a>，忽略其他查找。</li>
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</ul>
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<p>这些初始化器表达式中，如果实体 <code><b><i>e</i></b></code> 的类型是左值引用（只会在 <span class="t-spar">引用运算符</span> 是 <code><b>&</b></code>，或它是 <code><b>&&</b></code> 且初始化器是左值时发生），那么 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 为左值，否则它是亡值（这实际上进行了一种完美转发），<span class="t-c"><span class="mw-geshi cpp source-cpp">i</span></span> 是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a></span></span> 的纯右值，而且始终将 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1"><</span>i<span class="sy1">></span></span></span> 解释为模板形参列表。</p>
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<p>该变量拥有与 <code><b><i>e</i></b></code> 相同的<a href="cpp-language-storage_duration.html">存储期</a>。</p>
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<p>然后该标识符变成指代与上述变量绑定的对象的左值。</p>
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<p>第 i 个标识符的<i>被引用类型</i>是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple-tuple_element.html"><span class="kw1099">std::<span class="me2">tuple_element</span></span></a><span class="sy1"><</span>i, E<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span></span></span>。</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw4">float</span> x<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">char</span> y<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">int</span> z<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span><span class="kw4">float</span><span class="sy3">&</span>,<span class="kw4">char</span><span class="sy3">&&</span>,<span class="kw4">int</span><span class="sy1">></span> tpl<span class="br0">(</span>x,std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>y<span class="br0">)</span>,z<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&</span> <span class="br0">[</span>a,b,c<span class="br0">]</span> <span class="sy1">=</span> tpl<span class="sy4">;</span>
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<span class="co1">// a 指名指代 x 的结构化绑定；decltype(a) 为 float&</span>
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<span class="co1">// b 指名指代 y 的结构化绑定；decltype(b) 为 char&&</span>
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<span class="co1">// c 指名指代 tpl 的第 3 元素的结构化绑定；decltype(c) 为 const int</span></pre></div>
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</div>
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<h4><span class="mw-headline" id=".E6.83.85.E5.86.B5_3.EF.BC.9A.E7.BB.91.E5.AE.9A.E5.88.B0.E6.95.B0.E6.8D.AE.E6.88.90.E5.91.98">情况 3：绑定到数据成员</span></h4>
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<p><code><b><i>E</i></b></code> 的所有非静态数据成员必须都是 <code><b><i>E</i></b></code> 或 <code><b><i>E</i></b></code> 的同一基类的直接成员，必须在指名为 <code><i><b>e</b></i>.<i>name</i></code> 时于结构化绑定的语境中是良构的。<code><b><i>E</i></b></code> 不能有匿名联合体成员。标识符的数量必须等于非静态数据成员的数量。</p>
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<p><span class="t-spar">标识符列表</span> 中的各个标识符，按声明顺序依次成为指代 <code><b><i>e</i></b></code> 的各个成员的左值的名字（支持位域）；该左值的类型是 <span class="t-c"><span class="mw-geshi cpp source-cpp">e.<span class="me1">m_i</span></span></span> 的类型，其中 <code>m_i</code> 指代第 i 个成员。</p>
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<p>第 i 个标识符的<i>被引用类型</i>是 <span class="t-c"><span class="mw-geshi cpp source-cpp">e.<span class="me1">m_i</span></span></span> ，如果它不是引用类型；否则是 <code>m_i</code> 的声明类型。</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
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mutable <span class="kw4">int</span> x1 <span class="sy4">:</span> <span class="nu0">2</span><span class="sy4">;</span>
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<span class="kw4">volatile</span> <span class="kw4">double</span> y1<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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S f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">auto</span> <span class="br0">[</span>x, y<span class="br0">]</span> <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// x 是标识 2 位位域的 int 左值</span>
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<span class="co1">// y 是 const volatile double 左值</span></pre></div>
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</div>
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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>对成员 <code>get</code> 的查找照常忽略可访问性，同时也忽略非类型模板形参的确切类型。出现私有的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span><span class="kw4">char</span><span class="sy2">*</span><span class="sy1">></span> <span class="kw4">void</span> get<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span> 成员将导致使用成员解释方案，即使这会导致程序非良构。</p>
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<p>声明中 <code><b>[</b></code> 之前的部分应用于隐藏变量 <code><b><i>e</i></b></code>，而非引入的各个标识符。</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">1</span>, b <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&</span> <span class="br0">[</span>x, y<span class="br0">]</span> <span class="sy1">=</span> <a href="cpp-utility-tuple-tie.html"><span class="kw1113">std::<span class="me2">tie</span></span></a><span class="br0">(</span>a, b<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// x 与 y 类型为 int&</span>
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<span class="kw4">auto</span> <span class="br0">[</span>z, w<span class="br0">]</span> <span class="sy1">=</span> <a href="cpp-utility-tuple-tie.html"><span class="kw1113">std::<span class="me2">tie</span></span></a><span class="br0">(</span>a, b<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// z 与 w 类型仍为 int&</span>
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<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span><span class="sy3">&</span>z <span class="sy1">==</span> <span class="sy3">&</span>a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 通过</span></pre></div>
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</div>
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<p>如果 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple-tuple_size.html"><span class="kw1101">std::<span class="me2">tuple_size</span></span></a><span class="sy1"><</span>E<span class="sy1">></span></span></span> 是完整类型，那么始终使用元组式解释方案，即使这会导致程序非良构：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span> <span class="kw4">int</span> x<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">namespace</span> std <span class="br0">{</span>
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<span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">struct</span> tuple_size<span class="sy1"><</span><span class="sy4">::</span><span class="me2">A</span><span class="sy1">></span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">auto</span> <span class="br0">[</span>x<span class="br0">]</span> <span class="sy1">=</span> A<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 错误；不考虑“数据成员”解释方案。</span></pre></div>
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</div>
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<p>如果存在 <span class="t-spar">引用运算符</span> 且 <span class="t-spar">表达式</span> 为纯右值，则应用将引用绑定到临时量的通常规则（包括生存期延续）。这些情况下，隐藏变量 <code><b><i>e</i></b></code> 是绑定到从纯右值表达式<a href="cpp-language-implicit_conversion.html#.E4.B8.B4.E6.97.B6.E9.87.8F.E5.AE.9E.E8.B4.A8.E5.8C.96">实质化</a>的临时变量，并延长其生存期的一个引用。与之前一样，如果 <code><b><i>e</i></b></code> 是非 const 左值引用，则绑定失败：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&</span> <span class="br0">[</span>x<span class="br0">]</span> <span class="sy1">=</span> <a href="cpp-utility-tuple-make_tuple.html"><span class="kw1112">std::<span class="me2">make_tuple</span></span></a><span class="br0">(</span>a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK，非悬垂引用</span>
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<span class="kw4">auto</span><span class="sy3">&</span> <span class="br0">[</span>y<span class="br0">]</span> <span class="sy1">=</span> <a href="cpp-utility-tuple-make_tuple.html"><span class="kw1112">std::<span class="me2">make_tuple</span></span></a><span class="br0">(</span>a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误，不能绑定 auto& 到右值 std::tuple</span>
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<span class="kw4">auto</span><span class="sy3">&&</span> <span class="br0">[</span>z<span class="br0">]</span> <span class="sy1">=</span> <a href="cpp-utility-tuple-make_tuple.html"><span class="kw1112">std::<span class="me2">make_tuple</span></span></a><span class="br0">(</span>a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 同样 OK</span></pre></div>
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</div>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span>x<span class="br0">)</span></span></span> 指名结构化绑定的<i>被引用类型</i>，其中 <code>x</code> 代表一个结构化绑定。在元组式的情况下，它是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple-tuple_element.html"><span class="kw1099">std::<span class="me2">tuple_element</span></span></a></span></span> 所返回的类型，它可以不是引用，即使在此情况下始终会引入隐藏的引用。这相当于模拟了绑定到其各个非静态数据成员具有 <code>tuple_element</code> 所返回的类型的结构体的行为，而绑定自身的引用性质则只是实现细节。</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="sy3">&</span><span class="sy1">></span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> <span class="br0">[</span>x, y<span class="br0">]</span> <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// decltype(x) 为 int</span>
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<span class="co1">// decltype(y) 为 int&</span>
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<span class="kw4">const</span> <span class="kw4">auto</span> <span class="br0">[</span>z, w<span class="br0">]</span> <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// decltype(z) 为 const int</span>
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<span class="co1">// decltype(w) 为 int&</span></pre></div>
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</div>
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<table class="t-rev-begin">
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<tr class="t-rev t-until-cxx20">
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<td>
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<p><a href="cpp-language-lambda.html">Lambda 表达式</a>不能捕获结构化绑定。</p>
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</td>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <set></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <iomanip></span>
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|
<span class="co2">#include <iostream></span>
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|
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|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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|
<a href="cpp-container-set.html"><span class="kw1277">std::<span class="me2">set</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> myset<span class="br0">{</span><span class="st0">"hello"</span><span class="br0">}</span><span class="sy4">;</span>
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|
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i<span class="br0">{</span><span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span> i<span class="sy4">;</span> <span class="sy2">--</span>i<span class="br0">)</span> <span class="br0">{</span>
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|
<span class="kw1">if</span> <span class="br0">(</span><span class="kw4">auto</span> <span class="br0">[</span>iter, success<span class="br0">]</span> <span class="sy1">=</span> myset.<span class="me1">insert</span><span class="br0">(</span><span class="st0">"Hello"</span><span class="br0">)</span><span class="sy4">;</span> success<span class="br0">)</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"插入成功。值为 "</span> <span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1802">std::<span class="me2">quoted</span></span></a><span class="br0">(</span><span class="sy2">*</span>iter<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"。<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<span class="kw1">else</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"集合中已存在值 "</span> <span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1802">std::<span class="me2">quoted</span></span></a><span class="br0">(</span><span class="sy2">*</span>iter<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"。<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<span class="kw1">struct</span> BitFields <span class="br0">{</span>
|
|
<span class="kw4">int</span> b <span class="sy4">:</span> <span class="nu0">4</span> <span class="br0">{</span><span class="nu0">1</span><span class="br0">}</span>, d <span class="sy4">:</span> <span class="nu0">4</span> <span class="br0">{</span><span class="nu0">2</span><span class="br0">}</span>, p <span class="sy4">:</span> <span class="nu0">4</span> <span class="br0">{</span><span class="nu0">3</span><span class="br0">}</span>, q <span class="sy4">:</span> <span class="nu0">4</span> <span class="br0">{</span><span class="nu0">4</span><span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="co1">// C++20 ：位域的默认成员初始化器</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw4">const</span> <span class="kw4">auto</span> <span class="br0">[</span>b, d, p, q<span class="br0">]</span> <span class="sy1">=</span> BitFields<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> b <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> d <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> p <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> q <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="br0">{</span>
|
|
<span class="kw4">const</span> <span class="kw4">auto</span> <span class="br0">[</span>b, d, p, q<span class="br0">]</span> <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">{</span> <span class="kw1">return</span> BitFields<span class="br0">{</span><span class="nu0">4</span>, <span class="nu0">3</span>, <span class="nu0">2</span>, <span class="nu0">1</span><span class="br0">}</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> b <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> d <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> p <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> q <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">插入成功。值为 "Hello"。
|
|
集合中已存在值 "Hello"。
|
|
1 2 3 4
|
|
4 3 2 1</pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
|
|
<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
|
|
<table class="dsctable" style="font-size:0.8em">
|
|
<tr>
|
|
<th>DR</th>
|
|
<th>应用于</th>
|
|
<th>出版时的行为</th>
|
|
<th>正确行为</th>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.link/P0961R1">P0961R1</a></td>
|
|
<td>C++17</td>
|
|
<td>元组式情况中，如果查找找到任何类型的 <code>get</code> 则使用成员 <code>get</code></td>
|
|
<td>仅当查找找到拥有非类型模板形参的函数模板才使用</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.link/P0969R0">P0969R0</a></td>
|
|
<td>C++17</td>
|
|
<td>绑定到成员情况中，要求这些成员公开</td>
|
|
<td>只需要在声明的语境中可访问</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue2312">CWG 2312</a></td>
|
|
<td>C++17</td>
|
|
<td><code>mutable</code> 的含义在绑定到成员情况中丢失</td>
|
|
<td>保持其含义</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue2386">CWG 2386</a></td>
|
|
<td>C++17</td>
|
|
<td>总是在 <code>tuple_size<E></code> 为完整类型时使用“元组式”绑定协议</td>
|
|
<td>只会在 <code>tuple_size<E></code> 拥有成员 <code>value</code> 时才使用</td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<table class="t-dsc-begin">
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-utility-tuple-tie.html"><span class="t-lines"><span>tie</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>创建左值引用的 <code>tuple</code>，或将 tuple 解包为独立对象<br />
|
|
<span class="t-mark">(函数模板)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
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</body>
|
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</html>
|