114 lines
11 KiB
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114 lines
11 KiB
HTML
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<title>std::ranges::view_interface<D>::operator bool</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_view_interface_operator_bool 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::ranges::view_interface<D>::</span>operator bool</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-ranges.html">ranges</a>‎ | <a href="cpp-ranges-view_interface.html">view interface</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span> <span class="kw4">constexpr</span> operator <span class="kw4">bool</span><span class="br0">(</span><span class="br0">)</span> requires <span class="coMULTI">/* see below */</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span> <span class="kw4">constexpr</span> operator <span class="kw4">bool</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> requires <span class="coMULTI">/* see below */</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp">operator <span class="kw4">bool</span></span></span> 成员函数的默认实现检查视图是否为非空。它使得派生类型<a href="cpp-language-implicit_conversion.html#.E6.8C.89.E8.AF.AD.E5.A2.83.E8.BD.AC.E6.8D.A2">可按语境转换成 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span></a> 。</p>
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<div class="t-li1"><span class="t-li">1)</span> 令 <code>derived</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1"><</span>D<span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span></span></span> 。 requires 子句中的表达式等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">requires <span class="br0">{</span> ranges<span class="sy4">::</span><span class="me2">empty</span><span class="br0">(</span>derived<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span></span></span> ，而函数体等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> ranges<span class="sy4">::</span><span class="me2">empty</span><span class="br0">(</span>derived<span class="br0">)</span><span class="sy4">;</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> ，除了 <code>derived</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">const</span> D<span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span></span></span> 。</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<p>（无）</p>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>若派生类型的值为空（由 <span class="t-lc">std::ranges::empty</span> 确定）则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> ，否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</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>C++20 中，标准库中无派生自 <span class="t-lc">std::ranges::view_interface</span> 的类型提供其自身的 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator <span class="kw4">bool</span></span></span> 。几乎所有这些类型都使用默认实现。</p>
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<p>一个值得注意的例外是 <span class="t-lc">std::ranges::basic_istream_view</span> 。由于其迭代器类型决不满足 <a href="cpp-iterator-forward_iterator.html"><code>forward_iterator</code></a> ，故视图不能使用继承的 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator <span class="kw4">bool</span></span></span> 。</p>
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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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <array></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <ranges></span>
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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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<span class="kw4">const</span> <a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a> ints <span class="br0">{</span><span class="nu0">0</span>, <span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span>, <span class="nu0">4</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> odds <span class="sy1">=</span> ints <span class="sy3">|</span> std<span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">filter</span><span class="br0">(</span><span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">0</span> <span class="sy3">!</span><span class="sy1">=</span> i <span class="sy2">%</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> negs <span class="sy1">=</span> ints <span class="sy3">|</span> std<span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">filter</span><span class="br0">(</span><span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> i <span class="sy1"><</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</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> <a href="cpp-io-manip-boolalpha.html"><span class="kw1764">std::<span class="me2">boolalpha</span></span></a>
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<span class="sy1"><<</span> <span class="st0">"Has odd numbers: "</span> <span class="sy1"><<</span> <span class="br0">(</span><span class="sy3">!!</span>odds<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"Has negative numbers: "</span> <span class="sy1"><<</span> <span class="br0">(</span><span class="sy3">!!</span>negs<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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<p>输出：</p>
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<div class="text source-text">
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<pre class="de1">Has odd numbers: true
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Has negative numbers: false</pre></div>
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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-ranges-empty.html"><span class="t-lines"><span>ranges::empty</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>检查范围是否为空<br />
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<span class="t-mark">(定制点对象)</span></td>
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<div><a href="cpp-ranges-view_interface-empty.html"><span class="t-lines"><span>empty</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回视图是否为空。若视图满足 <a href="cpp-ranges-forward_range.html"><code>forward_range</code></a> 则提供。<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div><a href="cpp-iterator-empty.html"><span class="t-lines"><span>empty</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>检查容器是否为空<br />
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<span class="t-mark">(函数模板)</span></td>
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