150 lines
13 KiB
HTML
150 lines
13 KiB
HTML
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<title>std::experimental::ranges::less</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_experimental_ranges_functional_less 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>less</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-functional.html"><experimental/ranges/functional></a></code></div>
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<td></td>
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<tr class="t-dcl t-since-ranges">
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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="sy1">=</span> <span class="kw4">void</span> <span class="sy1">></span><br /></span>
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<p> requires <a href="cpp-experimental-ranges-concepts-StrictTotallyOrdered.html"><span class="kw2545">ranges::<span class="me2">StrictTotallyOrdered</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> <span class="sy3">||</span><br />
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<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="kw4">void</span><span class="sy1">></span> <span class="sy3">||</span><br />
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<span class="coMULTI">/* 二个 const T 左值上的 < 调用比较指针的内建运算符 */</span><br /></p>
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<span class="kw1">struct</span> less<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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><></span><br />
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<span class="kw1">struct</span> less<span class="sy1"><</span><span class="kw4">void</span><span class="sy1">></span><span class="sy4">;</span></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>进行比较的函数对象。初等模板在 <code>T</code> 类型左值上调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1"><</span></span></span> 。特化 <code>less<void></code> 从实参推导函数调用运算符的形参类型（但非返回类型）。</p>
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<p><code>less</code> 的所有特化均为 <a href="cpp-experimental-ranges-concepts-Semiregular.html"><code>Semiregular</code></a> 。</p>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">成员类型</span></h3>
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<td>成员类型</td>
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<td>定义</td>
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<td><code>is_transparent</code> <span class="t-mark">(仅为 <code>less<void></code> 特化的成员)</span></td>
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<td>/* 未指定 */</td>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
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<div><span class="t-lines"><span>operator()</span></span></div>
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<td>检查第一参数是否<i>小于</i>第二个<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<h2><span class="mw-headline" id="std::experimental::ranges::less::operator.28.29"><span style="font-size:0.7em; line-height:130%">std::experimental::ranges::less::</span>operator()</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span> x, <span class="kw4">const</span> T<span class="sy3">&</span> y<span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
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<td><span class="t-mark">(仅为初等 <code>less<T></code> 模板的成员)</span></td>
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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> requires <a href="cpp-experimental-ranges-concepts-StrictTotallyOrdered.html"><span class="kw2546">ranges::<span class="me2">StrictTotallyOrderedWith</span></span></a><span class="sy1"><</span>T, U<span class="sy1">></span> <span class="sy3">||</span><br />
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<span class="coMULTI">/* std::declval<T>() < std::declval<U>()<br />
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解析成比较指针的内建运算符 */</span><br /></p>
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>T<span class="sy3">&&</span> t, U<span class="sy3">&&</span> u<span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></div>
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<td>(2)</td>
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<td><span class="t-mark">(仅为 <code>less<void></code> 特化的成员)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 比较 <code>x</code> 与 <code>y</code> 。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">less</span><span class="sy1"><></span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>x, y<span class="br0">)</span></span></span> ；</div>
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<div class="t-li1"><span class="t-li">2)</span> 比较 <code>t</code> 与 <code>u</code> ，等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> <a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span> <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="sy4">;</span></span></span> ，除非该表达式解析成调用比较指针的内建 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1"><</span></span></span> 。</div>
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<p>当调用 <span class="t-v">(1)</span> 或 <span class="t-v">(2)</span> 会调用比较 <code>P</code> 类型指针的内建运算符时，替而按下列方式确定结果：</p>
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<ul>
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<li>若在所有 <code>P</code> 类型指针值上的实现定义的严格全序中，第一参数的（可能转换的）值先于第二参数的（可能转换的）值，则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。此严格全序与内建运算符 <code><</code> 、 <code>></code> 、 <code><=</code> 及 <code>>=</code> 所蕴含的偏序一致。</li>
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<li>否则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</li>
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<p>除非从 <code>T</code> 和 <code>U</code> 到 <code>P</code> 的转换序列均保持相等性（见后述），否则行为未定义。</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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<ul>
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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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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p>不同于 <span class="t-lc"><a href="cpp-utility-functional-less.html">std::less</a></span> ， <code>ranges::less</code> 要求六个比较运算符 <code><</code> 、 <code><=</code> 、 <code>></code> 、 <code>>=</code> 、 <code>==</code> 和 <code>!=</code> 均合法（通过 <a href="cpp-experimental-ranges-concepts-StrictTotallyOrdered.html"><code>StrictTotallyOrdered</code></a> 与 <a href="cpp-experimental-ranges-concepts-StrictTotallyOrdered.html"><code>StrictTotallyOrderedWith</code></a> 制约）。</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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<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
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<td class="mbox-text" style="">本节未完成<br />
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原因：暂无示例</td>
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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-utility-functional-less.html"><span class="t-lines"><span>less</span></span></a></div>
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<td>实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">x <span class="sy1"><</span> y</span></span> 的函数对象<br />
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<span class="t-mark">(类模板)</span></td>
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