160 lines
22 KiB
HTML
160 lines
22 KiB
HTML
<!DOCTYPE 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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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<title>std::ranges::rend</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_rend skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::ranges::</span>rend</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-ranges.html">ranges</a></span></div>
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<table class="t-dcl-begin">
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<div>定义于头文件 <code><a href="cpp-header-ranges.html"><ranges></a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span> <span class="kw1">namespace</span> <span class="coMULTI">/*unspecified*/</span> <span class="br0">{</span><br /></span>
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<p> <span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="coMULTI">/*unspecified*/</span> rend <span class="sy1">=</span> <span class="coMULTI">/*unspecified*/</span><span class="sy4">;</span><br /></p>
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<span class="br0">}</span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span><br />
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<span class="t-mark">(定制点对象)</span></td>
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<div>调用签名</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> T <span class="sy1">></span><br /></span>
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<p> requires <span class="coMULTI">/* see below */</span><br /></p>
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<span class="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">sentinel_for</span><span class="sy1"><</span>decltype<span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">rbegin</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>T<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy1">></span> <span class="kw4">auto</span> rend<span class="br0">(</span>T<span class="sy3">&&</span> t<span class="br0">)</span><span class="sy4">;</span></div>
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<p>获得指示逆向范围末尾的迭代器。</p>
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<div class="t-image">
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<p><img alt="range-rbegin-rend.svg" src="range-rbegin-rend.svg" width="589" height="250" /></p>
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<p>令 <code>t</code> 为 <code>T</code> 类型对象。若实参为左值或 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">enable_borrowed_range</span><span class="sy1"><</span><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span>T<span class="sy1">>></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，则对 <code>ranges::rend</code> 的调用表达式等价于：</p>
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<ol>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><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="me1">rend</span><span class="br0">(</span><span class="br0">)</span></span></span> 转换到其<a href="cpp-types-decay.html">衰变类型</a>，若该带转换的表达式合法，且其转换后的类型实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">sentinel_for</span><span class="sy1"><</span>decltype<span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">rbegin</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>T<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy1">></span></span></span> 。</li>
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<li>否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp">rend<span class="br0">(</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="br0">)</span></span></span> 转换到其<a href="cpp-types-decay.html">衰变类型</a>，若 <code>T</code> 为类或枚举类型，带转换的前述无限定调用合法，且其转换后的类型实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">sentinel_for</span><span class="sy1"><</span>decltype<span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">rbegin</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>T<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy1">></span></span></span> ，其中以下列候选进行<a href="cpp-language-overload_resolution.html">重载决议</a>：
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> rend<span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&</span><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> rend<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&</span><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span></span></span></li>
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<li><a href="cpp-language-adl.html">实参依赖查找</a>所找到的任何 <code>rend</code> 声明。</li>
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</ul>
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</li>
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<li>否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-make_reverse_iterator.html"><span class="kw663">std::<span class="me2">make_reverse_iterator</span></span></a><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</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="br0">)</span><span class="br0">)</span></span></span> ，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</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="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">end</span><span class="br0">(</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="br0">)</span></span></span> 均为合法表达式，拥有相同类型，且该类型实现 <span class="t-lc">std::bidirectional_iterator</span> 。</li>
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</ol>
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<p>所有其他情况下，调用 <code>ranges::rend</code> 为非良构，这能在 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">rend</span><span class="br0">(</span>t<span class="br0">)</span></span></span> 出现于模板实例化的立即语境时导致<a href="cpp-language-sfinae.html">替换失败</a>。</p>
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<h3><span class="mw-headline" id=".E8.A1.A8.E8.BE.BE.E5.BC.8F.E7.AD.89.E4.BB.B7">表达式等价</span></h3>
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<p>表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> <i>表达式等价</i>于表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> ，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> 拥有相同效果，均为<a href="cpp-language-noexcept_spec.html">潜在抛出</a>或均非潜在抛出（即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span><span class="br0">(</span>e<span class="br0">)</span> <span class="sy1">==</span> <span class="kw1">noexcept</span><span class="br0">(</span>f<span class="br0">)</span></span></span> ），且均为<a href="cpp-language-constant_expression.html">常量子表达式</a>或均非常量子表达式。</p>
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<h3><span class="mw-headline" id=".E5.AE.9A.E5.88.B6.E7.82.B9.E5.AF.B9.E8.B1.A1">定制点对象</span></h3>
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<p>名字 <code>ranges::rend</code> 代表一个<i>定制点对象</i>，它是<a href="cpp-named_req-LiteralType.html">字面</a> <a href="cpp-concepts-semiregular.html"><code>semiregular</code></a> 类类型的 const <a href="cpp-named_req-FunctionObject.html">函数对象</a>。为说明目的，以 <code><i>__rend_fn</i></code> 表示其类型的 cv 无限定版本。</p>
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<p><code><i>__rend_fn</i></code> 的所有实例均相等。在相同参数上调用类型 <code><i>__rend_fn</i></code> 的不同实例的效果是等价的，无关乎指代该实例的表达式是左值还是右值，以及是否为 const 限定（然而不要求 volatile 限定的实例可调用）。从而能自由地复制 <code>ranges::rend</code> 并且能彼此替代地使用其副本。</p>
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<p>给定类型集合 <code>Args...</code> ，若 <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>Args<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>...</span></span> 满足上面对于 <code>ranges::rend</code> 的参数要求，则 <code><i>__rend_fn</i></code> 实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span>__rend_fn, Args...<span class="sy1">></span></span></span>、 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span><span class="kw4">const</span> __rend_fn, Args...<span class="sy1">></span></span></span>、 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span>__rend_fn<span class="sy3">&</span>, Args...<span class="sy1">></span></span></span> 及 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span><span class="kw4">const</span> __rend_fn<span class="sy3">&</span>, Args...<span class="sy1">></span></span></span> 。否则， <code><i>__rend_fn</i></code> 的函数调用运算符不参与重载决议。</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>若实参为右值（即 <code>T</code> 为对象类型）且 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">enable_borrowed_range</span><span class="sy1"><</span><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span>T<span class="sy1">>></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> ，则调用 <code>ranges::rend</code> 非良构，这亦导致替换失败。</p>
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<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">rend</span><span class="br0">(</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="br0">)</span></span></span> 合法，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">rend</span><span class="br0">(</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="br0">)</span><span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</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="br0">)</span><span class="br0">)</span></span></span> 在所有情况下实现 <span class="t-lc">std::sentinel_for</span> ，同时 <code>T</code> 实现 <span class="t-lc">std::ranges::range</span> 。</p>
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<p>C++20 标准要求若底层 <code>rend</code> 函数调用返回纯右值，则从实质化的临时对象移动构造返回值。所有实现均直接返回纯右值。 C++20 后的提案 <a rel="nofollow" class="external text" href="https://wg21.link/P0849R8">P0849R8</a> 更正了该要求以匹配实现。</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="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;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <algorithm></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="co2">#include <vector></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v <span class="sy1">=</span> <span class="br0">{</span> <span class="nu0">3</span>, <span class="nu0">1</span>, <span class="nu0">4</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">namespace</span> ranges <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">find</span><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">rbegin</span><span class="br0">(</span>v<span class="br0">)</span>, ranges<span class="sy4">::</span><span class="me2">rend</span><span class="br0">(</span>v<span class="br0">)</span>, <span class="nu0">5</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> ranges<span class="sy4">::</span><span class="me2">rend</span><span class="br0">(</span>v<span class="br0">)</span><span class="br0">)</span> <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">"found a 5 in vector 'v'!<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">int</span> a<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span> <span class="nu0">5</span>, <span class="nu0">10</span>, <span class="nu0">15</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">find</span><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">rbegin</span><span class="br0">(</span>a<span class="br0">)</span>, ranges<span class="sy4">::</span><span class="me2">rend</span><span class="br0">(</span>a<span class="br0">)</span>, <span class="nu0">5</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> ranges<span class="sy4">::</span><span class="me2">rend</span><span class="br0">(</span>a<span class="br0">)</span><span class="br0">)</span> <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">"found a 5 in array 'a'!<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">found a 5 in array 'a'!</pre></div>
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</div>
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</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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<table class="t-dsc-begin">
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-ranges-crend.html"><span class="t-lines"><span>ranges::crend</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>返回指向只读范围的逆向尾迭代器<br />
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<span class="t-mark">(定制点对象)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-ranges-rbegin.html"><span class="t-lines"><span>ranges::rbegin</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>返回指向范围的逆向迭代器<br />
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<span class="t-mark">(定制点对象)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-rend.html"><span class="t-lines"><span>rend</span><span>crend</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx14">(C++14)</span></span></span></div>
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</div>
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</td>
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<td>返回容器或数组的逆向尾迭代器<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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