129 lines
12 KiB
HTML
129 lines
12 KiB
HTML
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<title>operator==,<=>(std::counted_iterator)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_counted_iterator_operator_cmp skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">operator==,<=><small>(std::counted_iterator)</small></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-iterator.html">iterator</a>‎ | <a href="cpp-iterator-counted_iterator.html">counted iterator</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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<table class="t-dcl-begin">
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<tr class="t-dcl t-since-cxx20">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> std<span class="sy4">::</span><span class="me2">common_with</span><span class="sy1"><</span>I<span class="sy1">></span> I2 <span class="sy1">></span><br /></span>
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<p> <span class="kw1">friend</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span><br /></p>
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<span class="kw4">const</span> counted_iterator<span class="sy3">&</span> x, <span class="kw4">const</span> counted_iterator<span class="sy1"><</span>I2<span class="sy1">></span><span class="sy3">&</span> y <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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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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</tr>
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<tr class="t-dcl t-since-cxx20">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> std<span class="sy4">::</span><span class="me2">common_with</span><span class="sy1"><</span>I<span class="sy1">></span> I2 <span class="sy1">></span><br /></span>
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<p> <span class="kw1">friend</span> <span class="kw4">constexpr</span> strong_ordering operator<span class="sy1"><=></span><span class="br0">(</span><br /></p>
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<span class="kw4">const</span> counted_iterator<span class="sy3">&</span> x, <span class="kw4">const</span> counted_iterator<span class="sy1"><</span>I2<span class="sy1">></span><span class="sy3">&</span> y <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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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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</tr>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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</tr>
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</tbody>
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</table>
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<p>比较底层长度（即到末尾的距离）。</p>
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<div class="t-li1"><span class="t-li">1)</span> 检查长度是否相等。</div>
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<div class="t-li1"><span class="t-li">2)</span> 用运算符 <code><=></code> 比较底层长度。</div>
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<p>若 <code>x</code> 与 <code>y</code> 不指向同一序列的元素则行为未定义。即必须存在某个 <span class="t-c"><span class="mw-geshi cpp source-cpp">n</span></span> 使得 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-next.html"><span class="kw674">std::<span class="me2">next</span></span></a><span class="br0">(</span>x.<span class="me1">base</span><span class="br0">(</span><span class="br0">)</span>, x.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">+</span> n<span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-next.html"><span class="kw674">std::<span class="me2">next</span></span></a><span class="br0">(</span>y.<span class="me1">base</span><span class="br0">(</span><span class="br0">)</span>, y.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">+</span> n<span class="br0">)</span></span></span> 指代同一元素。</p>
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<p><code><</code>、 <code><=</code>、 <code>></code>、 <code>>=</code> 及 <code>!=</code> 运算符分别从 <code>operator<=></code> 与 <code>operator==</code> <a href="cpp-language-operators.html#.E5.85.B3.E7.B3.BB.E8.BF.90.E7.AE.97.E7.AC.A6">合成</a>。</p>
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<p>此函数模板对通常<a href="cpp-language-unqualified_lookup.html">无限定</a>或<a href="cpp-language-qualified_lookup.html">有限定查找</a>不可见，而只能在 <code>std::counted_iterator<I></code> 为参数的关联类时由<a href="cpp-language-adl.html">实参依赖查找</a>找到。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>x, y</td>
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<td>-</td>
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<td>迭代器适配器</td>
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</tr>
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</table>
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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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<div class="t-li1"><span class="t-li">1)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">x.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> y.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span></span></span></div>
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<div class="t-li1"><span class="t-li">2)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">y.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><=></span> x.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span></span></span></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>由于<i>长度</i>向下计数，而非向上， <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1"><=></span></span></span> 的参数在底层比较表达式中的顺序是逆转的，即 <code>y</code> 为 <i>lhs</i> ， <code>x</code> 为 <i>rhs</i> 。</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 <initializer_list></span>
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<span class="co2">#include <iterator></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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<span class="kw4">static</span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> v <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span>, <span class="nu0">4</span>, <span class="nu0">5</span>, <span class="nu0">6</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">counted_iterator</span><span class="sy1"><</span><a href="cpp-utility-initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">iterator</span><span class="sy1">></span>
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it1 <span class="br0">{</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, <span class="nu0">5</span><span class="br0">}</span>,
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it2 <span class="br0">{</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, <span class="nu0">5</span><span class="br0">}</span>,
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it3 <span class="br0">{</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">+</span> <span class="nu0">1</span>, <span class="nu0">4</span><span class="br0">}</span>,
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it4 <span class="br0">{</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, <span class="nu0">0</span><span class="br0">}</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span> it1 <span class="sy1">==</span> it2 <span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span> it2 <span class="sy3">!</span><span class="sy1">=</span> it3 <span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span> it2 <span class="sy1"><</span> it3 <span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span> it1 <span class="sy1"><=</span> it2 <span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span> it3 <span class="sy3">!</span><span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">default_sentinel</span> <span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span> it4 <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">default_sentinel</span> <span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// it2 == std::counted_iterator{v.begin(), 4}; // UB ：运算数不指代同一序列的元素</span>
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<span class="br0">}</span></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-iterator-counted_iterator-operator_cmp2.html"><span class="t-lines"><span>operator==<span class="t-dsc-small">(std::default_sentinel)</span></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>检查到末尾的距离是否等于 <code>​0​</code><br />
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<span class="t-mark">(函数模板)</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-counted_iterator-operator+.html"><span class="t-lines"><span>operator+</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>令迭代器前进<br />
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<span class="t-mark">(函数模板)</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-counted_iterator-operator-.html"><span class="t-lines"><span>operator-</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>计算两个迭代器适配器间的距离<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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</div>
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</div>
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</div>
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</body>
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</html>
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