139 lines
15 KiB
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139 lines
15 KiB
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<title>operator==(std::common_iterator)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_common_iterator_operator_cmp skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">operator==<small>(std::common_iterator)</small></h1>
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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-common_iterator.html">common iterator</a></span></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> I2, std<span class="sy4">::</span><span class="me2">sentinel_for</span><span class="sy1"><</span>I<span class="sy1">></span> S2<span class="sy1">></span><br /></span>
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<p> requires std<span class="sy4">::</span><span class="me2">sentinel_for</span><span class="sy1"><</span>S, I2<span class="sy1">></span><br />
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<span class="kw1">friend</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span> <span class="kw4">const</span> common_iterator<span class="sy3">&</span> x,<br /></p>
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<span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">common_iterator</span><span class="sy1"><</span>I2, S2<span class="sy1">></span><span class="sy3">&</span> y <span class="br0">)</span><span class="sy4">;</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">template</span> <span class="sy1"><</span><span class="kw1">class</span> I2, std<span class="sy4">::</span><span class="me2">sentinel_for</span><span class="sy1"><</span>I<span class="sy1">></span> S2<span class="sy1">></span><br /></span>
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<p> requires std<span class="sy4">::</span><span class="me2">sentinel_for</span><span class="sy1"><</span>S, I2<span class="sy1">></span> <span class="sy3">&&</span> std<span class="sy4">::</span><span class="me2">equality_comparable_with</span><span class="sy1"><</span>I, I2<span class="sy1">></span><br />
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<span class="kw1">friend</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span> <span class="kw4">const</span> common_iterator<span class="sy3">&</span> x,<br /></p>
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<span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">common_iterator</span><span class="sy1"><</span>I2, S2<span class="sy1">></span><span class="sy3">&</span> y <span class="br0">)</span><span class="sy4">;</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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<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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<p>比较底层 <span class="t-lc"><a href="cpp-utility-variant.html">std::variant</a></span> 成员对象 <code><i>var</i></code> 所保有的迭代器与/或哨位。认为二个不可比较的迭代器或二个哨位相等。</p>
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<p>若 <code>x</code> 或 <code>y</code> 在非法状态，即 <span class="t-c"><span class="mw-geshi cpp source-cpp">x.<span class="me1">var</span>.<span class="me1">valueless_by_exception</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">||</span> y.<span class="me1">var</span>.<span class="me1">valueless_by_exception</span><span class="br0">(</span><span class="br0">)</span></span></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 则行为未定义。</p>
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<p>令 <code>i</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp">x.<span class="me1">var</span>.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span></span></span> ， <code>j</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp">y.<span class="me1">var</span>.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span></span></span> 。</p>
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<div class="t-li1"><span class="t-li">1)</span> 若 <span class="t-c"><span class="mw-geshi cpp source-cpp">i <span class="sy1">==</span> j</span></span> （即 <span class="t-c"><span class="mw-geshi cpp source-cpp">x</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">y</span></span> 均保有迭代器或哨位）则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，否则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span>i<span class="sy1">></span><span class="br0">(</span>x.<span class="me1">var</span><span class="br0">)</span> <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span>j<span class="sy1">></span><span class="br0">(</span>y.<span class="me1">var</span><span class="br0">)</span></span></span> ；</div>
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<div class="t-li1"><span class="t-li">2)</span> 若 <span class="t-c"><span class="mw-geshi cpp source-cpp">i <span class="sy1">==</span> <span class="nu0">1</span> <span class="sy3">&&</span> j <span class="sy1">==</span> <span class="nu0">1</span></span></span> （即 <span class="t-c"><span class="mw-geshi cpp source-cpp">x</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">y</span></span> 均保有哨位）则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，否则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span>i<span class="sy1">></span><span class="br0">(</span>x.<span class="me1">var</span><span class="br0">)</span> <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span>j<span class="sy1">></span><span class="br0">(</span>y.<span class="me1">var</span><span class="br0">)</span></span></span> 。</div>
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<p><code>!=</code> 运算符从 <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>这些函数模板对通常<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::common_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>要比较的迭代器适配器</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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<p>若底层迭代器与/或哨位相等则为 <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=".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 <iterator></span>
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<span class="co2">#include <initializer_list></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">int</span> a<span class="br0">[</span><span class="br0">]</span> <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="br0">}</span><span class="sy4">;</span>
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<span class="kw1">using</span> CI <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">common_iterator</span><span class="sy1"><</span>
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std<span class="sy4">::</span><span class="me2">counted_iterator</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy2">*</span><span class="sy1">></span>,
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std<span class="sy4">::</span><span class="me2">default_sentinel_t</span>
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<span class="sy1">></span><span class="sy4">;</span>
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CI i1<span class="br0">{</span> std<span class="sy4">::</span><span class="me2">counted_iterator</span><span class="br0">{</span> a <span class="sy2">+</span> <span class="nu0">0</span>, <span class="nu0">2</span> <span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
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CI i2<span class="br0">{</span> std<span class="sy4">::</span><span class="me2">counted_iterator</span><span class="br0">{</span> a <span class="sy2">+</span> <span class="nu0">1</span>, <span class="nu0">2</span> <span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
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CI i3<span class="br0">{</span> std<span class="sy4">::</span><span class="me2">counted_iterator</span><span class="br0">{</span> a <span class="sy2">+</span> <span class="nu0">0</span>, <span class="nu0">3</span> <span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
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CI i4<span class="br0">{</span> std<span class="sy4">::</span><span class="me2">counted_iterator</span><span class="br0">{</span> a <span class="sy2">+</span> <span class="nu0">0</span>, <span class="nu0">0</span> <span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
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CI s1<span class="br0">{</span> std<span class="sy4">::</span><span class="me2">default_sentinel</span> <span class="br0">}</span><span class="sy4">;</span>
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CI s2<span class="br0">{</span> std<span class="sy4">::</span><span class="me2">default_sentinel</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="br0">(</span>i1 <span class="sy1">==</span> i2<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="co1">// true</span>
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<span class="sy1"><<</span> <span class="br0">(</span>i1 <span class="sy1">==</span> i3<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="co1">// false</span>
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<span class="sy1"><<</span> <span class="br0">(</span>i2 <span class="sy1">==</span> i3<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="co1">// false</span>
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<span class="sy1"><<</span> <span class="br0">(</span>s1 <span class="sy1">==</span> s2<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="co1">// true</span>
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<span class="sy1"><<</span> <span class="br0">(</span>i1 <span class="sy1">==</span> s1<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="co1">// false</span>
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<span class="sy1"><<</span> <span class="br0">(</span>i4 <span class="sy1">==</span> s1<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// true</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">true false false true false true</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<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>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3574">LWG 3574</a></td>
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<td>C++20</td>
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<td><code>variant</code> 为完全 constexpr (P2231R1) 但 <code>common_iterator</code> 不是</td>
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<td>亦使之为 constexpr</td>
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</tr>
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</table>
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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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-common_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>计算二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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