122 lines
12 KiB
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122 lines
12 KiB
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<title>std::common_iterator<I,S>::operator=</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_common_iterator_operator 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::common_iterator<I,S>::</span>operator=</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-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, <span class="kw1">class</span> S2 <span class="sy1">></span><br /></span>
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<p>requires std<span class="sy4">::</span><span class="me2">convertible_to</span><span class="sy1"><</span><span class="kw4">const</span> I2<span class="sy3">&</span>, I<span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">convertible_to</span><span class="sy1"><</span><span class="kw4">const</span> S2<span class="sy3">&</span>, S<span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">assignable_from</span><span class="sy1"><</span>I<span class="sy3">&</span>, <span class="kw4">const</span> I2<span class="sy3">&</span><span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">assignable_from</span><span class="sy1"><</span>S<span class="sy3">&</span>, <span class="kw4">const</span> S2<span class="sy3">&</span><span class="sy1">></span><br /></p>
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<span class="kw4">constexpr</span> common_iterator<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span> <span class="kw4">const</span> common_iterator<span class="sy1"><</span>I2, S2<span class="sy1">></span><span class="sy3">&</span> x <span class="br0">)</span><span class="sy4">;</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></td>
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<td></td>
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<td></td>
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<td></td>
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<p>从 <code>x</code> 的底层 <span class="t-lc"><a href="cpp-utility-variant.html">std::variant</a></span> 成员对象 <code><i>var</i></code> 赋值给 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 的。</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> 。则此赋值运算符等价于：</p>
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<ul>
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<li><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>var<span class="br0">)</span> <span class="sy1">=</span> 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></span> ，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">var.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> i</span></span> ，</li>
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<li>否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp">var.<span class="me1">emplace</span><span class="sy1"><</span>i<span class="sy1">></span><span class="br0">(</span>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="br0">)</span></span></span> 。</li>
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</ul>
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<p>若 <code>x</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></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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<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</td>
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<td>-</td>
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<td>作为复制来源的迭代器</td>
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</tr>
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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><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0_2">参数</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> <span class="br0">{</span>
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<span class="kw4">const</span> <span class="kw4">auto</span> il <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="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><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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std<span class="sy4">::</span><span class="me2">default_sentinel_t</span><span class="sy1">></span><span class="sy4">;</span>
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CI first <span class="br0">{</span> std<span class="sy4">::</span><span class="me2">counted_iterator</span><span class="br0">{</span><a href="cpp-iterator-next.html"><span class="kw674">std::<span class="me2">next</span></span></a><span class="br0">(</span>begin<span class="br0">(</span>il<span class="br0">)</span>, <span class="nu0">1</span><span class="br0">)</span>, ssize<span class="br0">(</span>il<span class="br0">)</span> <span class="sy2">-</span> <span class="nu0">1</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">const</span> CI first2 <span class="br0">{</span> std<span class="sy4">::</span><span class="me2">counted_iterator</span><span class="br0">{</span><a href="cpp-iterator-next.html"><span class="kw674">std::<span class="me2">next</span></span></a><span class="br0">(</span>begin<span class="br0">(</span>il<span class="br0">)</span>, <span class="nu0">2</span><span class="br0">)</span>, ssize<span class="br0">(</span>il<span class="br0">)</span> <span class="sy2">-</span> <span class="nu0">2</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">const</span> CI last <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-algorithm-copy.html"><span class="kw1582">std::<span class="me2">copy</span></span></a><span class="br0">(</span>first, last, <a href="cpp-iterator-ostream_iterator.html"><span class="kw670">std::<span class="me2">ostream_iterator</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">{</span><a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a>, <span class="st0">" "</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> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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first <span class="sy1">=</span> first2<span class="sy4">;</span>
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<a href="cpp-algorithm-copy.html"><span class="kw1582">std::<span class="me2">copy</span></span></a><span class="br0">(</span>first, last, <a href="cpp-iterator-ostream_iterator.html"><span class="kw670">std::<span class="me2">ostream_iterator</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">{</span><a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a>, <span class="st0">" "</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> <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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</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">2 3 4 5 6
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3 4 5 6</pre></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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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-iterator-common_iterator-common_iterator.html"><span class="t-lines"><span>(构造函数)</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>构造新的迭代器适配器<br />
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<span class="t-mark">(公开成员函数)</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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