171 lines
15 KiB
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171 lines
15 KiB
HTML
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<title>std::map<Key,T,Compare,Allocator>::extract</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_container_map_extract skin-cppreference2 action-view cpp-page-template cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::map<Key,T,Compare,Allocator>::</span>extract</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-container.html">container</a>‎ | <a href="cpp-container-map.html">map</a></span></div>
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<div><span class="mw-geshi cpp source-cpp">node_type extract<span class="br0">(</span> const_iterator position <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp">node_type extract<span class="br0">(</span> <span class="kw4">const</span> key_type<span class="sy3">&</span> k <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 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> K <span class="sy1">></span><br />
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node_type extract<span class="br0">(</span> K<span class="sy3">&&</span> x <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 解链含 <code>position</code> 所指向元素的结点并返回占有它的<a href="cpp-container-node_handle.html">结点柄</a>。</div>
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<div class="t-li1"><span class="t-li">2)</span> 若容器拥有元素而其关键等于 <code>x</code> ，则从容器解链该元素并返回占有它的<a href="cpp-container-node_handle.html">结点柄</a>。否则，返回空结点柄。</div>
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<div class="t-li1"><span class="t-li">3)</span> 同 <span class="t-v">(2)</span> 。此重载仅若有限定标识 <span class="t-c"><span class="mw-geshi cpp source-cpp">Compare<span class="sy4">::</span><span class="me2">is_transparent</span></span></span> 合法并指代类型，且 <code>iterator</code> 与 <code>const_iterator</code> 均不可从 <code>K</code> 隐式转换才参与重载决议。它允许无需构造 <code>Key</code> 的实例就调用此函数。</div>
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<p>任何情况下，均不复制或移动元素，只重指向容器结点的内部指针（可能出现再平衡，和 <span class="t-lc"><a href="cpp-container-map-erase.html">erase()</a></span> 一样）。</p>
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<p>释出结点只会非法化指向被释出元素的迭代器。指向被释出元素的指针和引用保持合法，但在结点柄占有该元素时不能使用：若元素被插入容器，就能使用它们。</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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<td>position</td>
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<td>-</td>
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<td>指向此容器中的合法迭代器</td>
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<td>k</td>
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<td>-</td>
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<td>鉴别要被释出的结点的键</td>
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<td>x</td>
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<td>-</td>
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<td>任何能与键通透比较的类型的值，鉴别要释出的结点</td>
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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>占有被释出元素的<a href="cpp-container-node_handle.html">结点柄</a>，或 <span class="t-v">(2,3)</span> 中找不到元素的情况下为空结点柄。</p>
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<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
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<div class="t-li1"><span class="t-li">1)</span> 均摊常数</div>
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<div class="t-li1"><span class="t-li">2,3)</span> log(<span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">size</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>extract 是更换 map 的键而不重分配的唯一方式：</p>
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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">map<span class="sy1"><</span><span class="kw4">int</span>, string<span class="sy1">></span> m<span class="br0">{</span><span class="br0">{</span><span class="nu0">1</span>, <span class="st0">"mango"</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">2</span>, <span class="st0">"papaya"</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">3</span>, <span class="st0">"guava"</span><span class="br0">}</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> nh <span class="sy1">=</span> m.<span class="me1">extract</span><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
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nh.<span class="me1">key</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">4</span><span class="sy4">;</span>
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m.<span class="me1">insert</span><span class="br0">(</span>move<span class="br0">(</span>nh<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// m == {{1, "mango"}, {3, "guava"}, {4, "papaya"}}</span></pre></div>
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</div>
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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 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 <map></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-map.html"><span class="kw1281">std::<span class="me2">map</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">char</span><span class="sy1">></span> cont<span class="br0">{</span><span class="br0">{</span><span class="nu0">1</span>, <span class="st0">'a'</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">2</span>, <span class="st0">'b'</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">3</span>, <span class="st0">'c'</span><span class="br0">}</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> print <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span><span class="kw4">const</span> <span class="kw4">int</span>, <span class="kw4">char</span><span class="sy1">></span><span class="sy3">&</span> n<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">" "</span> <span class="sy1"><<</span> n.<span class="me1">first</span> <span class="sy1"><<</span> <span class="st0">'('</span> <span class="sy1"><<</span> n.<span class="me1">second</span> <span class="sy1"><<</span> <span class="st0">')'</span><span class="sy4">;</span>
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<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">"Start:"</span><span class="sy4">;</span>
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<a href="cpp-algorithm-for_each.html"><span class="kw1568">std::<span class="me2">for_each</span></span></a><span class="br0">(</span>cont.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, cont.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, print<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="co1">// 释出结点柄并更改键</span>
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<span class="kw4">auto</span> nh <span class="sy1">=</span> cont.<span class="me1">extract</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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nh.<span class="me1">key</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">4</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">"After extract and before insert:"</span><span class="sy4">;</span>
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<a href="cpp-algorithm-for_each.html"><span class="kw1568">std::<span class="me2">for_each</span></span></a><span class="br0">(</span>cont.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, cont.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, print<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="co1">// 往回插入结点柄</span>
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cont.<span class="me1">insert</span><span class="br0">(</span>move<span class="br0">(</span>nh<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">"End:"</span><span class="sy4">;</span>
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<a href="cpp-algorithm-for_each.html"><span class="kw1568">std::<span class="me2">for_each</span></span></a><span class="br0">(</span>cont.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, cont.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, print<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">Start: 1(a) 2(b) 3(c)
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After extract and before insert: 2(b) 3(c)
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End: 2(b) 3(c) 4(a)</pre></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-container-map-merge.html"><span class="t-lines"><span>merge</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>从另一容器接合结点<br />
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<span class="t-mark">(公开成员函数)</span></td>
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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-container-map-insert.html"><span class="t-lines"><span>insert</span></span></a></div>
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</div>
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</td>
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<td>插入元素<span class="t-rev-inl t-since-cxx17"><span>或结点</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span><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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<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-container-map-erase.html"><span class="t-lines"><span>erase</span></span></a></div>
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</div>
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</td>
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<td>擦除元素<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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