190 lines
22 KiB
HTML
190 lines
22 KiB
HTML
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<title>std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::operator[]</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_container_unordered_map_operator_at 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::unordered_map<Key,T,Hash,KeyEqual,Allocator>::</span>operator[]</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-unordered_map.html">unordered map</a></span></div>
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<div class="t-page-template" style="">
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<p></p>
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</div>
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<table class="t-dcl-begin">
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<tr class="t-dcl t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <span class="kw4">const</span> Key<span class="sy3">&</span> key <span class="br0">)</span><span class="sy4">;</span></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-cxx11">(C++11 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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<td>
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<div><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> Key<span class="sy3">&&</span> key <span class="br0">)</span><span class="sy4">;</span></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-cxx11">(C++11 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>返回到映射到等于 <code>key</code> 的键的值的引用，若这种键不存在则进行插入。</p>
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<div class="t-li1"><span class="t-li">1)</span> 若键不存在，则插入从 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-piecewise_construct.html"><span class="kw1106">std::<span class="me2">piecewise_construct</span></span></a>, <a href="cpp-utility-tuple-forward_as_tuple.html"><span class="kw1114">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span>key<span class="br0">)</span>, <a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><></span><span class="br0">(</span><span class="br0">)</span></span></span> 原位构造的 <code>value_type</code> 对象。<span class="t-rev-inl t-since-cxx17"><span>此函数等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> this<span class="sy2">-</span><span class="sy1">></span>try_emplace<span class="br0">(</span>key<span class="br0">)</span>.<span class="me1">first</span><span class="sy2">-</span><span class="sy1">></span>second<span class="sy4">;</span></span></span> 。</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span><br />
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使用默认分配器时，这导致从 <code>key</code> 复制构造关键，并<a href="cpp-language-value_initialization.html">值初始化</a>被映射值。
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<table class="t-par-begin">
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<tr class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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<code>value_type</code> 必须从 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-piecewise_construct.html"><span class="kw1106">std::<span class="me2">piecewise_construct</span></span></a>, <a href="cpp-utility-tuple-forward_as_tuple.html"><span class="kw1114">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span>key<span class="br0">)</span>, <a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><></span><span class="br0">(</span><span class="br0">)</span></span></span> <a href="cpp-named_req-EmplaceConstructible.html"><span style="font-style:italic">可就位构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(EmplaceConstructible)</span></a> 。使用默认分配器时，这表明 <code>key_type</code> 必须<a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 而 <code>mapped_type</code> 必须<a href="cpp-named_req-DefaultConstructible.html"><span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span></a> 。</td>
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</tr>
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</table>
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</div>
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<div class="t-li1"><span class="t-li">2)</span> 若键不存在，则插入从 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-piecewise_construct.html"><span class="kw1106">std::<span class="me2">piecewise_construct</span></span></a>, <a href="cpp-utility-tuple-forward_as_tuple.html"><span class="kw1114">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>key<span class="br0">)</span><span class="br0">)</span>, <a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><></span><span class="br0">(</span><span class="br0">)</span></span></span> 原位构造的 <code>value_type</code> 对象。<span class="t-rev-inl t-since-cxx17"><span>此函数等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> this<span class="sy2">-</span><span class="sy1">></span>try_emplace<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>key<span class="br0">)</span><span class="br0">)</span>.<span class="me1">first</span><span class="sy2">-</span><span class="sy1">></span>second<span class="sy4">;</span></span></span> 。</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span><br />
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使用默认分配器时，这导致从 <code>key</code> 移动构造关键，并<a href="cpp-language-value_initialization.html">值初始化</a>被映射值。
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<table class="t-par-begin">
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<tr class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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<code>value_type</code> 必须从 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-piecewise_construct.html"><span class="kw1106">std::<span class="me2">piecewise_construct</span></span></a>, <a href="cpp-utility-tuple-forward_as_tuple.html"><span class="kw1114">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>key<span class="br0">)</span><span class="br0">)</span>, <a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><></span><span class="br0">(</span><span class="br0">)</span></span></span> <a href="cpp-named_req-EmplaceConstructible.html"><span style="font-style:italic">可就位构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(EmplaceConstructible)</span></a> 。使用默认分配器时，这表明 <code>key_type</code> 必须为<a href="cpp-named_req-MoveConstructible.html"><span style="font-style:italic">可移动构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveConstructible)</span></a> 而 <code>mapped_type</code> 必须为<a href="cpp-named_req-DefaultConstructible.html"><span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span></a> 。</td>
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</tr>
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</table>
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</div>
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<p>若插入发生且导致容器的重哈希，则所有迭代器被非法化。否则迭代器不受影响。重哈希仅若新元素数量大于 <span class="t-c"><span class="mw-geshi cpp source-cpp">max_load_factor<span class="br0">(</span><span class="br0">)</span><span class="sy2">*</span>bucket_count<span class="br0">(</span><span class="br0">)</span></span></span> 才发生。</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>key</td>
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<td>-</td>
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<td>要寻找的元素键</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>若不存在拥有关键 <code>key</code> 的元素，则为到新元素被映射值的引用。否则为到既存的关键等价于 <code>key</code> 的元素的被映射值的引用。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>若任何操作抛出异常，则插入无效果。</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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<p>平均情况：常数，最坏情况：与大小成线性。</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>出版的 C++11 和 C++14 标准中，指定此函数要求 <code>mapped_type</code> 为<a href="cpp-named_req-DefaultInsertable.html"><span style="font-style:italic">可默认插入</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultInsertable)</span></a> 且 <code>key_type</code> 为<a href="cpp-named_req-CopyInsertable.html"><span style="font-style:italic">可复制插入</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyInsertable)</span></a> 或<a href="cpp-named_req-MoveInsertable.html"><span style="font-style:italic">可移动插入</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveInsertable)</span></a> 到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 。此规定有缺陷并为 <a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2469">LWG 问题 2469</a> 所修复，而上面的描述合并了该问题的解决方案。</p>
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<p>然而，已知一个实现（ libc++ ）通过二个分离的分配器 <code>construct()</code> 调用构造 <code>key_type</code> 和 <code>mapped_type</code> 对象，可认为如发布时的标准所要求，而非原位构造 <code>value_type</code> 对象。</p>
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<p><code>operator[]</code> 非 const ，因为若不关键不存在则它插入关键。若此行为非所欲或容器为 <code>const</code> ，则可用 <a href="cpp-container-unordered_map-at.html"><code>at()</code></a> 。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx17">
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<td>
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<p><a href="cpp-container-unordered_map-insert_or_assign.html"><code>insert_or_assign()</code></a> 返回的信息多于 <code>operator[]</code> ，而且不要求 <code>mapped_type</code> 可默认构造。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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</table>
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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 <iostream></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <vector></span>
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<span class="co2">#include <unordered_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-unordered_map.html"><span class="kw1293">std::<span class="me2">unordered_map</span></span></a><span class="sy1"><</span><span class="kw4">char</span>, <span class="kw4">int</span><span class="sy1">></span> letter_counts <span class="br0">{</span><span class="br0">{</span><span class="st0">'a'</span>, <span class="nu0">27</span><span class="br0">}</span>, <span class="br0">{</span><span class="st0">'b'</span>, <span class="nu0">3</span><span class="br0">}</span>, <span class="br0">{</span><span class="st0">'c'</span>, <span class="nu0">1</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">"initially:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&</span>pair <span class="sy4">:</span> letter_counts<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> pair.<span class="me1">first</span> <span class="sy1"><<</span> <span class="st0">": "</span> <span class="sy1"><<</span> pair.<span class="me1">second</span> <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>
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letter_counts<span class="br0">[</span><span class="st0">'b'</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span> <span class="co1">// 更新既存值</span>
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letter_counts<span class="br0">[</span><span class="st0">'x'</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu0">9</span><span class="sy4">;</span> <span class="co1">// 插入新值</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 modifications:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&</span>pair <span class="sy4">:</span> letter_counts<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> pair.<span class="me1">first</span> <span class="sy1"><<</span> <span class="st0">": "</span> <span class="sy1"><<</span> pair.<span class="me1">second</span> <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>
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<span class="co1">// 统计每个词的出现数</span>
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<span class="co1">// （首次调用 operator[] 以零初始化计数器）</span>
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<a href="cpp-container-unordered_map.html"><span class="kw1293">std::<span class="me2">unordered_map</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a>, size_t<span class="sy1">></span> word_map<span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&</span>w <span class="sy4">:</span> <span class="br0">{</span> <span class="st0">"this"</span>, <span class="st0">"sentence"</span>, <span class="st0">"is"</span>, <span class="st0">"not"</span>, <span class="st0">"a"</span>, <span class="st0">"sentence"</span>,
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<span class="st0">"this"</span>, <span class="st0">"sentence"</span>, <span class="st0">"is"</span>, <span class="st0">"a"</span>, <span class="st0">"hoax"</span><span class="br0">}</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="sy2">++</span>word_map<span class="br0">[</span>w<span class="br0">]</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&</span>pair <span class="sy4">:</span> word_map<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> pair.<span class="me1">second</span> <span class="sy1"><<</span> <span class="st0">" occurrences of word '"</span> <span class="sy1"><<</span> pair.<span class="me1">first</span> <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>
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<span class="br0">}</span></pre></div>
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</div>
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<p>可能的输出：</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">initially:
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a: 27
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b: 3
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c: 1
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after modifications:
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a: 27
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b: 42
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c: 1
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x: 9
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2 occurrences of word 'a'
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1 occurrences of word 'hoax'
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2 occurrences of word 'is'
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1 occurrences of word 'not'
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3 occurrences of word 'sentence'
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2 occurrences of word 'this'</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-container-unordered_map-at.html"><span class="t-lines"><span>at</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>访问指定的元素，同时进行越界检查<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-unordered_map-insert_or_assign.html"><span class="t-lines"><span>insert_or_assign</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>插入元素，或若键已存在则赋值给当前元素<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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<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-unordered_map-try_emplace.html"><span class="t-lines"><span>try_emplace</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>若键不存在则原位插入，若键存在则不做任何事<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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</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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