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141 lines
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<title>std::multimap 的推导指引</title>
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<h1 id="firstHeading" class="firstHeading"><code>std::multimap</code> 的推导指引</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-multimap.html">multimap</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-map.html"><map></a></code></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> InputIt,<br /></span>
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<p> <span class="kw1">class</span> Comp <span class="sy1">=</span> <a href="cpp-utility-functional-less.html"><span class="kw1039">std::<span class="me2">less</span></span></a><span class="sy1"><</span>iter_key_t<span class="sy1"><</span>InputIt<span class="sy1">>></span>,<br />
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<span class="kw1">class</span> Alloc <span class="sy1">=</span> <a href="cpp-memory-allocator.html"><span class="kw700">std::<span class="me2">allocator</span></span></a><span class="sy1"><</span>iter_to_alloc_t<span class="sy1"><</span>InputIt<span class="sy1">>>></span><br />
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multimap<span class="br0">(</span>InputIt, InputIt, Comp <span class="sy1">=</span> Comp<span class="br0">(</span><span class="br0">)</span>, Alloc <span class="sy1">=</span> Alloc<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><br /></p>
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<span class="sy2">-</span><span class="sy1">></span> multimap<span class="sy1"><</span>iter_key_t<span class="sy1"><</span>InputIt<span class="sy1">></span>, iter_val_t<span class="sy1"><</span>InputIt<span class="sy1">></span>, Comp, Alloc<span class="sy1">></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-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> Key,<br /></span>
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<p> <span class="kw1">class</span> T,<br />
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<span class="kw1">class</span> Comp <span class="sy1">=</span> <a href="cpp-utility-functional-less.html"><span class="kw1039">std::<span class="me2">less</span></span></a><span class="sy1"><</span>Key<span class="sy1">></span>,<br />
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<span class="kw1">class</span> Alloc <span class="sy1">=</span> <a href="cpp-memory-allocator.html"><span class="kw700">std::<span class="me2">allocator</span></span></a><span class="sy1"><</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> Key, T<span class="sy1">>>></span><br />
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multimap<span class="br0">(</span><a href="cpp-utility-initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a><span class="sy1"><</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> Key, T<span class="sy1">>></span>, Comp <span class="sy1">=</span> Comp<span class="br0">(</span><span class="br0">)</span>, Alloc <span class="sy1">=</span> Alloc<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><br /></p>
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<span class="sy2">-</span><span class="sy1">></span> multimap<span class="sy1"><</span>Key, T, Comp, Alloc<span class="sy1">></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-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> InputIt, <span class="kw1">class</span> Alloc<span class="sy1">></span><br /></span>
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<p>multimap<span class="br0">(</span>InputIt, InputIt, Alloc<span class="br0">)</span><br />
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<span class="sy2">-</span><span class="sy1">></span> multimap<span class="sy1"><</span>iter_key_t<span class="sy1"><</span>InputIt<span class="sy1">></span>, iter_val_t<span class="sy1"><</span>InputIt<span class="sy1">></span>,<br /></p>
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<a href="cpp-utility-functional-less.html"><span class="kw1039">std::<span class="me2">less</span></span></a><span class="sy1"><</span>iter_key_t<span class="sy1"><</span>InputIt<span class="sy1">>></span>, Alloc<span class="sy1">></span><span class="sy4">;</span></div>
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<td>(3)</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> Key, <span class="kw1">class</span> T, <span class="kw1">class</span> Allocator<span class="sy1">></span><br /></span>
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<p>multimap<span class="br0">(</span><a href="cpp-utility-initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a><span class="sy1"><</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> Key, T<span class="sy1">>></span>, Allocator<span class="br0">)</span><br /></p>
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<span class="sy2">-</span><span class="sy1">></span> multimap<span class="sy1"><</span>Key, T, <a href="cpp-utility-functional-less.html"><span class="kw1039">std::<span class="me2">less</span></span></a><span class="sy1"><</span>Key<span class="sy1">></span>, Allocator<span class="sy1">></span><span class="sy4">;</span></div>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<p>其中类型别名 <code>iter_key_t</code> 、 <code>iter_val_t</code> 、 <code>iter_to_alloc_t</code> 如同定义如下</p>
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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> InputIt<span class="sy1">></span><br /></span>
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<p><span class="kw1">using</span> iter_key_t <span class="sy1">=</span> <a href="cpp-types-remove_cv.html"><span class="kw598">std::<span class="me2">remove_const_t</span></span></a><span class="sy1"><</span><br /></p>
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<span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span><span class="sy4">::</span><span class="me2">first_type</span><span class="sy1">></span><span class="sy4">;</span></div>
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<td><span class="t-mark">(仅为说明)</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> InputIt<span class="sy1">></span><br />
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<span class="kw1">using</span> iter_val_t <span class="sy1">=</span> <span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span><span class="sy4">::</span><span class="me2">second_type</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark">(仅为说明)</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> InputIt<span class="sy1">></span><br /></span>
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<p><span class="kw1">using</span> iter_to_alloc_t <span class="sy1">=</span> <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span><br />
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<a href="cpp-types-add_cv.html"><span class="kw604">std::<span class="me2">add_const_t</span></span></a><span class="sy1"><</span><span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span><span class="sy4">::</span><span class="me2">first_type</span><span class="sy1">></span>,<br />
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<span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span><span class="sy4">::</span><span class="me2">second_type</span><br /></p>
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<span class="sy1">></span></div>
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<td><span class="t-mark">(仅为说明)</span></td>
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<p>为 multimap 提供此<a href="cpp-language-class_template_argument_deduction.html">推导指引</a>，以允许从迭代器范围（重载 <span class="t-v">(1,3)</span> ）和 <span class="t-lc"><a href="cpp-utility-initializer_list.html">std::initializer_list</a></span> （重载 <span class="t-v">(2,4)</span> ）推导。这些重载仅若 <code>InputIt</code> 满足<a href="cpp-named_req-InputIterator.html"><span style="font-style:italic">老式输入迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyInputIterator)</span></a> 、 <code>Alloc</code> 满足<a href="cpp-named_req-Allocator.html"><span style="font-style:italic">分配器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Allocator)</span></a> 且 <code>Comp</code> 不满足<a href="cpp-named_req-Allocator.html"><span style="font-style:italic">分配器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Allocator)</span></a> 才参与重载决议。</p>
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<p>注意：库确定类型是否满足<a href="cpp-named_req-InputIterator.html"><span style="font-style:italic">老式输入迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyInputIterator)</span></a> 的程度是未指定的，除了最低要求是整数类型不具备输入迭代器的条件。类似地，确定类型是否满足<a href="cpp-named_req-Allocator.html"><span style="font-style:italic">分配器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Allocator)</span></a> 是未指定的，除了最低要求是成员类型 <code>Alloc::value_type</code> 必须存在，且表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>Alloc<span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>.<span class="me1">allocate</span><span class="br0">(</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="br0">{</span><span class="br0">}</span><span class="br0">)</span></span></span> 在作为不求值操作数时必须为良构。</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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <map></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="co1">// std::multimap m1 = {{"foo", 1}, {"bar", 2}}; // 错误：花括号初始化器列表无类型；</span>
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<span class="co1">// 不能推导 pair<const Key, T></span>
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<span class="co1">// 自 {"foo", 1} 或 {"bar", 2}</span>
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<a href="cpp-container-multimap.html"><span class="kw1285">std::<span class="me2">multimap</span></span></a> m1 <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>
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<a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span><span class="kw4">char</span> <span class="kw4">const</span><span class="sy2">*</span> <span class="kw4">const</span>, <span class="kw4">int</span><span class="sy1">>></span><span class="br0">(</span><span class="br0">{</span><span class="br0">{</span><span class="st0">"foo"</span>, <span class="nu0">2</span><span class="br0">}</span>, <span class="br0">{</span><span class="st0">"bar"</span>, <span class="nu0">3</span><span class="br0">}</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 指引 #2</span>
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<a href="cpp-container-multimap.html"><span class="kw1285">std::<span class="me2">multimap</span></span></a> m2<span class="br0">(</span>m1.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, m1.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 指引 #1</span>
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<span class="br0">}</span></pre></div>
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