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<header data-kunai-mdinfo="{"meta": {"header": ["functional"], "namespace": ["std"], "id-type": ["class template"], "cpp": ["cpp11"]}, "sources": [{"id": "2efa0b047069ba0bafb6389c002a46ce2aab8329", "source": "#include <iostream>\n#include <functional>\n\nenum class E { A = 1, B = 42, C = 100 };\n\nint main()\n{\n int x;\n\n // char\u578b\u306e\u5024'C'\u306b\u5bfe\u3059\u308b\u30cf\u30c3\u30b7\u30e5\u5024\u3092\u6c42\u3081\u308b\n std::cout << std::hash<char>()('C') << std::endl;\n\n // int\u578b\u306e\u5024100\u306b\u5bfe\u3059\u308b\u30cf\u30c3\u30b7\u30e5\u5024\u3092\u6c42\u3081\u308b\n std::cout << std::hash<int>()(100) << std::endl;\n\n // double\u578b\u306e\u50243.14\u306b\u5bfe\u3059\u308b\u30cf\u30c3\u30b7\u30e5\u5024\u3092\u6c42\u3081\u308b\n std::cout << std::hash<double>()(3.14) << std::endl;\n\n // int*\u578b\u306e\u30a2\u30c9\u30ec\u30b9\u5024(&x)\u306b\u5bfe\u3059\u308b\u30cf\u30c3\u30b7\u30e5\u5024\u3092\u6c42\u3081\u308b\n std::cout << std::hash<int*>()(&x) << std::endl;\n\n // \u5217\u6319\u578b\u306e\u5024E::B\u306b\u5bfe\u3059\u308b\u30cf\u30c3\u30b7\u30e5\u5024\u3092\u6c42\u3081\u308b\uff08C++14\u304b\u3089\uff09\n E e = E::B;\n std::cout << std::hash<E>()(e) << std::endl;\n}\n"}, {"id": "59a3bffa639b8f6b714cc2ca564b8233247d61ff", "source": "#include <iostream>\n#include <unordered_map>\n#include <string>\n#include <string_view>\n\nstruct string_hash {\n using is_transparent = void;\n // string/string_view/const char*\u5171\u7528\u30cf\u30c3\u30b7\u30e5\u8a08\u7b97\n size_t operator()(std::string_view sv) const {\n return std::hash<std::string_view>{}(sv);\n }\n};\n\nint main()\n{\n // \u30ad\u30fc\u578b=std::string, \u5024\u578b=int\n std::unordered_map<std::string, int, string_hash, std::equal_to<>> um = {\n {\"Alice\", 3},\n {\"Bob\", 1},\n {\"Carol\", 4}\n };\n\n // string_hash\u304a\u3088\u3073std::equal_to<>\u306f\u3044\u305a\u308c\u3082\u30e1\u30f3\u30d0\u578b\u306bis_transparent\u3092\u6301\u3064\u305f\u3081\u3001\n // find()\u306a\u3069\u306e\u691c\u7d22\u95a2\u6570\u306b\u5024\u3092\u6e21\u3059\u5834\u5408\u306b\u3001\u30ad\u30fc\u578b(std::string)\u4e00\u6642\u30aa\u30d6\u30b8\u30a7\u30af\u30c8\u304c\u4f5c\u3089\u308c\u306a\u3044\n auto it = um.find(\"Alice\");\n if (it != um.end()) {\n std::cout << \"found : \" << it->second << std::endl;\n }\n}\n"}], "page_id": ["reference", "functional", "hash"]}">
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<div class="identifier-type">class template</div><div class="header"><functional></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">hash</span><span class="cpp cpp11" title="C++11で追加">(C++11)</span></h1>
<div itemprop="articleBody"><p><div class="codehilite"><pre><span></span><code><span class="k">namespace</span><span class="w"> </span><span class="nn">std</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="o">></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// ハッシュ関数の特殊化</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">bool</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">char</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">signed</span><span class="w"> </span><span class="kt">char</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">char</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">char8_t</span><span class="o">></span><span class="p">;</span><span class="w"> </span><span class="c1">// C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">char16_t</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">char32_t</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">wchar_t</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">short</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">short</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">long</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">long</span><span class="w"> </span><span class="kt">long</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">long</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">long</span><span class="w"> </span><span class="kt">long</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">float</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">double</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="kt">long</span><span class="w"> </span><span class="kt">double</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="n"><a href="../cstddef/nullptr_t.html">nullptr_t</a></span><span class="o">></span><span class="p">;</span><span class="w"> </span><span class="c1">// C++17</span>
<span class="w"> </span><span class="k">template</span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="o">></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">hash</span><span class="o"><</span><span class="n">T</span><span class="o">*></span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p>クラステンプレート<code>hash</code>は、非順序連想コンテナ（<code><a href="../unordered_map/unordered_map.html">unordered_map</a></code>/<code><a href="../unordered_map/unordered_multimap.html">unordered_multimap</a></code>/<code><a href="../unordered_set/unordered_set.html">unordered_set</a></code>/<code><a href="../unordered_set/unordered_multiset.html">unordered_multiset</a></code>）のキーとなる型のためのハッシュ値を計算する関数オブジェクトである。</p>
<p>このクラスはそのものにデフォルトの定義は存在せず、ユーザーが任意の型で特殊化する際の要件を定義する。<code>hash</code>クラステンプレートを特殊化する場合、後述するメンバ関数を持たせる必要がある。</p>
<h3>基本型のハッシュサポート</h3>
<p><code><functional></code>ヘッダでは、基本型に対する特殊化を提供する。
<code>std::string</code>などC++標準ライブラリ定義の型に対する特殊化は、対象型を定義する各種ヘッダファイルにて提供される。</p>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>型</th>
<th>対応バージョン</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>bool</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>char</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>signed char</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>unsigned char</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>char8_t</code></td>
<td>C++20</td>
</tr>
<tr>
<td><code>char16_t</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>char32_t</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>wchar_t</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>short</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>unsigned short</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>int</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>unsigned int</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>long</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>long long</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>unsigned long</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>unsigned long long</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>float</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>double</code></td>
<td>C++11</td>
</tr>
<tr>
<td><code>long double</code></td>
<td>C++11</td>
</tr>
<tr>
<td>全ての型へのポインタ</td>
<td>C++11</td>
</tr>
<tr>
<td>全ての列挙型</td>
<td>C++14</td>
</tr>
<tr>
<td><code>nullptr_t</code></td>
<td>C++17</td>
</tr>
</tbody>
</table>
<h2>メンバ関数</h2>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>(constructor)</code></td>
<td>デフォルトコンストラクタ、コピーコンストラクタ、ムーブコンストラクタを持つ</td>
</tr>
<tr>
<td><code>(destructor)</code></td>
<td>デストラクタを持つ</td>
</tr>
<tr>
<td><code>operator=</code></td>
<td>コピー代入演算子とムーブ代入演算子を持つ</td>
</tr>
<tr>
<td><code>operator()</code></td>
<td>関数呼び出し演算子によって、キー値<code>k</code>に対応する<code>size_t</code>型のハッシュ値を返す。<code>const</code>修飾メンバ関数。</td>
</tr>
</tbody>
</table>
<p>関数呼び出し演算子に関する制約は次の通り :</p>
<ul>
<li>引数に指定するキー値<code>k</code>を変更してはならない。</li>
<li>ハッシュ計算は引数で指定したキー値以外に依存してはいけない（状態をもってはいけない）。</li>
<li>式<code>k1 == k2</code>を満たす2個のキー値に対しては、同一ハッシュ値を返すこと。<ul>
<li>異なる2個のキーに対して同一ハッシュ値を返すこと（collision; 衝突）は許容されるが、入力に対する出力ハッシュ値は<code>size_t</code>型の値域において均等分布することが好ましい。</li>
</ul>
</li>
</ul>
<h2>メンバ型</h2>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
<th>対応バージョン</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>is_transparent</code></td>
<td>省略可。ハッシュ計算を行う関数オブジェクトがこの型を持っている場合、非順序連想コンテナの透過的な検索関数が有効になる。<br />この型は、例えば関数オブジェクトが<code>string</code>型／<code>string_view</code>型／<a class="cpprefjp-defined-word" data-desc="null-terminated。文字列においてヌル文字 '\0' を用いてその終端を表すこと。ヌル文字自体は文字列に含まない。広義には線形データ構造においてヌルまたは値 0 を番兵としてその終わりを示すこと">ヌル終端</a>文字列(<code>const char*</code>)に対して等価なハッシュ値を生成できる場合に定義される。</td>
<td>C++20</td>
</tr>
</tbody>
</table>
<h2>例</h2>
<h3>基本的な使い方</h3>
<p><div class="yata" id="2efa0b047069ba0bafb6389c002a46ce2aab8329"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../functional.html"><functional></a></span>
<span class="k">enum</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">E</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">A</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">42</span><span class="p">,</span><span class="w"> </span><span class="n">C</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">100</span><span class="w"> </span><span class="p">};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">x</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// char型の値'C'に対するハッシュ値を求める</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::hash</span></span><span class="o"><</span><span class="kt">char</span><span class="o">></span><span class="p">()(</span><span class="sc">'C'</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="c1">// int型の値100に対するハッシュ値を求める</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::hash</span></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">()(</span><span class="mi">100</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="c1">// double型の値3.14に対するハッシュ値を求める</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::hash</span></span><span class="o"><</span><span class="kt">double</span><span class="o">></span><span class="p">()(</span><span class="mf">3.14</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="c1">// int*型のアドレス値(&x)に対するハッシュ値を求める</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::hash</span></span><span class="o"><</span><span class="kt">int</span><span class="o">*></span><span class="p">()(</span><span class="o">&</span><span class="n">x</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="c1">// 列挙型の値E::Bに対するハッシュ値を求める（C++14から）</span>
<span class="w"> </span><span class="n">E</span><span class="w"> </span><span class="n">e</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">E</span><span class="o">::</span><span class="n">B</span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::hash</span></span><span class="o"><</span><span class="n">E</span><span class="o">></span><span class="p">()(</span><span class="n">e</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h4>出力例</h4>
<p><pre><code>67
100
1427109137
3219530756
42
</code></pre></p>
<h3>透過的にハッシュ値を計算できる場合 (C++20)</h3>
<p>メンバ型<code>is_transparent</code>が定義される場合、以下のようなコードにおいて、<code><a href="../unordered_map/unordered_map/find.html">find</a></code>メンバ関数に文字列リテラルを指定しても、一時的な<code><a href="../string/basic_string.html">string</a></code>オブジェクトが作成されず、パフォーマンス向上が期待できる。</p>
<p><div class="yata" id="59a3bffa639b8f6b714cc2ca564b8233247d61ff"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../unordered_map.html"><unordered_map></a></span>
<span class="cp">#include <a href="../string.html"><string></a></span>
<span class="cp">#include <a href="../string_view.html"><string_view></a></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">string_hash</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">is_transparent</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="kt">void</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// string/<a href="../string_view/basic_string_view.html">string_view</a>/const char*共用ハッシュ計算</span>
<span class="w"> </span><span class="kt">size_t</span><span class="w"> </span><span class="nf">operator</span><span class="p">()(</span><span class="n"><a href="../string_view/basic_string_view.html">std::string_view</a></span><span class="w"> </span><span class="n">sv</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">hash</span><span class="o"><</span><span class="n"><a href="../string_view/basic_string_view.html">std::string_view</a></span><span class="o">></span><span class="p">{}(</span><span class="n">sv</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="c1">// キー型=<a href="../string/basic_string.html">std::string</a>, 値型=int</span>
<span class="w"> </span><span class="n"><a href="../unordered_map/unordered_map.html">std::unordered_map</a></span><span class="o"><</span><span class="n"><a href="../string/basic_string.html">std::string</a></span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="n">string_hash</span><span class="p">,</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">equal_to</span><span class="o"><>></span><span class="w"> </span><span class="n">um</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="p">{</span><span class="s">"Alice"</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="s">"Bob"</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="s">"Carol"</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">}</span>
<span class="w"> </span><span class="p">};</span>
<span class="w"> </span><span class="c1">// string_hashおよびstd::equal_to<>はいずれもメンバ型にis_transparentを持つため、</span>
<span class="w"> </span><span class="c1">// find()などの検索関数に値を渡す場合に、キー型(<a href="../string/basic_string.html">std::string</a>)一時オブジェクトが作られない</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">it</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../unordered_map/unordered_map/find.html">um.find</a></span><span class="p">(</span><span class="s">"Alice"</span><span class="p">);</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">it</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n"><a href="../unordered_map/unordered_map/end.html">um.end()</a></span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"found : "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">it</span><span class="o">-></span><span class="n">second</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>found : 3
</code></pre></p>
<h2>バージョン</h2>
<h3>言語</h3>
<ul>
<li>C++11</li>
</ul>
<h3>処理系</h3>
<ul>
<li><a href="../../implementation.html#clang">Clang</a>: ??</li>
<li><a href="../../implementation.html#gcc">GCC</a>: 4.7.0 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../implementation.html#icc">ICC</a>: ??</li>
<li><a href="../../implementation.html#visual_cpp">Visual C++</a>: ??</li>
</ul>
<h2>関連項目</h2>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>ヘッダ</th>
<th>特殊化対象</th>
</tr>
</thead>
<tbody>
<tr>
<td><code><a href="../coroutine.html"><coroutine></a></code></td>
<td><code><a href="../coroutine/coroutine_handle.html">std::coroutine_handle</a></code></td>
</tr>
<tr>
<td><code><a href="../bitset.html"><bitset></a></code></td>
<td><code><a href="../bitset/bitset.html">std::bitset</a></code></td>
</tr>
<tr>
<td><code><a href="../filesystem.html"><filesystem></a></code></td>
<td><code><a href="../filesystem/path.html">std::filesystem::path</a></code></td>
</tr>
<tr>
<td><code><a href="../optional.html"><optional></a></code></td>
<td><code><a href="../optional/optional.html">std::optional</a></code></td>
</tr>
<tr>
<td><code><a href="../memory.html"><memory></a></code></td>
<td><code><a href="../memory/shared_ptr.html">std::shared_ptr</a></code>, <code><a href="../memory/unique_ptr.html">std::unique_ptr</a></code></td>
</tr>
<tr>
<td><code><a href="../stacktrace.html"><stacktrace></a></code></td>
<td><code><a href="../stacktrace/basic_stacktrace.html">std::basic_stacktrace</a></code>, <code><a href="../stacktrace/stacktrace_entry.html">std::stacktrace_entry</a></code></td>
</tr>
<tr>
<td><code><a href="../string.html"><string></a></code></td>
<td><code>std::string</code>など</td>
</tr>
<tr>
<td><code><a href="../string_view.html"><string_view></a></code></td>
<td><code>std::string_view</code>など</td>
</tr>
<tr>
<td><code><a href="../system_error.html"><system_error></a></code></td>
<td><code><a href="../system_error/error_code.html">std::error_code</a></code>, <code><a href="../system_error/error_condition.html">std::error_condition</a></code></td>
</tr>
<tr>
<td><code><a href="../thread.html"><thread></a></code></td>
<td><code><a href="../thread/thread/id.html">std::thread::id</a></code></td>
</tr>
<tr>
<td><code><a href="../typeindex.html"><typeindex></a></code></td>
<td><code><a href="../typeindex/type_index.html">std::type_index</a></code></td>
</tr>
<tr>
<td><code><a href="../variant.html"><variant></a></code></td>
<td><code><a href="../variant/variant.html">std::variant</a></code>, <code><a href="../variant/monostate.html">std::monostate</a></code></td>
</tr>
<tr>
<td><code><a href="../vector.html"><vector></a></code></td>
<td><code><a href="../vector/vector.html">std::vector<bool></a></code></td>
</tr>
</tbody>
</table>
<h2>参照</h2>
<ul>
<li><a href="https://ja.wikipedia.org/wiki/ハッシュ関数" target="_blank">ハッシュ関数 - Wikipedia</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2148" target="_blank">LWG 2148 - Hashing enums should be supported directly by <code>std::hash</code></a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2291" target="_blank">LWG 2291 - std::hash is vulnerable to collision DoS attack</a></li>
<li><a href="https://cplusplus.github.io/LWG/issue2543" target="_blank">LWG 2543 - LWG 2148 (hash support for enum types) seems under-specified</a></li>
<li><a href="https://cplusplus.github.io/LWG/issue2817" target="_blank">LWG 2817 - <code>std::hash</code> for <code>nullptr_t</code></a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0919r3.html" target="_blank">P0919R3 Heterogeneous lookup for unordered containers</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1690r1.html" target="_blank">P1690R1 Refinement Proposal for P0919 Heterogeneous lookup for unordered containers</a></li>
</ul></div>
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