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<div class="identifier-type">function</div><div class="header"><map></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="class" title="class map">map::</span><span class="token">コンストラクタ</span></h1>
<div itemprop="articleBody"><p><div class="codehilite"><pre><span></span><code><span class="n">map</span><span class="p">();</span><span class="w"> </span><span class="c1">// (1) C++14 から</span>
<span class="k">constexpr</span><span class="w"> </span><span class="n">map</span><span class="p">();</span><span class="w"> </span><span class="c1">// (1) C++26</span>
<span class="k">explicit</span><span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">Compare</span><span class="o">&</span><span class="w"> </span><span class="n">comp</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Allocator</span><span class="p">());</span><span class="w"> </span><span class="c1">// (2) C++14 から</span>
<span class="k">constexpr</span><span class="w"> </span><span class="k">explicit</span>
<span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">Compare</span><span class="o">&</span><span class="w"> </span><span class="n">comp</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Allocator</span><span class="p">());</span><span class="w"> </span><span class="c1">// (2) C++26</span>
<span class="k">explicit</span><span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">Compare</span><span class="o">&</span><span class="w"> </span><span class="n">comp</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Compare</span><span class="p">(),</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Allocator</span><span class="p">());</span><span class="w"> </span><span class="c1">// (1) + (2) C++11 まで</span>
<span class="k">explicit</span><span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (3) C++11 から</span>
<span class="k">constexpr</span><span class="w"> </span><span class="k">explicit</span><span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (3) C++26</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">InputIterator</span><span class="o">></span>
<span class="n">map</span><span class="p">(</span><span class="n">InputIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">InputIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Compare</span><span class="o">&</span><span class="w"> </span><span class="n">comp</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Compare</span><span class="p">(),</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Allocator</span><span class="p">());</span><span class="w"> </span><span class="c1">// (4) C++03</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">InputIterator</span><span class="o">></span>
<span class="k">constexpr</span>
<span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="n">InputIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">InputIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Compare</span><span class="o">&</span><span class="w"> </span><span class="n">comp</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Compare</span><span class="p">(),</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Allocator</span><span class="p">());</span><span class="w"> </span><span class="c1">// (4) C++26</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">InputIterator</span><span class="o">></span>
<span class="n">map</span><span class="p">(</span><span class="n">InputIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">InputIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (5) C++14</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">InputIterator</span><span class="o">></span>
<span class="k">constexpr</span>
<span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="n">InputIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">InputIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (5) C++26</span>
<span class="n">map</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">map</span><span class="o">&</span><span class="w"> </span><span class="n">x</span><span class="p">);</span><span class="w"> </span><span class="c1">// (6) C++03</span>
<span class="k">constexpr</span><span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">map</span><span class="o">&</span><span class="w"> </span><span class="n">x</span><span class="p">);</span><span class="w"> </span><span class="c1">// (6) C++26</span>
<span class="n">map</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">map</span><span class="o">&</span><span class="w"> </span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (7) C++11</span>
<span class="n">map</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">map</span><span class="o">&</span><span class="w"> </span><span class="n">x</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n"><a href="../../type_traits/type_identity.html">type_identity_t</a></span><span class="o"><</span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (7) C++23</span>
<span class="k">constexpr</span>
<span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">map</span><span class="o">&</span><span class="w"> </span><span class="n">x</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n"><a href="../../type_traits/type_identity.html">type_identity_t</a></span><span class="o"><</span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (7) C++26</span>
<span class="n">map</span><span class="p">(</span><span class="n">map</span><span class="o">&&</span><span class="w"> </span><span class="n">y</span><span class="p">);</span><span class="w"> </span><span class="c1">// (8) C++11</span>
<span class="k">constexpr</span><span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="n">map</span><span class="o">&&</span><span class="w"> </span><span class="n">y</span><span class="p">);</span><span class="w"> </span><span class="c1">// (8) C++26</span>
<span class="n">map</span><span class="p">(</span><span class="n">map</span><span class="o">&&</span><span class="w"> </span><span class="n">y</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (9) C++11</span>
<span class="n">map</span><span class="p">(</span><span class="n">map</span><span class="o">&&</span><span class="w"> </span><span class="n">y</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n"><a href="../../type_traits/type_identity.html">type_identity_t</a></span><span class="o"><</span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (9) C++23</span>
<span class="k">constexpr</span>
<span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="n">map</span><span class="o">&&</span><span class="w"> </span><span class="n">y</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n"><a href="../../type_traits/type_identity.html">type_identity_t</a></span><span class="o"><</span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (9) C++26</span>
<span class="n">map</span><span class="p">(</span><span class="n"><a href="../../initializer_list.html">initializer_list</a></span><span class="o"><</span><span class="n">value_type</span><span class="o">></span><span class="w"> </span><span class="n">init</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Compare</span><span class="o">&</span><span class="w"> </span><span class="n">comp</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Compare</span><span class="p">(),</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Allocator</span><span class="p">());</span><span class="w"> </span><span class="c1">// (10) C++11</span>
<span class="k">constexpr</span>
<span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="n"><a href="../../initializer_list.html">initializer_list</a></span><span class="o"><</span><span class="n">value_type</span><span class="o">></span><span class="w"> </span><span class="n">init</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Compare</span><span class="o">&</span><span class="w"> </span><span class="n">comp</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Compare</span><span class="p">(),</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Allocator</span><span class="p">());</span><span class="w"> </span><span class="c1">// (10) C++26</span>
<span class="n">map</span><span class="p">(</span><span class="n"><a href="../../initializer_list.html">initializer_list</a></span><span class="o"><</span><span class="n">value_type</span><span class="o">></span><span class="w"> </span><span class="n">init</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (11) C++14</span>
<span class="k">constexpr</span>
<span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="n"><a href="../../initializer_list.html">initializer_list</a></span><span class="o"><</span><span class="n">value_type</span><span class="o">></span><span class="w"> </span><span class="n">init</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (11) C++26</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="n">container</span><span class="o">-</span><span class="n">compatible</span><span class="o">-</span><span class="n"><a href="../../ranges/range.html">range</a></span><span class="w"> </span><span class="o"><</span><span class="n">value_type</span><span class="o">></span><span class="w"> </span><span class="n">R</span><span class="o">></span>
<span class="n">map</span><span class="p">(</span><span class="n"><a href="../../ranges/from_range_t.html">from_range_t</a></span><span class="p">,</span><span class="w"> </span><span class="n">R</span><span class="o">&&</span><span class="w"> </span><span class="n">rg</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Compare</span><span class="o">&</span><span class="w"> </span><span class="n">comp</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Compare</span><span class="p">(),</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Allocator</span><span class="p">());</span><span class="w"> </span><span class="c1">// (12) C++23</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="n">container</span><span class="o">-</span><span class="n">compatible</span><span class="o">-</span><span class="n"><a href="../../ranges/range.html">range</a></span><span class="w"> </span><span class="o"><</span><span class="n">value_type</span><span class="o">></span><span class="w"> </span><span class="n">R</span><span class="o">></span>
<span class="k">constexpr</span>
<span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="n"><a href="../../ranges/from_range_t.html">from_range_t</a></span><span class="p">,</span><span class="w"> </span><span class="n">R</span><span class="o">&&</span><span class="w"> </span><span class="n">rg</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Compare</span><span class="o">&</span><span class="w"> </span><span class="n">comp</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Compare</span><span class="p">(),</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Allocator</span><span class="p">());</span><span class="w"> </span><span class="c1">// (12) C++26</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="n">container</span><span class="o">-</span><span class="n">compatible</span><span class="o">-</span><span class="n"><a href="../../ranges/range.html">range</a></span><span class="w"> </span><span class="o"><</span><span class="n">value_type</span><span class="o">></span><span class="w"> </span><span class="n">R</span><span class="o">></span>
<span class="n">map</span><span class="p">(</span><span class="n"><a href="../../ranges/from_range_t.html">from_range_t</a></span><span class="p">,</span><span class="w"> </span><span class="n">R</span><span class="o">&&</span><span class="w"> </span><span class="n">rg</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (13) C++23</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="n">container</span><span class="o">-</span><span class="n">compatible</span><span class="o">-</span><span class="n"><a href="../../ranges/range.html">range</a></span><span class="w"> </span><span class="o"><</span><span class="n">value_type</span><span class="o">></span><span class="w"> </span><span class="n">R</span><span class="o">></span>
<span class="k">constexpr</span>
<span class="w"> </span><span class="n">map</span><span class="p">(</span><span class="n"><a href="../../ranges/from_range_t.html">from_range_t</a></span><span class="p">,</span><span class="w"> </span><span class="n">R</span><span class="o">&&</span><span class="w"> </span><span class="n">rg</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">Allocator</span><span class="o">&</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span><span class="w"> </span><span class="c1">// (13) C++26</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p><code>map</code> オブジェクトの構築</p>
<h2>効果</h2>
<ul>
<li>(1) デフォルトコンストラクタ。空の <code>map</code> オブジェクトを構築する。<code>map(Compare())</code> と等価。（C++14 から）</li>
<li>(2) 比較オブジェクト <code>comp</code>、およびアロケータ <code>alloc</code> で空の <code>map</code> オブジェクトを構築する。（C++14 から）</li>
<li>(1) + (2) デフォルトコンストラクタ。比較オブジェクト <code>comp</code>、およびアロケータ <code>alloc</code> で空の <code>map</code> オブジェクトを構築する。（C++11 まで）</li>
<li>(3) アロケータ <code>alloc</code> で空の <code>map</code> オブジェクトを構築する。</li>
<li>(4) <a class="cpprefjp-defined-word" data-desc="2つのイテレータで表現される半開区間を表す。C++20からイテレータと番兵で異なる型であっても指定可能になる">イテレータ範囲</a>コンストラクタ。比較オブジェクト <code>comp</code>、アロケータ <code>alloc</code> 、および範囲 <code>[first, last)</code> の要素で <code>map</code> オブジェクトを構築する。</li>
<li>(5) <a class="cpprefjp-defined-word" data-desc="2つのイテレータで表現される半開区間を表す。C++20からイテレータと番兵で異なる型であっても指定可能になる">イテレータ範囲</a>コンストラクタ。アロケータ <code>alloc</code> 、および範囲 <code>[first, last)</code> の要素で <code>map</code> オブジェクトを構築する。（C++14 から）</li>
<li>(6) コピーコンストラクタ。<code>x</code> の要素のコピーで <code>map</code> オブジェクトを構築する。アロケータは <code>std::<a href="../../memory/allocator_traits.html">allocator_traits</a><allocator_type>::<a href="../../memory/allocator_traits/select_on_container_copy_construction.html">select_on_container_copy_construction</a>(x.<a href="get_allocator.html">get_allocator</a>())</code> の呼び出しによって取得する。</li>
<li>(7) アロケータ <code>alloc</code> を指定したコピーコンストラクタ。<code>x</code> の要素のコピーで <code>map</code> オブジェクトを構築する。</li>
<li>(8) ムーブコンストラクタ。<code>y</code> の要素をムーブすることで <code>map</code> オブジェクトを構築する。アロケータは <code>y</code> に属しているアロケータをムーブして取得する。（C++11 から）</li>
<li>(9) アロケータ <code>alloc</code> を指定したムーブコンストラクタ。<code>y</code> の要素をムーブすることで <code>map</code> オブジェクトを構築する。（C++11 から）</li>
<li>(10) 比較オブジェクト <code>comp</code>、アロケータ <code>alloc</code> 、および初期化リスト <code>init</code> の要素で <code>map</code> オブジェクトを構築する。<code>map(init.<a href="../../initializer_list/initializer_list/begin.html">begin</a>(), init.<a href="../../initializer_list/initializer_list/end.html">end</a>(), comp, alloc)</code> と等価。（C++11 から）</li>
<li>(11) アロケータ <code>alloc</code> 、および初期化リスト <code>init</code> の要素で <code>map</code> オブジェクトを構築する。<code>map(init, Compare(), alloc)</code> と等価。（C++14 から）</li>
<li>(12) Rangeコンストラクタ。比較オブジェクト <code>comp</code>、アロケータ <code>alloc</code> 、およびRange <code>rg</code> の要素で <code>map</code> オブジェクトを構築する。</li>
<li>(13) Rangeコンストラクタ。アロケータ <code>alloc</code> 、およびRange <code>rg</code> の要素で <code>map</code> オブジェクトを構築する。</li>
</ul>
<h2><a class="cpprefjp-defined-word" data-desc="関数等の意味論を構成する要素の1つ。Complexity。関数が使うアルゴリズムの空間計算量・時間計算量">計算量</a></h2>
<ul>
<li>(1), (2), (3) 定数時間</li>
<li>(4), (5) <code>N = <a href="../../iterator/distance.html">distance</a>(first, last)</code> とすると、範囲 <code>[first, last)</code> が比較オブジェクト <code>comp</code> によって既にソート済みである場合は <code>N</code> に比例、そうでなければ <code>N log N</code> に比例</li>
<li>(6), (7) <code>x.<a href="size.html">size</a>()</code> に対して線形時間</li>
<li>(8) 定数時間</li>
<li>(9) 定数時間。但し、<code>alloc == y.<a href="get_allocator.html">get_allocator</a>()</code> でなければ <code>y.<a href="size.html">size</a>()</code> に対して線形時間</li>
<li>(10), (11) <code>N = init.<a href="../../initializer_list/initializer_list/size.html">size</a>()</code> とすると、範囲 <code>[init.<a href="../../initializer_list/initializer_list/begin.html">begin</a>(), init.<a href="../../initializer_list/initializer_list/end.html">end</a>())</code> が比較オブジェクト <code>comp</code> によって既にソート済みである場合は <code>N</code> に比例、そうでなければ <code>N log N</code> に比例</li>
<li>(12), (13) <code>N = <a href="../../iterator/ranges_distance.html">ranges::distance</a>(rg)</code> とすると、<code>rg</code> が比較オブジェクト <code>comp</code> によって既にソート済みである場合は <code>N</code> に比例、そうでなければ <code>N log N</code> に比例</li>
</ul>
<h2>備考</h2>
<ul>
<li>
<p>C++14 では、デフォルトコンストラクタを (1) + (2) の形式から (1) の形式に分離して残りを (2) の形式（<code>comp</code> のデフォルト引数を削除）にした。
これは、デフォルトコンストラクタに <code>explicit</code> が付いていると、</p>
<p><div class="codehilite"><pre><span></span><code><span class="n"><a href="../map.html">std::map</a></span><span class="o"><</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">char</span><span class="o">></span><span class="w"> </span><span class="n">m</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{};</span>
</code></pre></div>
</p>
<p>のようなコード（C++11 から導入された、コピーリスト初期化によるデフォルトコンストラクタ呼び出し）がエラーになってしまうためである。</p>
</li>
<li>
<p>C++14 では、(5) に形式が新たに追加された。
これは、<a class="cpprefjp-defined-word" data-desc="2つのイテレータで表現される半開区間を表す。C++20からイテレータと番兵で異なる型であっても指定可能になる">イテレータ範囲</a> <code>[first, last)</code> のみを引数にとるアロケータ使用構築（uses-allocator construction）に失敗してしまうためである。
具体的には、C++11 では以下のようなコードがエラーになってしまう。</p>
<p><div class="yata" id="7aadfecd28493cf3a88197bea405c97416a60132"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../list.html"><list></a></span>
<span class="cp">#include <a href="../../map.html"><map></a></span>
<span class="cp">#include <a href="../../scoped_allocator.html"><scoped_allocator></a></span>
<span class="cp">#include <a href="../../iterator.html"><iterator></a></span>
<span class="cp">#include <a href="../../utility.html"><utility></a></span>
<span class="cp">#include <a href="../../memory.html"><memory></a></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="k">using</span><span class="w"> </span><span class="n">mii</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../map.html">std::map</a></span><span class="o"><</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../../list/list.html">std::list</a></span><span class="o"><</span><span class="n">mii</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../../scoped_allocator/scoped_allocator_adaptor.html">std::scoped_allocator_adaptor</a></span><span class="o"><</span><span class="n"><a href="../../memory/allocator.html">std::allocator</a></span><span class="o"><</span><span class="n">mii</span><span class="o">>>></span><span class="w"> </span><span class="n">ls</span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../../utility/pair.html">std::pair</a></span><span class="o"><</span><span class="k">const</span><span class="w"> </span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">a</span><span class="p">[]</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="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="mi">5</span><span class="p">,</span><span class="w"> </span><span class="mi">6</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">};</span>
<span class="w"> </span><span class="n"><a href="../../list/list/emplace_back.html">ls.emplace_back</a></span><span class="p">(</span><span class="n"><a href="../../iterator/begin.html">std::begin</a></span><span class="p">(</span><span class="n">a</span><span class="p">),</span><span class="w"> </span><span class="n"><a href="../../iterator/end.html">std::end</a></span><span class="p">(</span><span class="n">a</span><span class="p">));</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<p>なお、C++14 では同様の理由で (11) の形式も新たに追加されているが、こちらは存在しなくてもエラーとはならない。<br />
（<code>map(init, alloc)</code> の形式の構築では、(11) の形式が無い場合でも (10) の形式を用いて <code>init</code> から一時 <code>map</code> が構築され、<code>alloc</code> と共に (9) の形式に引き渡される）</p>
</li>
<li>
<p>C++23 では、(7) と (9) のアロケータパラメータの型が <code>const Allocator&</code> から <code>const type_identity_t<Allocator>&</code> に変更された。
これは、クラステンプレートのテンプレート引数推論 (CTAD) の際に、コピー/ムーブ元の <code>map</code> から推論される <code>Allocator</code> と、アロケータ引数から推論される型が異なる場合に推論が失敗する問題を修正するためである。<code><a href="../../type_traits/type_identity.html">type_identity_t</a></code> で包むことで、アロケータ引数が非推論コンテキストとなり、アロケータの型はコピー/ムーブ元のみから推論されるようになる。</p>
</li>
</ul>
<h2>例</h2>
<p><div class="yata" id="3a15d2508cf26d1dac112b394cb32d9e5a8f4937"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../../map.html"><map></a></span>
<span class="cp">#include <a href="../../utility.html"><utility></a></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="n"><a href="../../utility/pair.html">std::pair</a></span><span class="o"><</span><span class="k">const</span><span class="w"> </span><span class="kt">int</span><span class="p">,</span><span class="kt">char</span><span class="o">></span><span class="w"> </span><span class="n">values</span><span class="p">[]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n"><a href="../../utility/make_pair.html">std::make_pair</a></span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="sc">'a'</span><span class="p">),</span><span class="w"> </span><span class="n"><a href="../../utility/make_pair.html">std::make_pair</a></span><span class="p">(</span><span class="mi">2</span><span class="p">,</span><span class="sc">'b'</span><span class="p">),</span><span class="w"> </span><span class="n"><a href="../../utility/make_pair.html">std::make_pair</a></span><span class="p">(</span><span class="mi">2</span><span class="p">,</span><span class="sc">'b'</span><span class="p">)</span><span class="w"> </span><span class="p">};</span>
<span class="w"> </span><span class="n"><a href="../map.html">std::map</a></span><span class="o"><</span><span class="kt">int</span><span class="p">,</span><span class="kt">char</span><span class="o">></span><span class="w"> </span><span class="n">m1</span><span class="p">(</span><span class="n">values</span><span class="p">,</span><span class="w"> </span><span class="n">values</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">3</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="../map.html">std::map</a></span><span class="o"><</span><span class="kt">int</span><span class="p">,</span><span class="kt">char</span><span class="o">></span><span class="w"> </span><span class="n">m2</span><span class="p">(</span><span class="n">m1</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">"Size of m1: "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">m1</span><span class="p">.</span><span class="n"><a href="size.html">size()</a></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="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">"Size of m2: "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">m2</span><span class="p">.</span><span class="n"><a href="size.html">size()</a></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>
<h3>出力</h3>
<p><pre><code>Size of m1: 2
Size of m2: 2
</code></pre></p>
<h3>処理系</h3>
<ul>
<li><a href="../../../implementation.html#clang">Clang</a>: ??</li>
<li><a href="../../../implementation.html#gcc">GCC</a>: ??</li>
<li><a href="../../../implementation.html#icc">ICC</a>: ??</li>
<li><a href="../../../implementation.html#visual_cpp">Visual C++</a>: 2012 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
</ul>
<h2>関連項目</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><a href="op_assign.html">operator=</a></code></td>
<td>代入演算子</td>
</tr>
<tr>
<td><code><a href="insert.html">insert</a></code></td>
<td>要素を挿入する</td>
</tr>
</tbody>
</table>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2679.pdf" target="_blank">N2679 Initializer Lists for Standard Containers(Revision 1)</a><ul>
<li>(10)の経緯となる提案文書</li>
</ul>
</li>
<li><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2193" target="_blank">LWG 2193. Default constructors for standard library containers are explicit</a><br />
(1) + (2) を 2 つの<a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a>に分割するきっかけとなったレポート</li>
<li><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2210" target="_blank">LWG 2210. Missing allocator-extended constructor for allocator-aware containers</a><br />
(5), (11) を追加するきっかけとなったレポート<br />
なお、Discussion の例はアロケータの型が誤っているので注意</li>
<li><a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1518r2.html" target="_blank">P1518R2 Stop Overconstraining Allocators in Container Deduction Guides</a><ul>
<li>C++23でのアロケータ引数を<code>type_identity_t</code>で包む変更</li>
</ul>
</li>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3372r3.html" target="_blank">P3372R3 constexpr containers and adaptors</a><ul>
<li>C++26で<code>map</code>のメンバが<code>constexpr</code>対応した</li>
</ul>
</li>
<li><a href="https://cplusplus.github.io/LWG/issue4291" target="_blank">LWG Issue 4291. <code>explicit map(const Allocator&)</code> should be constexpr</a><ul>
<li>C++26で、他のコンストラクタと同様に(3) <code>explicit map(const Allocator&)</code>にも<code>constexpr</code>が追加された（P3372で付け忘れられていたものの修正）</li>
</ul>
</li>
</ul></div>
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