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<header data-kunai-mdinfo="{"meta": {"cpp": ["cpp11"]}, "sources": [{"id": "094233b424ecefe3fb55532e50b78184615fdab6", "source": "#include <vector>\n#include <initializer_list>\n\ntemplate <class T>\nclass MyVector {\n std::vector<T> data_;\npublic:\n // \u30ea\u30b9\u30c8\u521d\u671f\u5316\u7528\u306e\u30b3\u30f3\u30b9\u30c8\u30e9\u30af\u30bf\n MyVector(std::initializer_list<T> init)\n : data_(init.begin(), init.end()) {}\n};\n\nint main()\n{\n MyVector<int> v = {1, 2, 3};\n}\n"}, {"id": "9a99bc8734f51d1764e9c9a175148c2268f07c63", "source": "#include <vector>\n#include <initializer_list>\n\ntemplate <class T>\nclass MyVector {\n std::vector<T> data_;\npublic:\n // \u30ea\u30b9\u30c8\u521d\u671f\u5316\u7528\u306e\u30b3\u30f3\u30b9\u30c8\u30e9\u30af\u30bf\n explicit MyVector(std::initializer_list<T> init)\n : data_(init.begin(), init.end()) {}\n};\n\nint main()\n{\n// MyVector<int> v = {1, 2, 3}; // \u30b3\u30f3\u30d1\u30a4\u30eb\u30a8\u30e9\u30fc\uff01\n MyVector<int> v {1, 2, 3}; // OK\n}\n"}, {"id": "073bfccb6f7b2718d281bc1a4a6ae36b15faeac8", "source": "#include <iostream>\n#include <initializer_list>\n\nstruct X {\n X()\n {\n std::cout << \"default constructor\" << std::endl;\n }\n\n X(std::initializer_list<int>)\n {\n std::cout << \"initializer-list constructor\" << std::endl;\n }\n};\n\nint main()\n{\n X x = {}; // \u300cdefault constructor\u300d\u304c\u51fa\u529b\u3055\u308c\u308b\n}\n"}, {"id": "4c934af9bbcfffc2fb73601aef1a94e034aa1146", "source": "#include <iostream>\n#include <initializer_list>\n\nint f()\n{\n std::cout << 1 << std::endl;\n return 1;\n}\n\nint g()\n{\n std::cout << 2 << std::endl;\n return 2;\n}\n\nint h()\n{\n std::cout << 3 << std::endl;\n return 3;\n}\n\nint main()\n{\n std::initializer_list<int> init = { f(), g(), h() }; // 1, 2, 3\u306e\u9806\u3067\u51fa\u529b\u3055\u308c\u308b\n}\n"}], "page_id": ["lang", "cpp11", "initializer_lists"]}">
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<h1 itemprop="name"><span class="token">初期化子リスト [N2672]</span><span class="cpp cpp11" title="C++11で追加">(C++11)</span></h1>
<div itemprop="articleBody"><p></p>
<p>このページはC++11に採用された言語機能の変更を解説しています。</p>
<p>のちのC++規格でさらに変更される場合があるため<a href="#relative-page">関連項目</a>を参照してください。</p>
<p></p>
<h2>概要</h2>
<p>「初期化子リスト (initializer lists)」は、<a class="cpprefjp-defined-word" data-desc="プログラム定義型。標準ライブラリの型を除く、ユーザー（プログラマ）によって定義された型のこと">ユーザー定義型</a>のオブジェクトに対して、波カッコによるリスト初期化を使用できるようにするよう<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>する機能である。</p>
<p>これによって、<code><a href="../../reference/vector.html">std::vector</a></code>のようなコンテナクラスに対しても、組み込み配列と同様に、波カッコによる簡易的な初期化構文を使用できる：</p>
<p><div class="codehilite"><pre><span></span><code><span class="c1">// 1, 2, 3の3要素を持つ配列オブジェクトを定義する</span>
<span class="kt">int</span><span class="w"> </span><span class="n">ar</span><span class="p">[]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span>
<span class="n"><a href="../../reference/vector/vector.html">std::vector</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">v1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span>
<span class="n"><a href="../../reference/vector/vector.html">std::vector</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">v2</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span>
<span class="c1">// 再代入</span>
<span class="n">v2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">4</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="p">};</span>
</code></pre></div>
</p>
<p><a class="cpprefjp-defined-word" data-desc="プログラム定義型。標準ライブラリの型を除く、ユーザー（プログラマ）によって定義された型のこと">ユーザー定義型</a>に対してリスト初期化を許可するためには、<code><a href="../../reference/initializer_list.html"><initializer_list></a></code>ヘッダで定義される<code><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></code>クラスのオブジェクトをパラメータとして、コンストラクタや代入演算子を<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>する：</p>
<p><div class="yata" id="094233b424ecefe3fb55532e50b78184615fdab6"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/vector.html"><vector></a></span>
<span class="cp">#include <a href="../../reference/initializer_list.html"><initializer_list></a></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="k">class</span><span class="w"> </span><span class="nc">MyVector</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/vector/vector.html">std::vector</a></span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="w"> </span><span class="n">data_</span><span class="p">;</span>
<span class="k">public</span><span class="o">:</span>
<span class="w"> </span><span class="c1">// リスト初期化用のコンストラクタ</span>
<span class="w"> </span><span class="n">MyVector</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="w"> </span><span class="n">init</span><span class="p">)</span>
<span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">data_</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list/begin.html">init.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../../reference/initializer_list/initializer_list/end.html">init.end()</a></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="n">MyVector</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<p>リスト初期化用のコンストラクタに<code>explicit</code>を付けた場合、代入演算子によるリスト初期化は許可せず、代入演算子を伴わないリスト初期化のみが許可される：</p>
<p><div class="yata" id="9a99bc8734f51d1764e9c9a175148c2268f07c63"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/vector.html"><vector></a></span>
<span class="cp">#include <a href="../../reference/initializer_list.html"><initializer_list></a></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="k">class</span><span class="w"> </span><span class="nc">MyVector</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/vector/vector.html">std::vector</a></span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="w"> </span><span class="n">data_</span><span class="p">;</span>
<span class="k">public</span><span class="o">:</span>
<span class="w"> </span><span class="c1">// リスト初期化用のコンストラクタ</span>
<span class="w"> </span><span class="k">explicit</span><span class="w"> </span><span class="n">MyVector</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="w"> </span><span class="n">init</span><span class="p">)</span>
<span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">data_</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list/begin.html">init.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../../reference/initializer_list/initializer_list/end.html">init.end()</a></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="c1">// MyVector<int> v = {1, 2, 3}; // コンパイルエラー！</span>
<span class="w"> </span><span class="n">MyVector</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span><span class="w"> </span><span class="c1">// OK</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h2>仕様</h2>
<ul>
<li>波カッコ <code>{ }</code> を使用した初期化子のリストによるオブジェクトもしくは参照の初期化を、「リスト初期化 (list initialization)」と呼び、その初期化子を「初期化子リスト (initializer list)」と呼ぶ。初期化子リストは、カンマ区切りで要素を列挙する</li>
<li>初期化子リストは、空であってもよい</li>
</ul>
<h3><a href="#initializer-list-contexts" id="initializer-list-contexts">初期化子リストを使用できる文脈</a></h3>
<p>初期化子リストは、以下の文脈で使用できる：</p>
<ul>
<li>
<p>変数定義での初期化子リストによる初期化</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
<span class="n">X</span><span class="w"> </span><span class="n">x1</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span><span class="w"> </span><span class="c1">// 直接初期化して変数定義</span>
<span class="n">X</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span><span class="w"> </span><span class="c1">// 直接初期化して一時オブジェクトを定義</span>
<span class="n">X</span><span class="w"> </span><span class="n">x2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span><span class="w"> </span><span class="c1">// コピー初期化して変数定義</span>
</code></pre></div>
</p>
</li>
<li>
<p><code>new</code>式での初期化子リストによる初期化</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">new</span><span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span><span class="w"> </span><span class="c1">// 動的記憶域でXオブジェクトを直接初期化</span>
</code></pre></div>
</p>
</li>
<li>
<p><code>return</code>文</p>
<p><div class="codehilite"><pre><span></span><code><span class="n">X</span><span class="w"> </span><span class="nf">f</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span>
<span class="p">}</span>
</code></pre></div>
</p>
</li>
<li>
<p>関数の引数</p>
<p><div class="codehilite"><pre><span></span><code><span class="kt">void</span><span class="w"> </span><span class="nf">f</span><span class="p">(</span><span class="n">X</span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="n">f</span><span class="p">({</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">});</span>
</code></pre></div>
</p>
</li>
<li>
<p>式の一部</p>
<p><div class="codehilite"><pre><span></span><code><span class="n"><a href="../../reference/vector/vector.html">std::vector</a></span><span class="o"><</span><span class="n">X</span><span class="o">></span><span class="w"> </span><span class="n">xs</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">},</span><span class="w"> </span><span class="c1">// 初期化子リストのなかでさらに初期化子リストを使用する</span>
<span class="w"> </span><span class="p">{</span><span class="mi">4</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="p">}</span>
<span class="p">};</span>
</code></pre></div>
</p>
</li>
<li>
<p>基底クラスやメンバの初期化子</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">Y</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/vector/vector.html">std::vector</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">values</span><span class="p">;</span>
<span class="w"> </span><span class="n">Y</span><span class="p">()</span>
<span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">},</span><span class="w"> </span><span class="n">values</span><span class="w"> </span><span class="p">{</span><span class="mi">4</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="p">}</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
</code></pre></div>
</p>
</li>
<li>
<p>代入演算子の右辺</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="o">&</span><span class="w"> </span><span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="o">*</span><span class="k">this</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="n">X</span><span class="w"> </span><span class="n">x</span><span class="p">;</span>
<span class="n">x</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span>
</code></pre></div>
</p>
</li>
</ul>
<h3><a href="#narrow-conversion" id="narrow-conversion">縮小変換</a></h3>
<ul>
<li>
<p>初期化子リストに縮小変換が要求された場合、プログラムは<a class="cpprefjp-defined-word" data-desc="プログラムが適格でないこと。コンパイルエラーなどになる" href="../../implementation-compliance.html#dfn-ill-formed">不適格</a>となる</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
<span class="n">X</span><span class="w"> </span><span class="n">x1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span><span class="w"> </span><span class="c1">// OK</span>
<span class="c1">//X x2 = {1, 2, 3.0}; // コンパイルエラー！3.0をint型に縮小変換できない</span>
</code></pre></div>
</p>
</li>
<li>
<p>縮小変換以外の型変換は許可される</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="kt">double</span><span class="o">></span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
<span class="n">X</span><span class="w"> </span><span class="n">x1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span><span class="w"> </span><span class="c1">// OK</span>
<span class="n">X</span><span class="w"> </span><span class="n">x2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mf">3.0</span><span class="p">};</span><span class="w"> </span><span class="c1">// OK</span>
</code></pre></div>
</p>
</li>
</ul>
<h3><a href="#initializer-list-constructor" id="initializer-list-constructor">初期化子リストコンストラクタ</a></h3>
<p>以下の条件を満たすコンストラクタを、「初期化子リストコンストラクタ (initializer-list constructor)」と呼ぶ：</p>
<ul>
<li>任意の型<code>E</code>を要素とする<code>std::initializer_list<E></code>型のパラメータをひとつだけとり、そのほかのパラメータを持たない</li>
<li>もしくは、<code>std::initializer_list<E></code>型のパラメータおよび、それ以降にデフォルト引数を持つ</li>
</ul>
<h3><a href="#overload-resolution" id="overload-resolution">オーバーロード解決</a></h3>
<ul>
<li>
<p>デフォルトコンストラクタと初期化子リストコンストラクタがある場合、空の初期化子リストが渡された際にはデフォルトコンストラクタが呼び出される</p>
<p><div class="yata" id="073bfccb6f7b2718d281bc1a4a6ae36b15faeac8"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../../reference/initializer_list.html"><initializer_list></a></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">()</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"default constructor"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">)</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"initializer-list constructor"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></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="n">X</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{};</span><span class="w"> </span><span class="c1">// 「default constructor」が出力される</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
</li>
<li>
<p>初期化子リストコンストラクタと、その初期化子リストの要素型と同じ型のパラメータリストを受け取るコンストラクタでは、初期化子リストコンストラクタが優先して呼び出される。そのような状況では、丸カッコでのコンストラクタ呼び出しが必要となる</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="kt">double</span><span class="o">></span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="kt">double</span><span class="w"> </span><span class="n">d</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="n">X</span><span class="w"> </span><span class="n">x1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mf">3.0</span><span class="p">};</span><span class="w"> </span><span class="c1">// 「1」が出力される</span>
</code></pre></div>
</p>
</li>
<li>
<p>異なる要素型を持つ<code>std::initializer_list</code>型同士で<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>ができる</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">)</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="kt">double</span><span class="o">></span><span class="p">)</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="n">X</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span><span class="w"> </span><span class="c1">// 「1」が出力される</span>
<span class="n">X</span><span class="w"> </span><span class="p">{</span><span class="mf">1.0</span><span class="p">,</span><span class="w"> </span><span class="mf">2.0</span><span class="p">,</span><span class="w"> </span><span class="mf">3.0</span><span class="p">};</span><span class="w"> </span><span class="c1">// 「2」が出力される</span>
</code></pre></div>
</p>
</li>
</ul>
<h3><a href="#life-time" id="life-time">初期化子リストオブジェクトの寿命</a></h3>
<ul>
<li>
<p>初期化子リストが暗黙的に<code>std::initializer_list<E></code>に型変換される際、実装は<code>E</code>型の要素を<code>N</code>個持つ配列を確保するかのように振る舞う。変換された<code>std::initializer_list<E></code>オブジェクトは、元となった初期化子リストの配列を参照する。以下のような初期化子リストの引数渡しがあった場合、</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="kt">double</span><span class="o">></span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
<span class="n">X</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span>
</code></pre></div>
</p>
<p>実装は以下と等価の初期化を行う (実装が用意した<code>std::initializer_list</code>クラスがポインタの組を受け取れると仮定する)：</p>
<p><div class="codehilite"><pre><span></span><code><span class="kt">double</span><span class="w"> </span><span class="n">__a</span><span class="p">[</span><span class="mi">3</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="kt">double</span><span class="p">{</span><span class="mi">1</span><span class="p">},</span><span class="w"> </span><span class="kt">double</span><span class="p">{</span><span class="mi">2</span><span class="p">},</span><span class="w"> </span><span class="kt">double</span><span class="p">{</span><span class="mi">3</span><span class="p">}};</span>
<span class="n">X</span><span class="w"> </span><span class="nf">x</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="kt">double</span><span class="o">></span><span class="p">(</span><span class="n">__a</span><span class="p">,</span><span class="w"> </span><span class="n">__a</span><span class="o">+</span><span class="mi">3</span><span class="p">));</span>
</code></pre></div>
</p>
<p>元となった配列の寿命は、変換先の<code>std::initializer_list</code>オブジェクトと同じとなる</p>
</li>
</ul>
<h3><a href="#type-deduction" id="type-deduction">型推論</a></h3>
<ul>
<li>
<p>初期化子リストを<code>auto</code>で受けた場合、<code>std::initializer_list</code>型に推論される。ただし、空の初期化子リストは推論に失敗する</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">auto</span><span class="w"> </span><span class="n">x1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span><span class="w"> </span><span class="c1">// x1の型は<a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a><int></span>
<span class="c1">//auto x2 = {}; // コンパイルエラー！x2の型を推論できない</span>
</code></pre></div>
</p>
</li>
<li>
<p>単一要素の初期化子リストを<code>auto</code>で受けた場合、C++11では<code>std::initializer_list<T></code>型に推論されるが、C++17では直接初期化の場合<code>T</code>型に推論されるよう仕様が変更されるので注意</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">auto</span><span class="w"> </span><span class="n">x</span><span class="p">{</span><span class="mi">1</span><span class="p">};</span><span class="w"> </span><span class="c1">// C++17ではxの型はintになる</span>
<span class="k">auto</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">};</span><span class="w"> </span><span class="c1">// C++11,17共に、xの型は<a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a><int>。</span>
</code></pre></div>
</p>
</li>
<li>
<p>関数テンプレートのパラメータとして初期化子リストを受けとった場合は、<code>std::initializer_list</code>型には推論されない</p>
<p><div class="codehilite"><pre><span></span><code><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="kt">void</span><span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="n">T</span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="kt">int</span><span class="w"> </span><span class="n">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n">f</span><span class="p">({</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">});</span><span class="w"> </span><span class="c1">// コンパイルエラー！Tの型を推論できない</span>
<span class="p">}</span>
</code></pre></div>
</p>
</li>
<li>
<p><code>std::initializer_list</code>の要素型のみを関数テンプレートで推論させることはできる</p>
<p><div class="codehilite"><pre><span></span><code><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="kt">void</span><span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="n">f</span><span class="p">({</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">});</span><span class="w"> </span><span class="c1">// OK : Tはint</span>
</code></pre></div>
</p>
</li>
</ul>
<h3><a href="#evaluation-order" id="evaluation-order">評価順序</a></h3>
<ul>
<li>
<p>初期化子リストに列挙した要素は、先頭から順番に評価されることが保証される</p>
<p><div class="yata" id="4c934af9bbcfffc2fb73601aef1a94e034aa1146"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../../reference/initializer_list.html"><initializer_list></a></span>
<span class="kt">int</span><span class="w"> </span><span class="nf">f</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span>
<span class="p">}</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">g</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">2</span><span class="p">;</span>
<span class="p">}</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">h</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">3</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">3</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="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">init</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">f</span><span class="p">(),</span><span class="w"> </span><span class="n">g</span><span class="p">(),</span><span class="w"> </span><span class="n">h</span><span class="p">()</span><span class="w"> </span><span class="p">};</span><span class="w"> </span><span class="c1">// 1, 2, 3の順で出力される</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
</li>
</ul>
<h2>この機能が必要になった背景・経緯</h2>
<p>C++の目標として、「組み込み型の振る舞いを<a class="cpprefjp-defined-word" data-desc="プログラム定義型。標準ライブラリの型を除く、ユーザー（プログラマ）によって定義された型のこと">ユーザー定義型</a>で定義できるようにする」というものがある。しかし、組み込み配列での波カッコを使用したリスト初期化は、<a class="cpprefjp-defined-word" data-desc="プログラム定義型。標準ライブラリの型を除く、ユーザー（プログラマ）によって定義された型のこと">ユーザー定義型</a>に対して<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>ができなかった。それにより、<code><a href="../../reference/vector.html">std::vector</a></code>のようなコンテナクラスの初期化が使いにくいものとなっていた：</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">const</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">N</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">3</span><span class="p">;</span>
<span class="kt">int</span><span class="w"> </span><span class="n">ar</span><span class="p">[</span><span class="n">N</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">};</span>
<span class="n"><a href="../../reference/vector/vector.html">std::vector</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">v</span><span class="p">(</span><span class="n">ar</span><span class="p">,</span><span class="w"> </span><span class="n">ar</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">N</span><span class="p">);</span>
</code></pre></div>
</p>
<p>この問題を解決するために、波カッコによるリスト初期化を<a class="cpprefjp-defined-word" data-desc="プログラム定義型。標準ライブラリの型を除く、ユーザー（プログラマ）によって定義された型のこと">ユーザー定義型</a>で<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>する機能が求められ、<code><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></code>クラスと<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>機能が導入された。</p>
<h2><a href="#relative-page" id="relative-page">関連項目</a></h2>
<ul>
<li><a href="uniform_initialization.html">C++11 一様初期化</a></li>
<li><a href="../cpp26/static_storage_for_braced_initializers.html">C++26 <code>std::initializer_list</code>の配列を静的記憶域に配置する</a></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1493.pdf" target="_blank">N1493 Braces Initialization Overloading</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1509.pdf" target="_blank">N1509 Generalized Initializer Lists</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1584.pdf" target="_blank">N1584 Regularizing Initialization Syntax</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1701.pdf" target="_blank">N1701 Regularizing Initialization Syntax (revision 1)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1824.htm" target="_blank">N1824 Extending Aggregate Initialization</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1890.pdf" target="_blank">N1890 Initialization and initializers</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1919.pdf" target="_blank">N1919 Initializer lists</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2100.pdf" target="_blank">N2100 Initializer lists (Rev. 2)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2215.pdf" target="_blank">N2215 Initializer lists (Rev. 3)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2531.pdf" target="_blank">N2531 Initializer lists WP wording (Revision 2)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2575.pdf" target="_blank">N2575 Initializer Lists - Alternative Mechanism and Rationale</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2640.pdf" target="_blank">N2640 Initializer Lists - Alternative Mechanism and Rationale (v. 2)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2672.htm" target="_blank">N2672 Initializer List proposed wording</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1030" target="_blank">CWG Issue 1030. Evaluation order in initializer-lists used in aggregate initialization</a></li>
</ul></div>
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