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2026年02月05日 04時53分18秒
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<div class="identifier-type">function template</div><div class="header"><memory></div><h1 itemprop="name"><span class="namespace" title="namespace std::ranges">std::ranges::</span><span class="token">uninitialized_default_construct</span><span class="cpp cpp20" title="C++20で追加">(C++20)</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="o">::</span><span class="nn">ranges</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="n"><a href="no-throw-forward-iterator.html">no-throw-forward-iterator</a></span><span class="w"> </span><span class="n">I</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="no-throw-sentinel.html">no-throw-sentinel</a></span><span class="o"><</span><span class="n">I</span><span class="o">></span><span class="w"> </span><span class="n">S</span><span class="o">></span>
<span class="w"> </span><span class="k">requires</span><span class="w"> </span><span class="n"><a href="../concepts/default_initializable.html">default_initializable</a></span><span class="o"><</span><span class="n"><a href="../iterator/iter_value_t.html">iter_value_t</a></span><span class="o"><</span><span class="n">I</span><span class="o">>></span>
<span class="w"> </span><span class="n">I</span>
<span class="w"> </span><span class="n">uninitialized_default_construct</span><span class="p">(</span><span class="n">I</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">S</span><span class="w"> </span><span class="n">last</span><span class="p">);</span><span class="w"> </span><span class="c1">// (1) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="n"><a href="no-throw-forward-iterator.html">no-throw-forward-iterator</a></span><span class="w"> </span><span class="n">I</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="no-throw-sentinel.html">no-throw-sentinel</a></span><span class="o"><</span><span class="n">I</span><span class="o">></span><span class="w"> </span><span class="n">S</span><span class="o">></span>
<span class="w"> </span><span class="k">requires</span><span class="w"> </span><span class="n"><a href="../concepts/default_initializable.html">default_initializable</a></span><span class="o"><</span><span class="n"><a href="../iterator/iter_value_t.html">iter_value_t</a></span><span class="o"><</span><span class="n">I</span><span class="o">>></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">I</span>
<span class="w"> </span><span class="n">uninitialized_default_construct</span><span class="p">(</span><span class="n">I</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">S</span><span class="w"> </span><span class="n">last</span><span class="p">);</span><span class="w"> </span><span class="c1">// (1) C++26</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="n"><a href="no-throw-forward-range.html">no-throw-forward-range</a></span><span class="w"> </span><span class="n">R</span><span class="o">></span>
<span class="w"> </span><span class="k">requires</span><span class="w"> </span><span class="n"><a href="../concepts/default_initializable.html">default_initializable</a></span><span class="o"><</span><span class="n"><a href="../ranges/range_value_t.html">range_value_t</a></span><span class="o"><</span><span class="n">R</span><span class="o">>></span>
<span class="w"> </span><span class="n"><a href="../ranges/borrowed_iterator_t.html">borrowed_iterator_t</a></span><span class="o"><</span><span class="n">R</span><span class="o">></span>
<span class="w"> </span><span class="n">uninitialized_default_construct</span><span class="p">(</span><span class="n">R</span><span class="o">&&</span><span class="w"> </span><span class="n">r</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="n"><a href="no-throw-forward-range.html">no-throw-forward-range</a></span><span class="w"> </span><span class="n">R</span><span class="o">></span>
<span class="w"> </span><span class="k">requires</span><span class="w"> </span><span class="n"><a href="../concepts/default_initializable.html">default_initializable</a></span><span class="o"><</span><span class="n"><a href="../ranges/range_value_t.html">range_value_t</a></span><span class="o"><</span><span class="n">R</span><span class="o">>></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n"><a href="../ranges/borrowed_iterator_t.html">borrowed_iterator_t</a></span><span class="o"><</span><span class="n">R</span><span class="o">></span>
<span class="w"> </span><span class="n">uninitialized_default_construct</span><span class="p">(</span><span class="n">R</span><span class="o">&&</span><span class="w"> </span><span class="n">r</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) C++26</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="n"><a href="../execution/execution-policy.html">execution-policy</a></span><span class="w"> </span><span class="n">Ep</span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../iterator/random_access_iterator.html">random_access_iterator</a></span><span class="w"> </span><span class="n">I</span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../iterator/sized_sentinel_for.html">sized_sentinel_for</a></span><span class="o"><</span><span class="n">I</span><span class="o">></span><span class="w"> </span><span class="n">S</span><span class="o">></span>
<span class="w"> </span><span class="k">requires</span><span class="w"> </span><span class="n"><a href="../concepts/default_initializable.html">default_initializable</a></span><span class="o"><</span><span class="n"><a href="../iterator/iter_value_t.html">iter_value_t</a></span><span class="o"><</span><span class="n">I</span><span class="o">>></span>
<span class="w"> </span><span class="n">I</span>
<span class="w"> </span><span class="n">uninitialized_default_construct</span><span class="p">(</span><span class="n">Ep</span><span class="o">&&</span><span class="w"> </span><span class="n">exec</span><span class="p">,</span>
<span class="w"> </span><span class="n">I</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">S</span><span class="w"> </span><span class="n">last</span><span class="p">);</span><span class="w"> </span><span class="c1">// (3) C++26</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="n"><a href="../execution/execution-policy.html">execution-policy</a></span><span class="w"> </span><span class="n">Ep</span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../ranges/sized-random-access-range.html">sized-random-access-range</a></span><span class="w"> </span><span class="n">R</span><span class="o">></span>
<span class="w"> </span><span class="k">requires</span><span class="w"> </span><span class="n"><a href="../concepts/default_initializable.html">default_initializable</a></span><span class="o"><</span><span class="n"><a href="../ranges/range_value_t.html">range_value_t</a></span><span class="o"><</span><span class="n">R</span><span class="o">>></span>
<span class="w"> </span><span class="n"><a href="../ranges/borrowed_iterator_t.html">borrowed_iterator_t</a></span><span class="o"><</span><span class="n">R</span><span class="o">></span>
<span class="w"> </span><span class="n">uninitialized_default_construct</span><span class="p">(</span><span class="n">Ep</span><span class="o">&&</span><span class="w"> </span><span class="n">exec</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">r</span><span class="p">);</span><span class="w"> </span><span class="c1">// (4) C++26</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p>未初期化領域の範囲 (<code>r</code>、<code>[first, last)</code>) の各要素を<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a>する。</p>
<ul>
<li>(1): <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>を指定する</li>
<li>(2): Rangeを直接指定する</li>
<li>(3): (1)の並列アルゴリズム版。実行ポリシーを指定する</li>
<li>(4): (2)の並列アルゴリズム版。実行ポリシーを指定する</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。Constraints。関数であればオーバーロード解決に参加するための条件、それ以外であれば受け付ける型の条件">テンプレートパラメータ制約</a></h2>
<ul>
<li>(1):<ul>
<li><code>I</code>が<code><a href="no-throw-forward-iterator.html">no-throw-forward-iterator</a></code>である</li>
<li><code>S</code>が<a href="no-throw-sentinel.html"><code>I</code>に対する例外を投げない番兵</a>である</li>
<li><code>I</code>の要素型が、<a href="../concepts/default_initializable.html">デフォルト構築可能</a>である</li>
</ul>
</li>
<li>(2):<ul>
<li><code>R</code>が<code><a href="no-throw-forward-range.html">no-throw-forward-range</a></code>である</li>
<li><code>R</code>の要素型が、<a href="../concepts/default_initializable.html">デフォルト構築可能</a>である</li>
</ul>
</li>
</ul>
<h2>効果</h2>
<p>説明用の関数<code>voidify</code>があるとして、</p>
<p><div class="codehilite"><pre><span></span><code><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="k">constexpr</span><span class="w"> </span><span class="kt">void</span><span class="o">*</span><span class="w"> </span><span class="n">voidify</span><span class="p">(</span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">obj</span><span class="p">)</span><span class="w"> </span><span class="k">noexcept</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="k">const_cast</span><span class="o"><</span><span class="kt">void</span><span class="o">*></span><span class="p">(</span><span class="k">static_cast</span><span class="o"><</span><span class="k">const</span><span class="w"> </span><span class="k">volatile</span><span class="w"> </span><span class="kt">void</span><span class="o">*></span><span class="p">(</span><span class="n"><a href="addressof.html">addressof</a></span><span class="p">(</span><span class="n">obj</span><span class="p">)));</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>以下と等価である：</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">for</span><span class="w"> </span><span class="p">(;</span><span class="w"> </span><span class="n">first</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">last</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">first</span><span class="p">)</span>
<span class="w"> </span><span class="o">::</span><span class="k">new</span><span class="w"> </span><span class="p">(</span><span class="n">voidify</span><span class="p">(</span><span class="o">*</span><span class="n">first</span><span class="p">))</span><span class="w"> </span><span class="n"><a href="../type_traits/remove_reference.html">remove_reference_t</a></span><span class="o"><</span><span class="n"><a href="../iterator/iter_reference_t.html">iter_reference_t</a></span><span class="o"><</span><span class="n">I</span><span class="o">>></span><span class="p">;</span>
<span class="k">return</span><span class="w"> </span><span class="n">first</span><span class="p">;</span>
</code></pre></div>
</p>
<h2><a class="cpprefjp-defined-word" data-desc="問題が発生したときに、現在実行位置を過去に通過・記録した位置に戻し、文脈情報を添えて紐づけられた処理(例外ハンドラー)を呼び出す仕組み。またはその事態">例外</a></h2>
<p>呼び出すコンストラクタなどから<a class="cpprefjp-defined-word" data-desc="問題が発生したときに、現在実行位置を過去に通過・記録した位置に戻し、文脈情報を添えて紐づけられた処理(例外ハンドラー)を呼び出す仕組み。またはその事態">例外</a>が送出された場合、その<a class="cpprefjp-defined-word" data-desc="問題が発生したときに、現在実行位置を過去に通過・記録した位置に戻し、文脈情報を添えて紐づけられた処理(例外ハンドラー)を呼び出す仕組み。またはその事態">例外</a>がこの関数の外側に伝播される前に、その時点で構築済のオブジェクトは全て<a class="cpprefjp-defined-word" data-desc="未規定の動作。事前の断りなく処理系によって異なる動作をする" href="../../implementation-compliance.html#dfn-unspecified-behavior">未規定</a>の順序で破棄される。すなわち、<a class="cpprefjp-defined-word" data-desc="問題が発生したときに、現在実行位置を過去に通過・記録した位置に戻し、文脈情報を添えて紐づけられた処理(例外ハンドラー)を呼び出す仕組み。またはその事態">例外</a>が送出された場合は初期化対象領域は未初期化のままとなる。</p>
<h2>備考</h2>
<ul>
<li><code><a href="../vector/vector.html">std::vector</a></code>クラスの要素数を変更する操作は、要素を<a class="cpprefjp-defined-word" data-desc="値初期化。`T x{};`構文での初期化。組み込み型の場合はゼロ相当の値で、クラス型の場合はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的に値初期化される">値構築</a>するためゼロ初期化が行われる。その<a class="cpprefjp-defined-word" data-desc="`T x{};`構文での初期化。組み込み型の場合はゼロ相当の値で、クラス型の場合はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的に値初期化される">値初期化</a>のコストが気になるような場合に、<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a>することでプログラマの責任で必要な分だけ任意に初期化でき、パフォーマンス向上が期待できるようになる。<ul>
<li>例としてBoost Container Libraryの<code>vector</code>クラスには、要素数を変更するメンバ関数に<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a>のオプションとして<code><a href="https://www.boost.org/doc/libs/release/doc/html/container/extended_functionality.html#container.extended_functionality.default_initialialization" target="_blank">default_init</a></code>がある</li>
</ul>
</li>
</ul>
<h2>例</h2>
<h3>基本的な使い方</h3>
<p><div class="yata" id="dcf72cd0a6acc00342c431cf36708269129ac9fd"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../memory.html"><memory></a></span>
<span class="cp">#include <a href="../algorithm.html"><algorithm></a></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Vector</span><span class="w"> </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="n">y</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="allocator.html">std::allocator</a></span><span class="o"><</span><span class="n">Vector</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">// メモリ確保。</span>
<span class="w"> </span><span class="c1">// この段階では、[p, p + size)の領域は未初期化</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n"><a href="../cstddef/size_t.html">std::size_t</a></span><span class="w"> </span><span class="n">size</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">Vector</span><span class="o">*</span><span class="w"> </span><span class="n">p</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="allocator/allocate.html">alloc.allocate</a></span><span class="p">(</span><span class="n">size</span><span class="p">);</span>
<span class="w"> </span><span class="c1">// 未初期化領域[p, p + size)の各要素をデフォルト構築</span>
<span class="w"> </span><span class="n"><span style="color:#ff0000">std::ranges::uninitialized_default_construct</span></span><span class="p">(</span><span class="n"><a href="../ranges/subrange.html">std::ranges::subrange</a></span><span class="p">{</span><span class="n">p</span><span class="p">,</span><span class="w"> </span><span class="n">p</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">size</span><span class="p">});</span>
<span class="w"> </span><span class="c1">// pの領域が初期化され、かつ範囲pの全ての要素が2で埋められているか確認</span>
<span class="w"> </span><span class="n"><a href="../algorithm/for_each.html">std::for_each</a></span><span class="p">(</span><span class="n">p</span><span class="p">,</span><span class="w"> </span><span class="n">p</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">size</span><span class="p">,</span><span class="w"> </span><span class="p">[](</span><span class="k">const</span><span class="w"> </span><span class="n">Vector</span><span class="o">&</span><span class="w"> </span><span class="n">v</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="n">v</span><span class="p">.</span><span class="n">x</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">','</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">v</span><span class="p">.</span><span class="n">y</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="w"> </span><span class="c1">// 要素を破棄</span>
<span class="w"> </span><span class="n"><a href="ranges_destroy.html">std::ranges::destroy</a></span><span class="p">(</span><span class="n">p</span><span class="p">,</span><span class="w"> </span><span class="n">p</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">size</span><span class="p">);</span>
<span class="w"> </span><span class="c1">// メモリ解放</span>
<span class="w"> </span><span class="n"><a href="allocator/deallocate.html">alloc.deallocate</a></span><span class="p">(</span><span class="n">p</span><span class="p">,</span><span class="w"> </span><span class="n">size</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h3>出力例</h3>
<p><pre><code>1445540552,1445540279
0,1445540279
0,1445540279
</code></pre></p>
<h3>並列アルゴリズムの例 (C++26)</h3>
<p><div class="yata" id="b027c75a86bf886293bad43780dac0d3feea68ce"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../memory.html"><memory></a></span>
<span class="cp">#include <a href="../execution.html"><execution></a></span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="allocator.html">std::allocator</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">alloc</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">p</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">alloc</span><span class="p">.</span><span class="n">allocate</span><span class="p">(</span><span class="mi">3</span><span class="p">);</span>
<span class="w"> </span><span class="c1">// 並列にデフォルト構築</span>
<span class="w"> </span><span class="n"><span style="color:#ff0000">std::ranges::uninitialized_default_construct</span></span><span class="p">(</span>
<span class="w"> </span><span class="n"><a href="../execution/execution/execution_policy.html">std::execution::par</a></span><span class="p">,</span><span class="w"> </span><span class="n">p</span><span class="p">,</span><span class="w"> </span><span class="n">p</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">alloc</span><span class="p">.</span><span class="n">deallocate</span><span class="p">(</span><span class="n">p</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"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"done"</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</h4>
<p><pre><code>done
</code></pre></p>
<h2>バージョン</h2>
<h3>言語</h3>
<ul>
<li>C++20</li>
</ul>
<h3>処理系</h3>
<ul>
<li><a href="../../implementation.html#clang">Clang</a>: 16.0 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../implementation.html#gcc">GCC</a>: 10.2.0 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../implementation.html#visual_cpp">Visual C++</a>: 2019 Update 10 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
</ul>
<h2>関連項目</h2>
<ul>
<li><code><a href="uninitialized_default_construct.html">uninitialized_default_construct</a></code></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0896r4.pdf" target="_blank">P0896R4 The One Ranges Proposal</a></li>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3508r0.html" target="_blank">P3508R0 Wording for "constexpr for specialized memory algorithms"</a></li>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3369r0.html" target="_blank">P3369R0 <code>constexpr</code> for <code>uninitialized_default_construct</code></a><ul>
<li>上記2文書で、C++26から<code>constexpr</code>がついた</li>
</ul>
</li>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3179r9.html" target="_blank">P3179R9 C++ parallel range algorithms</a></li>
</ul></div>
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