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<div class="identifier-type">function template</div><div class="header"><functional></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">bind_front</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="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n"><i>unspecified</i></span><span class="w"> </span><span class="n">bind_front</span><span class="p">(</span><span class="n">F</span><span class="o">&&</span><span class="w"> </span><span class="n">f</span><span class="p">,</span><span class="w"> </span><span class="n">Args</span><span class="o">&&</span><span class="p">...</span><span class="w"> </span><span class="n">args</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="k">auto</span><span class="w"> </span><span class="n">f</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n"><i>unspecified</i></span><span class="w"> </span><span class="n">bind_front</span><span class="p">(</span><span class="n">Args</span><span class="o">&&</span><span class="p">...</span><span class="w"> </span><span class="n">args</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) C++26</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p>関数の引数を先頭から順に部分適用する。</p>
<p>この関数は、メンバ関数とレシーバー (<code>*this</code>) を束縛して持ち運び、引数をあとで渡して呼び出す、というユースケースで使用する。そのようなケースでは、<code>const</code>性の伝播が難しいラムダ式や<code><a href="bind.html">std::bind()</a></code>よりも容易になる。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">using</span><span class="w"> </span><span class="k">namespace</span><span class="w"> </span><span class="nn"><a href="placeholders.html">std::placeholders</a></span><span class="p">;</span>
<span class="c1">// 4引数をもつメンバ関数Strategy::process</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Strategy</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="nf">process</span><span class="p">(</span><span class="n"><a href="../string/basic_string.html">std::string</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../string/basic_string.html">std::string</a></span><span class="p">,</span><span class="w"> </span><span class="kt">double</span><span class="p">,</span><span class="w"> </span><span class="kt">double</span><span class="p">);</span><span class="w"> </span><span class="p">};</span>
<span class="c1">// 1. ラムダ式の場合、非constメンバ関数を呼び出すためにmutableを付けなければならないため、</span>
<span class="c1">// シンプルなメンバ関数のラップができない。</span>
<span class="c1">// コーナーケースとしては、<a href="../../lang/cpp14/decltype_auto.html">decltype(auto)</a>の考慮もときに必要になる</span>
<span class="k">auto</span><span class="w"> </span><span class="n">f1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">[</span><span class="n">s</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Strategy</span><span class="p">{}]</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="o">&&</span><span class="p">...</span><span class="w"> </span><span class="n">args</span><span class="p">)</span><span class="w"> </span><span class="k">mutable</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">s</span><span class="p">.</span><span class="n">process</span><span class="p">(</span><span class="n"><a href="../utility/forward.html">std::forward</a></span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">args</span><span class="p">)</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)...);</span>
<span class="p">};</span>
<span class="c1">// 2. <a href="bind.html">std::bind</a>()の場合、引数の数をユーザーが知っており、</span>
<span class="c1">// プレースホルダーを適切に設定しなければならない</span>
<span class="k">auto</span><span class="w"> </span><span class="n">f2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="bind.html">std::bind</a></span><span class="p">(</span><span class="o">&</span><span class="n">Strategy</span><span class="o">::</span><span class="n">process</span><span class="p">,</span><span class="w"> </span><span class="n">Strategy</span><span class="p">{},</span><span class="w"> </span><span class="n"><a href="placeholders.html">_1</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="placeholders.html">_2</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="placeholders.html">_3</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="placeholders.html">_4</a></span><span class="p">);</span>
<span class="c1">// 3. std::bind_front()の場合、const性の伝播が自動的に行われ、</span>
<span class="c1">// プレースホルダーを指定する必要がない</span>
<span class="k">auto</span><span class="w"> </span><span class="n">f3</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">bind_front</span><span class="p">(</span><span class="o">&</span><span class="n">Strategy</span><span class="o">::</span><span class="n">process</span><span class="p">,</span><span class="w"> </span><span class="n">Strategy</span><span class="p">{});</span>
</code></pre></div>
</p>
<p>ただし、この関数はメンバ関数とレシーバーを受け取る専用にはなっておらず、「引数を先頭から順に束縛する」という汎用的な機能になっているため、「メンバ関数ポインタのみを束縛」「関数と引数の一部を束縛」といった利用もできる。</p>
<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。Preconditions。関数呼び出し時に満たされていると関数が想定する条件。満たさなければ未定義の動作。契約属性の`[[expects]]`に相当">事前条件</a></h2>
<p><code><a href="../type_traits/decay.html">decay_t</a><F></code>を適用した型を<code>FD</code>、<code><a href="../type_traits/decay.html">decay_t</a><Args>...</code>を適用した型パラメータパックを<code>BoundArgs</code>として</p>
<ul>
<li>(1) : <ul>
<li><code>FD</code>がCpp17MoveConstructible要件を満たすこと</li>
<li><code>BoundArgs</code>のそれぞれの型<code>Ti</code>が<a href="../type_traits/is_object.html">オブジェクト型</a>である場合、Cpp17MoveConstructible要件を満たすこと</li>
</ul>
</li>
<li>(2) :<ul>
<li><code>BoundArgs</code>のそれぞれの型<code>Ti</code>がCpp17MoveConstructible要件を満たすこと</li>
</ul>
</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。Mandates。満たさなければプログラムが不適格となる要件。この要件は、オーバーロード解決によって選択された関数内での`static_assert`の定数条件式や、関数に対するdelete宣言などで表現できる">適格要件</a></h2>
<ul>
<li>(1) :<ul>
<li><code><a href="../type_traits/is_constructible.html">is_constructible_v</a><FD, F> && <a href="../type_traits/is_move_constructible.html">is_move_constructible_v</a><FD> && (<a href="../type_traits/is_constructible.html">is_constructible_v</a><BoundArgs, Args> && ...) && (<a href="../type_traits/is_move_constructible.html">is_move_constructible_v</a><BoundArgs> && ...)</code>が<code>true</code>であること</li>
</ul>
</li>
<li>(2) : <code>F</code>を<code>f</code>の型として<ul>
<li><code>(<a href="../type_traits/is_constructible.html">is_constructible_v</a><BoundArgs, Args> && ...)</code>が<code>true</code>、かつ</li>
<li><code>(<a href="../type_traits/is_move_constructible.html">is_move_constructible_v</a><BoundArgs> && ...)</code>が<code>true</code>、かつ</li>
<li>もし<code><a href="../type_traits/is_pointer.html">is_pointer_v</a><F> || <a href="../type_traits/is_member_pointer.html">is_member_pointer_v</a><F></code>が<code>true</code>ならば、<code>f != nullptr</code>であること</li>
</ul>
</li>
</ul>
<h2><a class="cpprefjp-defined-word" data-desc="関数呼び出し式の評価結果となるオブジェクト・値">戻り値</a></h2>
<p>呼び出し可能な<code>f</code>を<code><a href="invoke.html">std::invoke()</a></code>で呼び出した時に必要な引数列に前方一致する<code>f</code>と<code>args...</code>を完全転送して保持し、後から残りの引数リストを渡すことで<code>f</code>を呼び出せる<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>の関数オブジェクトを返す。</p>
<p>返される関数オブジェクトは渡された引数（<code>f, args...</code>）を参照として保持せず、適切にコピー/ムーブして保持する。</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>
<ul>
<li>(1) : 関数オブジェクト<code>f</code>のムーブによって任意の<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>が送出される可能性がある</li>
<li>(2) : <code>bound_args</code>の初期化による任意の<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>が送出される可能性がある</li>
</ul>
<h2>例</h2>
<p><div class="yata" id="679d60a543709cfe31a5e2f408e88e2d997534b9"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../functional.html"><functional></a></span>
<span class="k">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="kt">void</span><span class="w"> </span><span class="nf">f</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">c</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">a</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">b</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">c</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="kt">void</span><span class="w"> </span><span class="nf">g</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">c</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">a</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">b</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">c</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>
<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="p">{</span>
<span class="w"> </span><span class="c1">// メンバ関数ポインタと*thisを束縛する</span>
<span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="n">x</span><span class="p">;</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::bind_front</span></span><span class="p">(</span><span class="o">&</span><span class="n">X</span><span class="o">::</span><span class="n">f</span><span class="p">,</span><span class="w"> </span><span class="n">x</span><span class="p">);</span>
<span class="w"> </span><span class="c1">// メンバ関数の引数を渡して呼び出す</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="p">}</span>
<span class="w"> </span><span class="c1">// 「メンバ関数ポインタと*thisの束縛」という機能に限定されてはおらず、</span>
<span class="w"> </span><span class="c1">// 先頭から順に束縛できる (関数オブジェクトの指定だけは必須)</span>
<span class="w"> </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="p">;</span>
<span class="w"> </span><span class="n"><span style="color:#ff0000">std::bind_front</span></span><span class="p">(</span><span class="o">&</span><span class="n">X</span><span class="o">::</span><span class="n">f</span><span class="p">)(</span><span class="n">x</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="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">);</span>
<span class="w"> </span><span class="n"><span style="color:#ff0000">std::bind_front</span></span><span class="p">(</span><span class="o">&</span><span class="n">X</span><span class="o">::</span><span class="n">f</span><span class="p">,</span><span class="w"> </span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">)(</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"><span style="color:#ff0000">std::bind_front</span></span><span class="p">(</span><span class="n">g</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"><span style="color:#ff0000">std::bind_front</span></span><span class="p">(</span><span class="n">g</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">)(</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="p">}</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h3>出力</h3>
<p><pre><code>1,2,3
1,2,3
1,2,3
1,2,3
1,2,3
</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>:</li>
<li><a href="../../implementation.html#gcc">GCC</a>: 9.1 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../implementation.html#visual_cpp">Visual C++</a>: ??</li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0356r5.html" target="_blank">P0356R5 Simplified partial function application</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1651r0.html" target="_blank">P1651R0 <code>bind_front</code> should not unwrap <code>reference_wrapper</code></a></li>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2714r1.html" target="_blank">P2714R1 Bind front and back to NTTP callables</a><ul>
<li>C++26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>(2)を追加</li>
</ul>
</li>
</ul></div>
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