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<header data-kunai-mdinfo="{"meta": {"cpp": ["cpp11"]}, "sources": [{"id": "01d40ce08881c1b4eb2a64b7d67bdcc5e22bb415", "source": "#include <iostream>\n#include <utility>\n\n// \u30d1\u30e9\u30e1\u30fc\u30bf\u30d1\u30c3\u30af\u304c\u7a7a\u306b\u306a\u3063\u305f\u3089\u7d42\u4e86\nvoid print() {}\n\n// \u3072\u3068\u3064\u4ee5\u4e0a\u306e\u30d1\u30e9\u30e1\u30fc\u30bf\u3092\u53d7\u3051\u53d6\u308b\u3088\u3046\u306b\u3057\u3001\n// \u53ef\u5909\u5f15\u6570\u3092\u5148\u982d\u3068\u305d\u308c\u4ee5\u5916\u306b\u5206\u5272\u3059\u308b\ntemplate <class Head, class... Tail>\nvoid print(Head&& head, Tail&&... tail)\n{\n std::cout << head << std::endl;\n\n // \u30d1\u30e9\u30e1\u30fc\u30bf\u30d1\u30c3\u30aftail\u3092\u3055\u3089\u306bhead\u3068tail\u306b\u5206\u5272\u3059\u308b\n print(std::forward<Tail>(tail)...);\n}\n\nint main()\n{\n print(1, 'a', \"hello\");\n}\n"}, {"id": "4966236b749ccd035c5aa737235f4225e6b60600", "source": "#include <cstddef>\n\ntemplate <class... Args>\nvoid f(Args... args)\n{\n constexpr std::size_t template_parameter_pack_size = sizeof...(Args);\n constexpr std::size_t function_parameter_pack_size = sizeof...(args);\n\n static_assert(template_parameter_pack_size == 3, \"\");\n static_assert(function_parameter_pack_size == 3, \"\");\n}\n\nint main()\n{\n f(1, 'a', \"hello\");\n}\n"}, {"id": "1906df53e083ccf3814466240e828424c642ca24", "source": "#include <tuple>\n\ntemplate <class... ResultTypes>\nvoid f(ResultTypes(*...funcs)(int, char))\n{\n // t is std::tuple<int, float>{1, 1.23f}\n auto t = std::make_tuple(funcs(3, 'a')...);\n}\n\nint a(int, char) { return 1; }\nfloat b(int, char) { return 1.23f; }\n\nint main()\n{\n f(a, b);\n}\n"}, {"id": "ab5b8214ccd7d0c7e098c725031b4939311b934d", "source": "#include <iostream>\n#include <string>\n#include <sstream>\n\ntemplate <class T>\nstd::string to_std_string(const T& x)\n{\n std::stringstream ss;\n ss << x;\n\n std::string result;\n ss >> result;\n return result;\n}\n\nvoid print(std::string a, std::string b, std::string c)\n{\n std::cout << a << std::endl;\n std::cout << b << std::endl;\n std::cout << c << std::endl;\n}\n\ntemplate <class... Args>\nvoid f(Args&&... args)\n{\n // \u30d1\u30e9\u30e1\u30fc\u30bf\u30d1\u30c3\u30af\u306e\u5404\u8981\u7d20\u3092\u6587\u5b57\u5217\u306b\u5909\u63db\u3057\u3066\u304b\u3089\n // print()\u95a2\u6570\u306b\u6e21\u3059\n print(to_std_string(args)...);\n}\n\nint main()\n{\n f(1, 'a', \"hello\");\n}\n"}, {"id": "aa7086cad231ccaf2ebf31fe35aefa5230251996", "source": "#include <utility>\n#include <tuple>\n#include <type_traits>\n\n// 2\u3064\u306e\u578b\u30ea\u30b9\u30c8\u3092\u7db4\u3058\u5408\u308f\u305b\u308b\ntemplate <class... Args1>\nstruct zip {\n template <class... Args2>\n struct with {\n using type = std::tuple<std::pair<Args1, Args2>...>;\n };\n};\n\nint main()\n{\n static_assert(std::is_same<\n zip<int, long, long long>::with<float, double, long double>::type,\n std::tuple<\n std::pair<int, float>,\n std::pair<long, double>,\n std::pair<long long, long double>\n >\n >::value, \"\"); // OK\n}\n"}], "page_id": ["lang", "cpp11", "variadic_templates"]}">
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2026年04月08日 23時58分08秒
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<h1 itemprop="name"><span class="token">可変引数テンプレート [N2242]</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>「可変引数テンプレート (variadic templates)」は、任意の型とそのオブジェクトを任意の数だけ受け取る機能である。これによって、「最大でN個のパラメータを受け取る関数テンプレートやクラステンプレート」を実装する際に、N個の<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>をユーザーが用意する必要なく、ひとつの実装だけで済むようになる。</p>
<p>可変引数テンプレートとしてテンプレートパラメータを受け取る場合、テンプレートパラメータを宣言する<code>class</code>または<code>typename</code>とテンプレートパラメータの間に、省略記号 <code>...</code> を入力する：</p>
<p><div class="codehilite"><pre><span></span><code><span class="c1">// 0個以上のテンプレートパラメータを受け取る</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</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="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="p">;</span>
<span class="c1">// 0個以上の任意の型のパラメータを受け取る</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</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="kt">void</span><span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="n">Args</span><span class="p">...</span><span class="w"> </span><span class="n">args</span><span class="p">);</span>
</code></pre></div>
</p>
<p>可変引数テンプレートで宣言したパラメータ(ここでは<code>Args</code>と<code>args</code>)は「パラメータパック (parameter pack)」と呼ばれ、複数の型またはオブジェクトがまとめられた状態となっている。</p>
<p>パラメータパックになっているパラメータは、「展開 (expansion)」して使用する必要がある。これは他の関数やクラステンプレートにパラメータパックを転送するような状況で必要となる。パラメータパックの展開には、パラメータパックの後ろに省略記号 <code>...</code> を入力する：</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="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></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="c1">// パラメータパックを ... で展開して、</span>
<span class="w"> </span><span class="c1">// <a href="../../reference/tuple/tuple.html">std::tuple</a>クラステンプレートの引数として渡す</span>
<span class="w"> </span><span class="n"><a href="../../reference/tuple/tuple.html">std::tuple</a></span><span class="o"><</span><span class="n">Args</span><span class="p">...</span><span class="o">></span><span class="w"> </span><span class="n">values</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="p">,</span><span class="w"> </span><span class="kt">char</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n"><a href="../../reference/string/basic_string.html">std::string</a></span><span class="o">&</span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</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="kt">void</span><span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="n">Args</span><span class="p">...</span><span class="w"> </span><span class="n">args</span><span class="p">)</span>
<span class="p">{</span>
<span class="w"> </span><span class="c1">// パラメータパックを ... で展開して、</span>
<span class="w"> </span><span class="c1">// 関数g()の引数として渡す</span>
<span class="w"> </span><span class="n">g</span><span class="p">(</span><span class="n">args</span><span class="p">...);</span>
<span class="p">}</span>
<span class="n">f</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="sc">'a'</span><span class="p">,</span><span class="w"> </span><span class="s">"hello"</span><span class="p">);</span>
</code></pre></div>
</p>
<p>パラメータパックを最初の要素から順番に処理していきたい場合には、「任意の型のパラメータをひとつと、任意の個数の任意の型のパラメータを受け取る」というような形式のパラメータリストとし、再帰によって処理をする：</p>
<p><div class="yata" id="01d40ce08881c1b4eb2a64b7d67bdcc5e22bb415"><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/utility.html"><utility></a></span>
<span class="c1">// パラメータパックが空になったら終了</span>
<span class="kt">void</span><span class="w"> </span><span class="nf">print</span><span class="p">()</span><span class="w"> </span><span class="p">{}</span>
<span class="c1">// ひとつ以上のパラメータを受け取るようにし、</span>
<span class="c1">// 可変引数を先頭とそれ以外に分割する</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">Head</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">Tail</span><span class="o">></span>
<span class="kt">void</span><span class="w"> </span><span class="n">print</span><span class="p">(</span><span class="n">Head</span><span class="o">&&</span><span class="w"> </span><span class="n">head</span><span class="p">,</span><span class="w"> </span><span class="n">Tail</span><span class="o">&&</span><span class="p">...</span><span class="w"> </span><span class="n">tail</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="n">head</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="c1">// パラメータパックtailをさらにheadとtailに分割する</span>
<span class="w"> </span><span class="n">print</span><span class="p">(</span><span class="n"><a href="../../reference/utility/forward.html">std::forward</a></span><span class="o"><</span><span class="n">Tail</span><span class="o">></span><span class="p">(</span><span class="n">tail</span><span class="p">)...);</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">print</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="sc">'a'</span><span class="p">,</span><span class="w"> </span><span class="s">"hello"</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<p>出力：</p>
<p><pre><code>1
a
hello
</code></pre></p>
<h2>仕様</h2>
<ul>
<li><code>template <class... Args></code>や<code>void f(Args... args)</code>での、<code>Args</code>および<code>args</code>は、複数のパラメータをまとめた状態となっている。このまとまった状態のパラメータを「パラメータパック (parameter pack)」という<ul>
<li>テンプレートパラメータのパラメータパックを「テンプレートパラメータパック (template parameter pack)」、関数のパラメータとしてのパラメータパックを「関数パラメータパック (function parameter pack)」という</li>
</ul>
</li>
<li>
<p>パラメータパックは複数のパラメータがまとめられた状態となっているため、そのままでは個々のパラメータを取り出せない。それらを取り出すには、パラメータパックを「展開 (expansion)」する必要がある。パラメータパックの展開には、パラメータパック名に続いて「<code>...</code>」を記述する。例：</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="p">...</span><span class="w"> </span><span class="n">Args</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">Args</span><span class="p">...</span><span class="w"> </span><span class="n">args</span><span class="p">)</span>
<span class="p">{</span>
<span class="w"> </span><span class="n">g</span><span class="p">(</span><span class="n">args</span><span class="p">...);</span><span class="w"> </span><span class="c1">// パラメータパックargsを...で展開して、</span>
<span class="w"> </span><span class="c1">// 関数g()にそれらのパラメータを転送する</span>
<span class="p">}</span>
</code></pre></div>
</p>
</li>
<li>
<p>パラメータパックの宣言、および展開に使用する「<code>...</code>」は、「省略記号 (ellipsis, エリプシス)」という</p>
</li>
<li>
<p>パラメータパックには、ゼロ個以上のパラメータが含まれる</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="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></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="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="o"><</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">char</span><span class="p">,</span><span class="w"> </span><span class="kt">double</span><span class="o">></span><span class="w"> </span><span class="n">a</span><span class="p">;</span><span class="w"> </span><span class="c1">// OK</span>
<span class="w"> </span><span class="n">X</span><span class="o"><></span><span class="w"> </span><span class="n">b</span><span class="p">;</span><span class="w"> </span><span class="c1">// OK</span>
<span class="p">}</span>
</code></pre></div>
</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="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></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="c1">// パラメータパックが0要素だった場合の特殊化</span>
<span class="k">template</span><span class="w"> </span><span class="o"><></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="o"><></span><span class="w"> </span><span class="p">{};</span>
</code></pre></div>
</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="p">...</span><span class="w"> </span><span class="n">Args</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">Args</span><span class="p">...</span><span class="w"> </span><span class="n">args</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="mf">3.14f</span><span class="p">,</span><span class="w"> </span><span class="s">"hello"</span><span class="p">);</span><span class="w"> </span><span class="c1">// OK</span>
<span class="w"> </span><span class="n">f</span><span class="p">();</span><span class="w"> </span><span class="c1">// OK</span>
<span class="p">}</span>
</code></pre></div>
</p>
</li>
<li>
<p>パラメータパックの宣言をする際、パラメータパックはパラメータリストの最後になければならない</p>
<p><div class="codehilite"><pre><span></span><code><span class="c1">// OK</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">Head</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">Tail</span><span class="o">></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="c1">// コンパイルエラー！パラメータパックは最後に置かなければならない</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="p">...</span><span class="w"> </span><span class="n">Init</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Last</span><span class="o">></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Y</span><span class="w"> </span><span class="p">{};</span>
</code></pre></div>
</p>
</li>
<li>
<p>関数パラメータパックは、型推論の補助として、パラメータパックの全ての型に対して共通の修飾を付加できる：</p>
<p><div class="codehilite"><pre><span></span><code><span class="c1">// パラメータパックArgsに含まれる全ての型のパラメータを、</span>
<span class="c1">// const左辺値参照として受け取る</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</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="kt">void</span><span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="k">const</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="p">{}</span>
</code></pre></div>
</p>
</li>
<li>
<p><code>sizeof...(identifier)</code>演算子にパラメータパックを指定することで、パラメータパックに含まれるパラメータの要素数を取得できる：</p>
<p><div class="yata" id="4966236b749ccd035c5aa737235f4225e6b60600"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/cstddef.html"><cstddef></a></span>
<span class="k">template</span><span class="w"> </span><span class="o"><</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="kt">void</span><span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="n">Args</span><span class="p">...</span><span class="w"> </span><span class="n">args</span><span class="p">)</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="constexpr.html">constexpr</a></span><span class="w"> </span><span class="n"><a href="../../reference/cstddef/size_t.html">std::size_t</a></span><span class="w"> </span><span class="n">template_parameter_pack_size</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">sizeof</span><span class="p">...(</span><span class="n">Args</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="constexpr.html">constexpr</a></span><span class="w"> </span><span class="n"><a href="../../reference/cstddef/size_t.html">std::size_t</a></span><span class="w"> </span><span class="n">function_parameter_pack_size</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">sizeof</span><span class="p">...(</span><span class="n">args</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="static_assert.html">static_assert</a></span><span class="p">(</span><span class="n">template_parameter_pack_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="s">""</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="static_assert.html">static_assert</a></span><span class="p">(</span><span class="n">function_parameter_pack_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="s">""</span><span class="p">);</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="sc">'a'</span><span class="p">,</span><span class="w"> </span><span class="s">"hello"</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
</li>
</ul>
<h3>パラメータパックの宣言ができる場所</h3>
<p>パラメータパックの宣言は、以下の場所でできる：</p>
<ul>
<li>
<p>関数のパラメータ</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="p">...</span><span class="w"> </span><span class="n">Args</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">Args</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">// ここ</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="sc">'a'</span><span class="p">,</span><span class="w"> </span><span class="s">"hello"</span><span class="p">);</span>
</code></pre></div>
</p>
<p><div class="yata" id="1906df53e083ccf3814466240e828424c642ca24"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/tuple.html"><tuple></a></span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="p">...</span><span class="w"> </span><span class="n">ResultTypes</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">ResultTypes</span><span class="p">(</span><span class="o">*</span><span class="p">...</span><span class="n">funcs</span><span class="p">)(</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">char</span><span class="p">))</span>
<span class="p">{</span>
<span class="w"> </span><span class="c1">// t is <a href="../../reference/tuple/tuple.html">std::tuple</a><int, float>{1, 1.23f}</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../../reference/tuple/make_tuple.html">std::make_tuple</a></span><span class="p">(</span><span class="n">funcs</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="sc">'a'</span><span class="p">)...);</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="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">char</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="mi">1</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="kt">float</span><span class="w"> </span><span class="n">b</span><span class="p">(</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">char</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="mf">1.23f</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="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
</li>
<li>
<p>テンプレートパラメータ</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="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></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="n">X</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="p">,</span><span class="w"> </span><span class="kt">double</span><span class="o">></span><span class="w"> </span><span class="n">x</span><span class="p">;</span>
</code></pre></div>
</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="kt">int</span><span class="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Y</span><span class="w"> </span><span class="p">{};</span>
<span class="n">Y</span><span class="o"><</span><span class="mi">3</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">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">2</span><span class="p">,</span><span class="w"> </span><span class="mi">6</span><span class="o">></span><span class="w"> </span><span class="n">y</span><span class="p">;</span>
</code></pre></div>
</p>
</li>
<li>
<p>テンプレートテンプレートパラメータ</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">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="p">...</span><span class="o">></span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Container</span><span class="o">></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">ContainerHolder</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">Container</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">cont</span><span class="p">;</span>
<span class="p">};</span>
<span class="n">ContainerHolder</span><span class="o"><</span><span class="n"><a href="../../reference/vector/vector.html">std::vector</a></span><span class="o">></span><span class="w"> </span><span class="n">v</span><span class="p">;</span>
<span class="n">ContainerHolder</span><span class="o"><</span><span class="n"><a href="../../reference/list/list.html">std::list</a></span><span class="o">></span><span class="w"> </span><span class="n">ls</span><span class="p">;</span>
</code></pre></div>
</p>
</li>
</ul>
<h3>パラメータパックの展開ができる場所</h3>
<p>パラメータパックの展開は、以下の場所でできる：</p>
<ul>
<li>
<p>関数の引数</p>
<p><div class="codehilite"><pre><span></span><code><span class="n">f</span><span class="p">(</span><span class="n">args</span><span class="p">...);</span>
</code></pre></div>
</p>
</li>
<li>
<p>テンプレートの引数</p>
<p><div class="codehilite"><pre><span></span><code><span class="n"><a href="../../reference/tuple/tuple.html">std::tuple</a></span><span class="o"><</span><span class="n">Args</span><span class="p">...</span><span class="o">></span><span class="w"> </span><span class="n">t</span><span class="p">;</span>
</code></pre></div>
</p>
</li>
<li>
<p>初期化子</p>
<p><div class="codehilite"><pre><span></span><code><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="w"> </span><span class="n">args</span><span class="p">...</span><span class="w"> </span><span class="p">};</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Person</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">id</span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../../reference/string/basic_string.html">std::string</a></span><span class="w"> </span><span class="n">name</span><span class="p">;</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">age</span><span class="p">;</span>
<span class="p">};</span>
<span class="n">Person</span><span class="w"> </span><span class="n">person</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">args</span><span class="p">...</span><span class="w"> </span><span class="p">};</span>
</code></pre></div>
</p>
</li>
<li>
<p>継承時の基底クラスリストの指定</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="p">...</span><span class="w"> </span><span class="n">Bases</span><span class="o">></span>
<span class="k">class</span><span class="w"> </span><span class="nc">Derived</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">Bases</span><span class="p">...;</span>
</code></pre></div>
</p>
</li>
<li>
<p>コンストラクタのメンバ初期化子</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="p">...</span><span class="w"> </span><span class="n">Bases</span><span class="o">></span>
<span class="k">class</span><span class="w"> </span><span class="nc">Derived</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">Bases</span><span class="p">...</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">Derived</span><span class="p">(</span><span class="n">Bases</span><span class="p">...</span><span class="w"> </span><span class="n">bases</span><span class="p">)</span>
<span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">Bases</span><span class="p">(</span><span class="n">bases</span><span class="p">)...</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
</code></pre></div>
</p>
</li>
<li>
<p>動的<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>仕様</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="p">...</span><span class="w"> </span><span class="n">ExceptionList</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="w"> </span><span class="k">throw</span><span class="p">(</span><span class="n">ExceptionList</span><span class="p">...);</span>
</code></pre></div>
</p>
</li>
</ul>
<h3>パラメータパックの拡張</h3>
<ul>
<li>
<p>パラメータパックは、<code>f(args...)</code>のように省略記号で展開するほかに、<code>f(g(args)...)</code>のようにパラメータパックの各要素に共通の処理を適用する式を書くこともできる。これをパラメータパックの「拡張 (extend)」という</p>
<ul>
<li>この例の場合、<code>args</code>パラメータパックの各要素に関数<code>g()</code>を適用してパラメータパックの要素に対して値と型の変換を行った結果のパラメータパックを生成し、その結果となるパラメータパックを関数<code>f()</code>に渡している</li>
</ul>
<p><div class="yata" id="ab5b8214ccd7d0c7e098c725031b4939311b934d"><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/string.html"><string></a></span>
<span class="cp">#include <a href="../../reference/sstream.html"><sstream></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="n"><a href="../../reference/string/basic_string.html">std::string</a></span><span class="w"> </span><span class="n">to_std_string</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">x</span><span class="p">)</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/sstream/basic_stringstream.html">std::stringstream</a></span><span class="w"> </span><span class="n">ss</span><span class="p">;</span>
<span class="w"> </span><span class="n">ss</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">x</span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../../reference/string/basic_string.html">std::string</a></span><span class="w"> </span><span class="n">result</span><span class="p">;</span>
<span class="w"> </span><span class="n">ss</span><span class="w"> </span><span class="o">>></span><span class="w"> </span><span class="n">result</span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">result</span><span class="p">;</span>
<span class="p">}</span>
<span class="kt">void</span><span class="w"> </span><span class="n">print</span><span class="p">(</span><span class="n"><a href="../../reference/string/basic_string.html">std::string</a></span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../../reference/string/basic_string.html">std::string</a></span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../../reference/string/basic_string.html">std::string</a></span><span class="w"> </span><span class="n">c</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="n">a</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="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="n">b</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="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="n">c</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="p">}</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</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="kt">void</span><span class="w"> </span><span class="n">f</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="p">{</span>
<span class="w"> </span><span class="c1">// パラメータパックの各要素を文字列に変換してから</span>
<span class="w"> </span><span class="c1">// print()関数に渡す</span>
<span class="w"> </span><span class="n">print</span><span class="p">(</span><span class="n">to_std_string</span><span class="p">(</span><span class="n">args</span><span class="p">)...);</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="sc">'a'</span><span class="p">,</span><span class="w"> </span><span class="s">"hello"</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
</li>
<li>
<p>複数のパラメータパックに対して拡張を行う場合、それらのパラメータパックは同じ要素数でなければならない。そうでない場合、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="yata" id="aa7086cad231ccaf2ebf31fe35aefa5230251996"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/utility.html"><utility></a></span>
<span class="cp">#include <a href="../../reference/tuple.html"><tuple></a></span>
<span class="cp">#include <a href="../../reference/type_traits.html"><type_traits></a></span>
<span class="c1">// 2つの型リストを綴じ合わせる</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="p">...</span><span class="w"> </span><span class="n">Args1</span><span class="o">></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">zip</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="p">...</span><span class="w"> </span><span class="n">Args2</span><span class="o">></span>
<span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">with</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">type</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../../reference/tuple/tuple.html">std::tuple</a></span><span class="o"><</span><span class="n"><a href="../../reference/utility/pair.html">std::pair</a></span><span class="o"><</span><span class="n">Args1</span><span class="p">,</span><span class="w"> </span><span class="n">Args2</span><span class="o">></span><span class="p">...</span><span class="o">></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"><a href="static_assert.html">static_assert</a></span><span class="p">(</span><span class="n"><a href="../../reference/type_traits/is_same.html">std::is_same</a></span><span class="o"><</span>
<span class="w"> </span><span class="n">zip</span><span class="o"><</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">long</span><span class="p">,</span><span class="w"> </span><span class="kt">long</span><span class="w"> </span><span class="kt">long</span><span class="o">>::</span><span class="n">with</span><span class="o"><</span><span class="kt">float</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">long</span><span class="w"> </span><span class="kt">double</span><span class="o">>::</span><span class="n">type</span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../../reference/tuple/tuple.html">std::tuple</a></span><span class="o"><</span>
<span class="w"> </span><span class="n"><a href="../../reference/utility/pair.html">std::pair</a></span><span class="o"><</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">float</span><span class="o">></span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../../reference/utility/pair.html">std::pair</a></span><span class="o"><</span><span class="kt">long</span><span class="p">,</span><span class="w"> </span><span class="kt">double</span><span class="o">></span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../../reference/utility/pair.html">std::pair</a></span><span class="o"><</span><span class="kt">long</span><span class="w"> </span><span class="kt">long</span><span class="p">,</span><span class="w"> </span><span class="kt">long</span><span class="w"> </span><span class="kt">double</span><span class="o">></span>
<span class="w"> </span><span class="o">></span>
<span class="w"> </span><span class="o">>::</span><span class="n">value</span><span class="p">,</span><span class="w"> </span><span class="s">""</span><span class="p">);</span><span class="w"> </span><span class="c1">// OK</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>可変引数テンプレートがなかったころ、任意の数のパラメータを受け取ってほかの処理に転送するといった処理をするために、パラメータの数だけ<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>を用意する必要があり、それによってコピー＆ペーストのコードができあがっていた。とくに関数テンプレートの場合、<code>const</code>左辺値参照と非<code>const</code>左辺値参照の全ての組み合わせを<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>として定義する必要があったため、このコードは非常に膨大になり、コンパイル時間が増えるひとつの原因ともなっていた。</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">T1</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T2</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T3</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">T1</span><span class="o">&</span><span class="w"> </span><span class="n">t1</span><span class="p">,</span><span class="w"> </span><span class="n">T2</span><span class="o">&</span><span class="w"> </span><span class="n">t2</span><span class="p">,</span><span class="w"> </span><span class="n">T3</span><span class="o">&</span><span class="w"> </span><span class="n">t3</span><span class="p">);</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">T1</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T2</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T3</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="k">const</span><span class="w"> </span><span class="n">T1</span><span class="o">&</span><span class="w"> </span><span class="n">t1</span><span class="p">,</span><span class="w"> </span><span class="n">T2</span><span class="o">&</span><span class="w"> </span><span class="n">t2</span><span class="p">,</span><span class="w"> </span><span class="n">T3</span><span class="o">&</span><span class="w"> </span><span class="n">t3</span><span class="p">);</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">T1</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T2</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T3</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="k">const</span><span class="w"> </span><span class="n">T1</span><span class="o">&</span><span class="w"> </span><span class="n">t1</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">T2</span><span class="o">&</span><span class="w"> </span><span class="n">t2</span><span class="p">,</span><span class="w"> </span><span class="n">T3</span><span class="o">&</span><span class="w"> </span><span class="n">t3</span><span class="p">);</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">T1</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T2</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T3</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="k">const</span><span class="w"> </span><span class="n">T1</span><span class="o">&</span><span class="w"> </span><span class="n">t1</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">T2</span><span class="o">&</span><span class="w"> </span><span class="n">t2</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">T3</span><span class="o">&</span><span class="w"> </span><span class="n">t3</span><span class="p">);</span>
<span class="err">…</span>
</code></pre></div>
</p>
<p>この問題を解決するために、任意の数だけ任意の型のパラメータを受け取る機能が必要とされ、可変引数テンプレートが導入された。</p>
<h2><a href="#relative-page" id="relative-page">関連項目</a></h2>
<ul>
<li><a href="variadic_macros.html">C++11 可変引数マクロ</a></li>
<li><a href="../cpp17/folding_expressions.html">C++17 畳み込み式</a></li>
<li><a href="../cpp26/pack_indexing.html">C++26 パラメータパックへのインデックスアクセスを許可</a></li>
<li><a href="../cpp26/variadic_friends.html">C++26 可変引数テンプレートで<code>friend</code>宣言をできるようにする</a></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1483.pdf" target="_blank">N1483 Typesafe Variable-length Function and Template Argument Lists</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1603.pdf" target="_blank">N1603 Variadic Templates</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1704.pdf" target="_blank">N1704 Variadic Templates: Exploring the Design Space</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2080.pdf" target="_blank">N2080 Variadic Templates (Revision 3)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2152.pdf" target="_blank">N2152 Proposed Wording for Variadic Templates</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2191.pdf" target="_blank">N2191 Proposed Wording for Variadic Templates (Revision 1)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2242.pdf" target="_blank">N2242 Proposed Wording for Variadic Templates (Revision 2)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2488.pdf" target="_blank">N2488 Extending Variadic Template Template Parameters</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2555.pdf" target="_blank">N2555 Extending Variadic Template Template Parameters (Revision 1)</a></li>
</ul></div>
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