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<header data-kunai-mdinfo="{"meta": {"header": ["utility"], "namespace": ["std"], "id-type": ["function template"], "cpp": ["cpp11"]}, "sources": [{"id": "33836758d2494b9ad0e9061d75a8ee541bae8776", "source": "#include <iostream>\n#include <utility>\n\n// \u3053\u306e\u95a2\u6570\u306f\u3001\u578bFrom\u304b\u3089\u578bTo\u3078\u306e\u660e\u793a\u7684\u306a\u578b\u5909\u63db\u304c\u5b9a\u7fa9\u3055\u308c\u3066\u3044\u308b\u5834\u5408\u306e\u307f\n// \u30aa\u30fc\u30d0\u30fc\u30ed\u30fc\u30c9\u89e3\u6c7a\u306e\u5019\u88dc\u3068\u306a\u308b\ntemplate <class To, class From>\ndecltype(static_cast<To>(std::declval<From>())) convert(From&& x)\n{\n return static_cast<To>(std::forward<From>(x));\n}\n\n// std::declval()\u306f\u5b9f\u969b\u306b\u8a55\u4fa1\u3057\u3066\u306f\u3044\u3051\u306a\u3044\n/*\ntemplate <class T>\nT f() { return std::declval<T>(); }\n\nvoid g()\n{\n int x = f<int>(); // \u30a8\u30e9\u30fc\uff01\n}\n*/\n\nint main()\n{\n int x = 3;\n double d = convert<double>(x);\n\n std::cout << d << std::endl;\n}\n"}], "page_id": ["reference", "utility", "declval"]}">
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最終更新日時(UTC):
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2025年07月11日 17時16分44秒
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<span itemprop="author" itemscope itemtype="http://schema.org/Person">
<span itemprop="name">Koichi Murase</span>
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<div class="identifier-type">function template</div><div class="header"><utility></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">declval</span><span class="cpp cpp11" title="C++11で追加">(C++11)</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">T</span><span class="o">></span>
<span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc"><a href="../type_traits/add_rvalue_reference.html">add_rvalue_reference</a></span><span class="o"><</span><span class="n">T</span><span class="o">>::</span><span class="n">type</span>
<span class="w"> </span><span class="n"><a href="">declval</a></span><span class="p">()</span><span class="w"> </span><span class="k">noexcept</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p>指定された型の値を得る。</p>
<p>この関数は、<code>decltype(expr)</code>や<code>noexcept(expr)</code> といった、実際には評価されない式の内部において、特定の型を持った式を作るためのヘルパ関数である。</p>
<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>この関数は呼び出されないので、値としては何も返さない。</p>
<p>実際には評価されない文脈(<code>decltype</code>, <code>sizeof</code>, <code>noexcept</code>)で使用することで、指定された型に対する特定の演算が可能かどうかを調べるためだけの「型の値」が返る。</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>
<p>投げない</p>
<h2>備考</h2>
<p>テンプレートパラメータ<code>T</code>は、<a class="cpprefjp-defined-word" data-desc="型のサイズを決定できない、完全には定義されていない型。例えば先行宣言のみのクラス型、要素数不明の配列型、`void`など。クラス定義内部ではそのクラス自身は不完全型">不完全型</a>であってもよい。</p>
<p>この関数が実際に評価されうる場合、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>
<h2>例</h2>
<p><div class="yata" id="33836758d2494b9ad0e9061d75a8ee541bae8776"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../utility.html"><utility></a></span>
<span class="c1">// この関数は、型Fromから型Toへの明示的な型変換が定義されている場合のみ</span>
<span class="c1">// オーバーロード解決の候補となる</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">To</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">From</span><span class="o">></span>
<span class="k">decltype</span><span class="p">(</span><span class="k">static_cast</span><span class="o"><</span><span class="n">To</span><span class="o">></span><span class="p">(</span><span class="n"><span style="color:#ff0000">std::declval</span></span><span class="o"><</span><span class="n">From</span><span class="o">></span><span class="p">()))</span><span class="w"> </span><span class="n">convert</span><span class="p">(</span><span class="n">From</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="k">return</span><span class="w"> </span><span class="k">static_cast</span><span class="o"><</span><span class="n">To</span><span class="o">></span><span class="p">(</span><span class="n"><a href="forward.html">std::forward</a></span><span class="o"><</span><span class="n">From</span><span class="o">></span><span class="p">(</span><span class="n">x</span><span class="p">));</span>
<span class="p">}</span>
<span class="c1">// <span style="color:#ff0000">std::declval</span>()は実際に評価してはいけない</span>
<span class="cm">/*</span>
<span class="cm">template <class T></span>
<span class="cm">T f() { return <span style="color:#ff0000">std::declval</span><T>(); }</span>
<span class="cm">void g()</span>
<span class="cm">{</span>
<span class="cm"> int x = f<int>(); // エラー！</span>
<span class="cm">}</span>
<span class="cm">*/</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="kt">int</span><span class="w"> </span><span class="n">x</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="kt">double</span><span class="w"> </span><span class="n">d</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">convert</span><span class="o"><</span><span class="kt">double</span><span class="o">></span><span class="p">(</span><span class="n">x</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">d</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h3>出力</h3>
<p><pre><code>3
</code></pre></p>
<h2>バージョン</h2>
<h3>言語</h3>
<ul>
<li>C++11</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>: 4.7.0 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../implementation.html#icc">ICC</a>: ??</li>
<li><a href="../../implementation.html#visual_cpp">Visual C++</a>: 2012 <span aria-label="検証済" role="img" title="検証済">✅</span>, 2013 <span aria-label="検証済" role="img" title="検証済">✅</span>, 2015 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#1255" target="_blank">LWG Issue 1255. <code>declval</code> should be added to the library</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3276.pdf" target="_blank">N3276 US22/DE9 Revisited: Decltype and Call Expressions</a></li>
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