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<h1 itemprop="name"><span class="token">if consteval [P1938R3]</span><span class="cpp cpp23" title="C++23で追加">(C++23)</span></h1>
<div itemprop="articleBody"><p></p>
<p>このページはC++23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>C++23では、<code>constexpr</code>関数がコンパイル時に呼ばれたかを判定するための構文として、<code>if consteval</code>を導入する。これは「consteval if文」と呼ばれる。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">constexpr</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="nf">my_fma</span><span class="p">(</span><span class="kt">double</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="kt">double</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="k">if</span><span class="w"> </span><span class="k">consteval</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// コンパイル時に実行される文脈</span>
<span class="w"> </span><span class="k">return</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">b</span><span class="w"> </span><span class="o">+</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="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// 実行時に実行される文脈</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">__builtin_fma</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="w"> </span><span class="n">c</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
<span class="cp">#include <a href="../../reference/iostream.html"><iostream></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="k">constexpr</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">static_fma</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">my_fma</span><span class="p">(</span><span class="mf">1.0</span><span class="p">,</span><span class="w"> </span><span class="mf">2.0</span><span class="p">,</span><span class="w"> </span><span class="mf">3.0</span><span class="p">);</span>
<span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">dynamic_fma</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">my_fma</span><span class="p">(</span><span class="mf">1.0</span><span class="p">,</span><span class="w"> </span><span class="mf">2.0</span><span class="p">,</span><span class="w"> </span><span class="mf">3.0</span><span class="p">);</span>
<span class="w"> </span><span class="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">static_fma</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">dynamic_fma</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>
</code></pre></div>
</p>
<p><code>if consteval</code>文内は即時関数 (<code>consteval</code>) の文脈であり、<code>consteval</code>関数を呼び出すことができる。</p>
<p><code>if consteval</code>の逆条件として、実行時かを判定するためには、<code>if !consteval</code>を使用する (<code>consteval</code>の前に<code>!</code>)。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">constexpr</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="nf">my_fma</span><span class="p">(</span><span class="kt">double</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="kt">double</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="k">if</span><span class="w"> </span><span class="o">!</span><span class="k">consteval</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// 実行時に実行される文脈</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">__builtin_fma</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="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="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// コンパイル時に実行される文脈</span>
<span class="w"> </span><span class="k">return</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">b</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">c</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p><code>if consteval</code>文は複合文でなければならない。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">constexpr</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="nf">my_fma</span><span class="p">(</span><span class="kt">double</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">b</span><span class="p">,</span><span class="w"> </span><span class="kt">double</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="k">if</span><span class="w"> </span><span class="o">!</span><span class="k">consteval</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">__builtin_fma</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="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="k">else</span>
<span class="w"> </span><span class="k">return</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">b</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">c</span><span class="p">;</span><span class="w"> </span><span class="c1">// コンパイルエラー！複合文でなければならない</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>この機能が必要になった背景・経緯</h2>
<h3>constexprとconstevalの相互作用での問題</h3>
<p><code>consteval</code>を使用したプログラムを考える。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">consteval</span><span class="w"> </span><span class="kt">int</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">i</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="n">i</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="k">constexpr</span><span class="w"> </span><span class="kt">int</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">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n"><a href="../../reference/type_traits/is_constant_evaluated.html">std::is_constant_evaluated</a></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="n">f</span><span class="p">(</span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span><span class="w"> </span><span class="c1">// <==</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</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">42</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
<span class="k">consteval</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="nf">h</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</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="n">f</span><span class="p">(</span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>定数式評価時には、関数<code>g()</code>と関数<code>h()</code>は同じことをするが、実行時には関数<code>h()</code>を呼び出すことはできない。また、<code>if (std::is_constant_evaluated())</code>文内は定数式ではあるが即時関数の文脈ではないため、即時関数<code>f()</code>を呼び出すことはできず、<code><==</code>コメントのところで<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。この文脈では、<code>f(42)</code>を呼び出すことはできるが、<code>f(i)</code>を呼び出すことはできない。</p>
<h3><code>if constexpr (std::is_constant_evaluated())</code>の問題</h3>
<p>もうひとつの問題は、<code><a href="../../reference/type_traits/is_constant_evaluated.html">std::is_constant_evaluated()</a></code>固有の問題だが、この関数は<code>if</code>文と使わなければならないが、誤って<code>if constexpr</code>で使用してしまう場合である。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">constexpr</span><span class="w"> </span><span class="kt">size_t</span><span class="w"> </span><span class="nf">strlen</span><span class="p">(</span><span class="kt">char</span><span class="w"> </span><span class="k">const</span><span class="o">*</span><span class="w"> </span><span class="n">s</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_constant_evaluated</span><span class="p">())</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="o">*</span><span class="n">p</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">s</span><span class="p">;</span><span class="w"> </span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">p</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="o">*</span><span class="n">p</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="sc">'\0'</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="k">static_cast</span><span class="o"><</span><span class="n"><a href="../../reference/cstddef/size_t.html">std::size_t</a></span><span class="o">></span><span class="p">(</span><span class="n">p</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">s</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="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">__asm__</span><span class="p">(</span><span class="s">"SSE 4.2で計算…"</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>この関数は常にコンパイル時の方の文脈が呼ばれてしまう。この使用間違いはコンパイラによっては警告を出力するが、誤用を防ぐのがむずかしいという問題があった。</p>
<h2><a href="#relative-page" id="relative-page">関連項目</a></h2>
<ul>
<li><code><a href="../../reference/type_traits/is_constant_evaluated.html">std::is_constant_evaluated()</a></code></li>
<li><a href="../cpp20/immediate_functions.html">C++20 常に定数式評価する<code>consteval</code></a></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1938r3.html" target="_blank">P1938R3 <code>if consteval</code></a></li>
</ul></div>
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