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<h1 itemprop="name"><span class="token">__has_include [P0061R1]</span><span class="cpp cpp17" title="C++17で追加">(C++17)</span></h1>
<div itemprop="articleBody"><p></p>
<p>このページはC++17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><code>__has_include</code>はインクルードするファイルが存在するかどうかを返す<a class="cpprefjp-defined-word" data-desc="boolを返す関数・関数オブジェクト。predicate">述語</a>である。</p>
<p>処理系によって異なるヘッダファイルをインクルードする場合、従来はコンパイラのみでヘッダファイルの有無を確認する方法がなく、下記例のように外部ツールにより補助する方法を取ることが多かった。</p>
<ul>
<li>外部ツールでヘッダファイルの存在を確認し、ヘッダファイルの有無を表すマクロ (例: <code>#define HAS_STDIO_H 1</code>など) を生成</li>
<li>生成されたマクロは固有のコンフィグヘッダ (例: config.hなど) に出力</li>
<li>プログラマはコンフィグヘッダのマクロに応じて<code>#if .. #include .. #endif</code>を記述</li>
</ul>
<p>C++17では標準的な方法でコンパイラがヘッダファイルの有無を確認できるため、ヘッダファイルの有無を確認するために外部ツールやコンフィグヘッダが不要となる。</p>
<p>ただし外部ツールの機能はヘッダファイルの有無を確認する以外にも、ライブラリのリンク可否、テストプログラムの実行結果取得など多くの機能を持つため、<code>__has_include</code>により外部ツールやコンフィグヘッダが完全に不要になるわけではない。</p>
<h2>仕様</h2>
<p><code>__has_include</code>に指定されたインクルードファイルが存在する場合は 1 として評価され、インクルードファイルが存在しない場合は 0 として評価される。</p>
<p>詳細な文法は下記の通り。</p>
<p><pre><code>defined-macro-expression:
defined identifier
defined ( identifier )
h-preprocessing-token:
any preprocessing-token other than >
h-pp-tokens:
h-preprocessing-token
h-pp-tokens h-preprocessing-token
has-include-expression:
__has_include ( <h-char-sequence> )
__has_include ( "q-char-sequence" )
__has_include ( string-literal )
__has_include ( < h-pp-tokens > )
</code></pre></p>
<p>誤解を恐れず言えば<code>#include</code>の後に指定できるトークンと、同様のトークンが<code>__has_include</code>にも指定できる。</p>
<h2>例</h2>
<p><div class="yata" id="c076f980c8c5f95c9201ec515402719e2e5511bf"><div class="codehilite"><pre><span></span><code><span class="cp">#if __has_include(<has_include.hpp>)</span>
<span class="cp"># warning <has_include.hpp> is found</span>
<span class="cp">#else</span>
<span class="cp"># warning <has_include.hpp> is not found</span>
<span class="cp">#endif</span>
<span class="cp">#if __has_include("has_include.hpp")</span>
<span class="cp"># warning "has_include.hpp" is found</span>
<span class="cp">#else</span>
<span class="cp"># warning "has_include.hpp" is not found</span>
<span class="cp">#endif</span>
<span class="cp">#if __has_include(<fuga>)</span>
<span class="cp"># warning <fuga> is found</span>
<span class="cp">#else</span>
<span class="cp"># warning <fuga> is not found</span>
<span class="cp">#endif</span>
<span class="cp">#if __has_include("fuga")</span>
<span class="cp"># warning "fuga" is found</span>
<span class="cp">#else</span>
<span class="cp"># warning "fuga" is not found</span>
<span class="cp">#endif</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>
</code></pre></div>
</div></p>
<h3>出力</h3>
<p><pre><code></code></pre></p>
<h3>警告例</h3>
<p>カレントディレクトリに has_include.hpp ファイルが存在する状態で、clang++ 5.0.0 にてコンパイルした場合。</p>
<p><pre><code>has_include.cpp:5:4: warning: <has_include.hpp> is not found [-W#warnings]
# warning <has_include.hpp> is not found
^
has_include.cpp:9:4: warning: "has_include.hpp" is found [-W#warnings]
# warning "has_include.hpp" is found
^
has_include.cpp:17:4: warning: <fuga> is not found [-W#warnings]
# warning <fuga> is not found
^
has_include.cpp:23:4: warning: "fuga" is not found [-W#warnings]
# warning "fuga" is not found
^
4 warnings generated.
</code></pre></p>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0061r1.html" target="_blank">P0061R1 __has_include for C++17</a></li>
</ul></div>
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