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<h1 itemprop="name"><span class="token">Pragma演算子 [N1653]</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>C99互換として、Pragma演算子が導入された。</p>
<p>Pragma演算子は、マクロ内で<a class="cpprefjp-defined-word" data-desc="処理系定義の動作。処理系によって事前に定めた動作をする" href="../../implementation-compliance.html#dfn-implementation-defined-behavior">処理系定義</a>の機能を使用するための機能である。<a class="cpprefjp-defined-word" data-desc="処理系定義の動作。処理系によって事前に定めた動作をする" href="../../implementation-compliance.html#dfn-implementation-defined-behavior">処理系定義</a>の機能を使用するために、従来はPragmaディレクティブを使用してきたが、その機能はマクロ内では使用できない。</p>
<p>Pragma演算子は、<code>_Pragma(文字列リテラル)</code>の形式で、<a class="cpprefjp-defined-word" data-desc="処理系定義の動作。処理系によって事前に定めた動作をする" href="../../implementation-compliance.html#dfn-implementation-defined-behavior">処理系定義</a>の機能を文字列リテラルとして引数指定して使用する。</p>
<h2>仕様</h2>
<p>Pragma演算子は、単項演算子の形式で、以下の構文を持つ：</p>
<p><pre><code>_Pragma (文字列リテラル)
</code></pre></p>
<p>これは、以下のように処理される：</p>
<ul>
<li>文字列リテラルの<code>L</code>プレフィックスを削除する</li>
<li>前後のダブルクォーテーションを削除する</li>
<li>エスケープシーケンス<code>\"</code>をダブルクォーテーションに置換する</li>
<li>エスケープシーケンス<code>\\</code>をバックスラッシュひとつに置換する</li>
</ul>
<p>処理結果のプリプロセッサトークンのシーケンスは、Pragmaディレクティブに指定した場合と同様に処理される。</p>
<h2>例</h2>
<p><div class="yata" id="2885a7b70c910f930f86b68cd1e402d57730a44c"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/iostream.html"><iostream></a></span>
<span class="c1">// OpenMPの言語拡張を使用して、for文を並列実行する</span>
<span class="cp">#define OMP_PARALLEL_FOR _Pragma("omp parallel for")</span>
<span class="c1">// 以下のように、Pragmaディレクティブでは書けない</span>
<span class="c1">//#define OMP_PARALLEL_FOR #pragma omp parallel for</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="k">constexpr</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">N</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">5</span><span class="p">;</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">a</span><span class="p">[</span><span class="n">N</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </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="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="kt">int</span><span class="w"> </span><span class="n">b</span><span class="p">[</span><span class="n">N</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </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="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="kt">int</span><span class="w"> </span><span class="n">c</span><span class="p">[</span><span class="n">N</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="c1">// OMP_PARALLEL_FORは、 _Pragma("omp parallel for") に展開される</span>
<span class="w"> </span><span class="n">OMP_PARALLEL_FOR</span>
<span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="n">N</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">c</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="n">a</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="n">b</span><span class="p">[</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">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="n">N</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">i</span><span class="p">)</span><span class="w"> </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="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="n"><a href="../../reference/ostream/endl.html">std::endl</a></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>2
4
6
8
10
</code></pre></p>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1653.htm" target="_blank">N1653 Working draft changes for C99 preprocessor synchronization</a></li>
</ul></div>
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