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最終更新日時:
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<div class="identifier-type">macro</div><div class="header"><cstdarg></div><h1 itemprop="name"><span class="token">va_start</span></h1>
<div itemprop="articleBody"><p><div class="codehilite"><pre><span></span><code><span class="cp">#define va_start(ap, parmN) <i>unspecified</i> </span><span class="c1">// (1) C++98</span>
<span class="cp">#define va_start(ap, ...) <i>unspecified</i> </span><span class="c1">// (1) C++26</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p>可変引数へのアクセスを開始し、<code><a href="va_list.html">va_list</a></code>オブジェクト<code>ap</code>を初期化する。</p>
<p><code>ap</code>を初期化した後は、<code><a href="va_arg.html">va_arg</a></code>マクロで各引数を順に取り出せる。使用後は、同じ<code>ap</code>に対して<code><a href="va_end.html">va_end</a></code>マクロを呼び出さなければならない。</p>
<ul>
<li>C++98 : 第2引数<code>parmN</code>には、可変引数（<code>...</code>）の直前にある関数の仮引数を指定する。</li>
<li>C++26 : 可変引数マクロとなり、<code>ap</code>以降の引数（<code>...</code>）は破棄される。歴史的経緯から第2引数を受け付けるが、その値は使用されない。</li>
</ul>
<h2>効果</h2>
<p><code>ap</code>を、関数に渡された可変引数にアクセスできる状態に初期化する。</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>なし</p>
<h2>備考</h2>
<ul>
<li>同じ関数内で、<code>va_start</code>で初期化した<code>ap</code>は、対応する<code><a href="va_end.html">va_end</a></code>で後始末しなければならない。</li>
<li>C++26では、第2引数以降のいずれかが不均衡なカッコを含む、またはトークンに変換されない前処理トークンに展開される場合、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（診断は要求されない）。</li>
</ul>
<h2>例</h2>
<p><div class="yata" id="24c7b76287b6ee6e0a4ef7cb008c097c96fac739"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../cstdarg.html"><cstdarg></a></span>
<span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="c1">// count個のint引数の合計を求める</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">sum</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">count</span><span class="p">,</span><span class="w"> </span><span class="p">...)</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="va_list.html">std::va_list</a></span><span class="w"> </span><span class="n">args</span><span class="p">;</span>
<span class="w"> </span><span class="n"><span style="color:#ff0000">va_start</span></span><span class="p">(</span><span class="n">args</span><span class="p">,</span><span class="w"> </span><span class="n">count</span><span class="p">);</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">total</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="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">count</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">total</span><span class="w"> </span><span class="o">+=</span><span class="w"> </span><span class="n"><a href="va_arg.html">va_arg</a></span><span class="p">(</span><span class="n">args</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="n"><a href="va_end.html">va_end</a></span><span class="p">(</span><span class="n">args</span><span class="p">);</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">total</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="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">sum</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">10</span><span class="p">,</span><span class="w"> </span><span class="mi">20</span><span class="p">,</span><span class="w"> </span><span class="mi">30</span><span class="p">)</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>60
</code></pre></p>
<h2>バージョン</h2>
<h3>言語</h3>
<ul>
<li>C++98</li>
</ul>
<h2>関連項目</h2>
<ul>
<li><code><a href="va_arg.html">va_arg</a></code>: 次の可変引数を取得する</li>
<li><code><a href="va_end.html">va_end</a></code>: 可変引数へのアクセスを終了する</li>
<li><code><a href="va_copy.html">va_copy</a></code>: <code>va_list</code>オブジェクトを複製する</li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3348r4.pdf" target="_blank">P3348R4 C++26 should refer to C23 not C17</a><ul>
<li>C++26でC23を参照するようになり、<code>va_start</code>が可変引数マクロ<code>va_start(ap, ...)</code>となった</li>
</ul>
</li>
<li><a href="https://cplusplus.github.io/LWG/issue4388" target="_blank">LWG Issue 4388. Align new definition of <code>va_start</code> with C23</a><ul>
<li>C++26で、<code>va_start</code>の第2引数以降は破棄されること、およびそれらが不均衡なカッコやトークンに変換されない前処理トークンを含む場合はプログラムが<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>（<a class="cpprefjp-defined-word" data-desc="処理系は規則違反に対してエラーメッセージや警告を出さないかもしれない" href="../../implementation-compliance.html#dfn-no-diagnostic-required">診断不要</a>）となることが明確化された</li>
</ul>
</li>
</ul></div>
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