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最終更新日時:
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2019年06月07日 18時32分57秒 (JST)
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<span class="fa fa-fw fa-clock-o fa-flip-horizontal"></span>履歴
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<div class="identifier-type">function</div><div class="header"><ios></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="class" title="class basic_ios">basic_ios::</span><span class="token">operator bool</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">explicit</span><span class="w"> </span><span class="k">operator</span><span class="w"> </span><span class="kt">bool</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="p">;</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p>現在の状態値が正常を示す値になっていることを判定する変換関数。</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><code>!<a href="fail.html">fail</a>()</code>。</p>
<h2>備考</h2>
<ul>
<li>本関数と <code><a href="good.html">good</a>()</code> は結果が異なる事に注意。本関数は <code>eofbit</code> が設定されていても <code>true</code> を返すが、<code><a href="good.html">good</a>()</code> は <code>eofbit</code> がセットされている場合 <code>false</code> を返す。</li>
<li>C++03 までは本関数とほぼ等価の機能が <code><a href="op_voidptr.html">operator void*</a>()</code> によって提供されていたが、C++11 でユーザ定義変換関数に <code>explicit</code> を指定することで当該変換の暗黙適用を回避できるようになったことから、より意図が明確となるように本関数が提供され、<code><a href="op_voidptr.html">operator void*</a>()</code> は廃止された。<br />
なお、<code>if</code> 文や <code>while</code> 文の条件式で使用している分には問題とならないが、この変更によって一部 C++03 までは問題の無かったコードがコンパイルエラーとなる等の非互換が生じている。
<br />
<div class="codehilite"><pre><span></span><code><span class="c1">// C++03 まではコンパイル可能だが C++11 からはコンパイルエラーになる例</span>
<span class="kt">bool</span><span class="w"> </span><span class="n">b1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../../iostream/cout.html">std::cout</a></span><span class="p">;</span>
<span class="kt">bool</span><span class="w"> </span><span class="n">b2</span><span class="w"> </span><span class="o">=</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="nb">NULL</span><span class="p">;</span>
</code></pre></div>
</li>
</ul>
<h2>実装例</h2>
<p><div class="codehilite"><pre><span></span><code><span class="k">explicit</span><span class="w"> </span><span class="k">operator</span><span class="w"> </span><span class="kt">bool</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="o">!</span><span class="n"><a href="fail.html">fail</a></span><span class="p">();</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>バージョン</h2>
<h3>言語</h3>
<ul>
<li>C++11</li>
</ul>
<h3>備考</h3>
<p>libc++ では C++03 モードでも本関数が使用可能である。（ただし、<code>explicit</code> ではない）</p>
<h2>参照</h2>
<ul>
<li>状態値の書き込み<ul>
<li><code><a href="setstate.html">setstate</a></code></li>
<li><code><a href="clear.html">clear</a></code></li>
</ul>
</li>
<li>状態値の読み取り<ul>
<li><code><a href="rdstate.html">rdstate</a></code></li>
<li><code><a href="good.html">good</a></code></li>
<li><code><a href="eof.html">eof</a></code></li>
<li><code><a href="fail.html">fail</a></code></li>
<li><code><a href="bad.html">bad</a></code></li>
<li><code>operator bool</code>（この関数）</li>
<li><code><a href="op_voidptr.html">operator void*</a></code></li>
<li><code><a href="op_not.html">operator!</a></code></li>
</ul>
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