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<title>explicit_operator_bool</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="core.explicit_operator_bool"></a><a class="link" href="explicit_operator_bool.html" title="explicit_operator_bool">explicit_operator_bool</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="explicit_operator_bool.html#core.explicit_operator_bool.overview">Overview</a></span></dt>
<dt><span class="section"><a href="explicit_operator_bool.html#core.explicit_operator_bool.examples">Examples</a></span></dt>
<dt><span class="section"><a href="explicit_operator_bool.html#core.explicit_operator_bool.history">History</a></span></dt>
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<div class="simplesect">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp93227056"></a>Authors</h3></div></div></div>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
Andrey Semashev
</li></ul></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="core.explicit_operator_bool.overview"></a><a class="link" href="explicit_operator_bool.html#core.explicit_operator_bool.overview" title="Overview">Overview</a>
</h3></div></div></div>
<p>
Header <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">core</span><span class="special">/</span><span class="identifier">explicit_operator_bool</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>
provides <code class="computeroutput"><span class="identifier">BOOST_EXPLICIT_OPERATOR_BOOL</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">BOOST_EXPLICIT_OPERATOR_BOOL_NOEXCEPT</span><span class="special">()</span></code> and <code class="computeroutput"><span class="identifier">BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL</span><span class="special">()</span></code> compatibility helper macros that expand
to an explicit conversion operator to <code class="computeroutput"><span class="keyword">bool</span></code>.
For compilers not supporting explicit conversion operators introduced in
C++11 the macros expand to a conversion operator that implements the <a href="http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Safe_bool" target="_top">safe bool
idiom</a>. In case if the compiler is not able to handle safe bool idiom
well the macros expand to a regular conversion operator to <code class="computeroutput"><span class="keyword">bool</span></code>.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="core.explicit_operator_bool.examples"></a><a class="link" href="explicit_operator_bool.html#core.explicit_operator_bool.examples" title="Examples">Examples</a>
</h3></div></div></div>
<p>
Both macros are intended to be placed within a user's class definition. The
generated conversion operators will be implemented in terms of <code class="computeroutput"><span class="keyword">operator</span><span class="special">!()</span></code>
that should be defined by user in this class. In case of <code class="computeroutput"><span class="identifier">BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL</span><span class="special">()</span></code> the generated conversion operator will
be declared <code class="computeroutput"><span class="keyword">constexpr</span></code> which
requires the corresponding <code class="computeroutput"><span class="keyword">operator</span><span class="special">!()</span></code> to also be <code class="computeroutput"><span class="keyword">constexpr</span></code>.
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">></span>
<span class="keyword">class</span> <span class="identifier">my_ptr</span>
<span class="special">{</span>
<span class="identifier">T</span><span class="special">*</span> <span class="identifier">m_p</span><span class="special">;</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">BOOST_EXPLICIT_OPERATOR_BOOL</span><span class="special">()</span>
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">!()</span> <span class="keyword">const</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="special">!</span><span class="identifier">m_p</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
Now <code class="computeroutput"><span class="identifier">my_ptr</span></code> can be used in
conditional expressions, similarly to a regular pointer:
</p>
<pre class="programlisting"><span class="identifier">my_ptr</span><span class="special"><</span> <span class="keyword">int</span> <span class="special">></span> <span class="identifier">p</span><span class="special">;</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">p</span><span class="special">)</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"true"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="core.explicit_operator_bool.history"></a><a class="link" href="explicit_operator_bool.html#core.explicit_operator_bool.history" title="History">History</a>
</h3></div></div></div>
<h5>
<a name="core.explicit_operator_bool.history.h0"></a>
<span class="phrase"><a name="core.explicit_operator_bool.history.boost_1_56"></a></span><a class="link" href="explicit_operator_bool.html#core.explicit_operator_bool.history.boost_1_56">boost
1.56</a>
</h5>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Added new macros <code class="computeroutput"><span class="identifier">BOOST_EXPLICIT_OPERATOR_BOOL_NOEXCEPT</span></code>
and <code class="computeroutput"><span class="identifier">BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL</span></code>
to define <code class="computeroutput"><span class="keyword">noexcept</span></code> and
<code class="computeroutput"><span class="keyword">constexpr</span></code> operators.
</li>
<li class="listitem">
The header moved to Boost.Core.
</li>
</ul></div>
<h5>
<a name="core.explicit_operator_bool.history.h1"></a>
<span class="phrase"><a name="core.explicit_operator_bool.history.boost_1_55"></a></span><a class="link" href="explicit_operator_bool.html#core.explicit_operator_bool.history.boost_1_55">boost
1.55</a>
</h5>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
The macro was extracted from Boost.Log.
</li></ul></div>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="right"><div class="copyright-footer">Copyright © 2014 Peter Dimov<br>Copyright © 2014 Glen Fernandes<br>Copyright © 2014 Andrey Semashev<p>
Distributed under the <a href="http://boost.org/LICENSE_1_0.txt" target="_top">Boost
Software License, Version 1.0</a>.
</p>
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