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| <div class="section" title="A note about optional<bool>"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_optional.a_note_about_optional_bool_"></a><a class="link" href="a_note_about_optional_bool_.html" title="A note about optional<bool>">A note about |
| optional<bool></a> |
| </h2></div></div></div> |
| <p> |
| <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> should |
| be used with special caution and consideration. |
| </p> |
| <p> |
| First, it is functionally similar to a tristate boolean (false,maybe,true) |
| —such as <a href="../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>— |
| except that in a tristate boolean, the maybe state <span class="underline">represents |
| a valid value</span>, unlike the corresponding state of an uninitialized |
| <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code>. It |
| should be carefully considered if an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> |
| instead of a <code class="computeroutput"><span class="identifier">tribool</span></code> is really |
| needed. |
| </p> |
| <p> |
| Second, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> |
| provides an implicit conversion to <code class="computeroutput"><span class="keyword">bool</span></code>. |
| This conversion refers to the initialization state and not to the contained |
| value. Using <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> can |
| lead to subtle errors due to the implicit <code class="computeroutput"><span class="keyword">bool</span></code> |
| conversion: |
| </p> |
| <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> |
| <span class="keyword">void</span> <span class="identifier">bar</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">v</span> <span class="special">=</span> <span class="keyword">try</span><span class="special">();</span> |
| |
| <span class="comment">// The following intended to pass the value of 'v' to foo(): |
| </span> <span class="identifier">foo</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> |
| <span class="comment">// But instead, the initialization state is passed |
| </span> <span class="comment">// due to a typo: it should have been foo(*v). |
| </span><span class="special">}</span> |
| </pre> |
| <p> |
| The only implicit conversion is to <code class="computeroutput"><span class="keyword">bool</span></code>, |
| and it is safe in the sense that typical integral promotions don't apply (i.e. |
| if <code class="computeroutput"><span class="identifier">foo</span><span class="special">()</span></code> |
| takes an <code class="computeroutput"><span class="keyword">int</span></code> instead, it won't |
| compile). |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2003 -2007 Fernando Luis Cacciola Carballal<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) |
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