ceph/src/boost/libs/optional/doc/html/boost_optional/tutorial/gotchas.html

   1 <html>
   2 <head>
   3 <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
   4 <title>Gotchas</title>
   5 <link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
   6 <meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
   7 <link rel="home" href="../../index.html" title="Boost.Optional">
   8 <link rel="up" href="../../optional/tutorial.html" title="Tutorial">
   9 <link rel="prev" href="in_place_factories.html" title="In-Place Factories">
  10 <link rel="next" href="gotchas/moved_from__optional_.html" title="Moved-from optional">
  11 </head>
  12 <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
  13 <table cellpadding="2" width="100%"><tr>
  14 <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
  15 <td align="center"><a href="../../../../../../index.html">Home</a></td>
  16 <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
  17 <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
  18 <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
  19 <td align="center"><a href="../../../../../../more/index.htm">More</a></td>
  20 </tr></table>
  21 <hr>
  22 <div class="spirit-nav">
  23 <a accesskey="p" href="in_place_factories.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="gotchas/moved_from__optional_.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
  24 </div>
  25 <div class="section">
  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_optional.tutorial.gotchas"></a><a class="link" href="gotchas.html" title="Gotchas">Gotchas</a>
  28 </h3></div></div></div>
  29 <div class="section">
  30 <div class="titlepage"><div><div><h4 class="title">
  31 <a name="boost_optional.tutorial.gotchas.a_note_about_optional_bool_"></a><a class="link" href="gotchas.html#boost_optional.tutorial.gotchas.a_note_about_optional_bool_" title="A note about optional&lt;bool&gt;">A
  32         note about optional&lt;bool&gt;</a>
  33 </h4></div></div></div>
  34 <p>
  35           <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span></code>
  36           should be used with special caution and consideration.
  37         </p>
  38 <p>
  39           First, it is functionally similar to a tristate boolean (false, maybe,
  40           true) &#8212;such as <a href="../../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>&#8212;
  41           except that in a tristate boolean, the maybe state <span class="underline">represents
  42           a valid value</span>, unlike the corresponding state of an uninitialized
  43           <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span></code>.
  44           It should be carefully considered if an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span></code> instead of a <code class="computeroutput"><span class="identifier">tribool</span></code>
  45           is really needed.
  46         </p>
  47 <p>
  48           Second, although <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> provides a contextual conversion
  49           to <code class="computeroutput"><span class="keyword">bool</span></code> in C++11, this falls
  50           back to an implicit conversion on older compilers. This conversion refers
  51           to the initialization state and not to the contained value. Using <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span></code>
  52           can lead to subtle errors due to the implicit <code class="computeroutput"><span class="keyword">bool</span></code>
  53           conversion:
  54         </p>
  55 <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>
  56 <span class="keyword">void</span> <span class="identifier">bar</span><span class="special">()</span>
  57 <span class="special">{</span>
  58     <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span> <span class="identifier">v</span> <span class="special">=</span> <span class="keyword">try</span><span class="special">();</span>
  59
  60     <span class="comment">// The following intended to pass the value of 'v' to foo():</span>
  61     <span class="identifier">foo</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
  62     <span class="comment">// But instead, the initialization state is passed</span>
  63     <span class="comment">// due to a typo: it should have been foo(*v).</span>
  64 <span class="special">}</span>
  65 </pre>
  66 <p>
  67           The only implicit conversion is to <code class="computeroutput"><span class="keyword">bool</span></code>,
  68           and it is safe in the sense that typical integral promotions don't apply
  69           (i.e. if <code class="computeroutput"><span class="identifier">foo</span><span class="special">()</span></code>
  70           takes an <code class="computeroutput"><span class="keyword">int</span></code> instead, it won't
  71           compile).
  72         </p>
  73 <p>
  74           Third, mixed comparisons with <code class="computeroutput"><span class="keyword">bool</span></code>
  75           work differently than similar mixed comparisons between pointers and <code class="computeroutput"><span class="keyword">bool</span></code>, so the results might surprise you:
  76         </p>
  77 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span> <span class="identifier">oEmpty</span><span class="special">(</span><span class="identifier">none</span><span class="special">),</span> <span class="identifier">oTrue</span><span class="special">(</span><span class="keyword">true</span><span class="special">),</span> <span class="identifier">oFalse</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
  78
  79 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span>  <span class="comment">// renders true</span>
  80 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false!</span>
  81 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span>  <span class="comment">// renders false!</span>
  82
  83 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span>  <span class="comment">// renders false</span>
  84 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders true!</span>
  85 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span>  <span class="comment">// renders false</span>
  86
  87 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span>   <span class="comment">// renders false</span>
  88 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span>  <span class="comment">// renders false</span>
  89 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span>   <span class="comment">// renders true</span>
  90 </pre>
  91 <p>
  92           In other words, for <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code>, the following assertion does not
  93           hold:
  94         </p>
  95 <pre class="programlisting"><span class="identifier">assert</span><span class="special">((</span><span class="identifier">opt</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">)</span> <span class="special">==</span> <span class="special">(!</span><span class="identifier">opt</span><span class="special">));</span>
  96 </pre>
  97 </div>
  98 </div>
  99 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 100 <td align="left"></td>
 101 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2016 Andrzej Krzemie&#324;ski<p>
 102         Distributed under the Boost Software License, Version 1.0. (See accompanying
 103         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>)
 104       </p>
 105 </div></td>
 106 </tr></table>
 107 <hr>
 108 <div class="spirit-nav">
 109 <a accesskey="p" href="in_place_factories.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="gotchas/moved_from__optional_.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
 110 </div>
 111 </body>
 112 </html>