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

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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_optional.tutorial.type_requirements"></a><a class="link" href="type_requirements.html" title="Type requirements">Type requirements</a>
  28 </h3></div></div></div>
  29 <p>
  30         The very minimum requirement of <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
  31         is that <code class="computeroutput"><span class="identifier">T</span></code> is a complete type
  32         and that it has a publicly accessible destructor. <code class="computeroutput"><span class="identifier">T</span></code>
  33         doesn't even need to be constructible. You can use a very minimum interface:
  34       </p>
  35 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">o</span><span class="special">;</span>     <span class="comment">// uninitialized</span>
  36 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">o</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// check if initialized</span>
  37 <span class="identifier">assert</span><span class="special">(!</span><span class="identifier">o</span><span class="special">);</span>        <span class="comment">//</span>
  38 <span class="identifier">o</span><span class="special">.</span><span class="identifier">value</span><span class="special">();</span>         <span class="comment">// always throws</span>
  39 </pre>
  40 <p>
  41         But this is practically useless. In order for <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
  42         to be able to do anything useful and offer all the spectrum of ways of accessing
  43         the contained value, <code class="computeroutput"><span class="identifier">T</span></code> needs
  44         to have at least one accessible constructor. In that case you need to initialize
  45         the optional object with function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>, or if your compiler does not support it,
  46         resort to <a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place
  47         Factories</a>:
  48       </p>
  49 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">o</span><span class="special">;</span>
  50 <span class="identifier">o</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="string">"T"</span><span class="special">,</span> <span class="string">"ctor"</span><span class="special">,</span> <span class="string">"params"</span><span class="special">);</span>
  51 </pre>
  52 <p>
  53         If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>,
  54         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> is
  55         also <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and
  56         can be easily initialized from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>
  57         and be passed by value:
  58       </p>
  59 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
  60 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;();</span>
  61 </pre>
  62 <p>
  63         If <code class="computeroutput"><span class="identifier">T</span></code> is <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> is
  64         also <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>
  65         and can be easily initialized from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>:
  66       </p>
  67 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
  68 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">v</span><span class="special">;</span>
  69 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">o</span><span class="special">;</span>
  70 </pre>
  71 <p>
  72         If <code class="computeroutput"><span class="identifier">T</span></code> is not <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>, it is still possible to
  73         reset the value of <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
  74         using function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>:
  75       </p>
  76 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
  77 <span class="identifier">o</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="identifier">make_another_T</span><span class="special">());</span>
  78 </pre>
  79 <p>
  80         If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Moveable</span></code>
  81         (both <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and
  82         <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>) then <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> is
  83         also <code class="computeroutput"><span class="identifier">Moveable</span></code> and additionally
  84         can be constructed and assigned from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
  85       </p>
  86 <p>
  87         Similarly, if <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Copyable</span></code> (both <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a> and <code class="computeroutput"><span class="identifier">CopyAssignable</span></code>) then <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
  88         is also <code class="computeroutput"><span class="identifier">Copyable</span></code> and additionally
  89         can be constructed and assigned from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
  90       </p>
  91 <p>
  92         <code class="computeroutput"><span class="identifier">T</span></code> <span class="emphasis"><em>is not</em></span>
  93         required to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a>.
  94       </p>
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  98 <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>
  99         Distributed under the Boost Software License, Version 1.0. (See accompanying
 100         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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