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| 3 | <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> | |
| 4 | <title>Type requirements</title> | |
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| 24 | </div> | |
| 25 | <div class="section"> | |
| 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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>();</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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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> | |
| 95 | </div> | |
| 96 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 97 | <td align="left"></td> | |
| 98 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<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>) | |
| 101 | </p> | |
| 102 | </div></td> | |
| 103 | </tr></table> | |
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