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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_optional.tutorial.type_requirements"></a><a class="link" href="type_requirements.html" title="Type requirements">Type requirements</a> |
| </h3></div></div></div> |
| <p> |
| 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> |
| is that <code class="computeroutput"><span class="identifier">T</span></code> is a complete type |
| and that it has a publicly accessible destructor. <code class="computeroutput"><span class="identifier">T</span></code> |
| doesn't even need to be constructible. You can use a very minimum interface: |
| </p> |
| <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> |
| <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> |
| <span class="identifier">assert</span><span class="special">(!</span><span class="identifier">o</span><span class="special">);</span> <span class="comment">//</span> |
| <span class="identifier">o</span><span class="special">.</span><span class="identifier">value</span><span class="special">();</span> <span class="comment">// always throws</span> |
| </pre> |
| <p> |
| 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> |
| to be able to do anything useful and offer all the spectrum of ways of accessing |
| the contained value, <code class="computeroutput"><span class="identifier">T</span></code> needs |
| to have at least one accessible constructor. In that case you need to initialize |
| 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, |
| resort to <a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place |
| Factories</a>: |
| </p> |
| <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">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> |
| </pre> |
| <p> |
| If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>, |
| <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is |
| also <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and |
| can be easily initialized from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code> |
| and be passed by value: |
| </p> |
| <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> |
| <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> |
| </pre> |
| <p> |
| 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 |
| also <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a> |
| and can be easily initialized from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>: |
| </p> |
| <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> |
| <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> |
| <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> |
| </pre> |
| <p> |
| 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 |
| 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> |
| using function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>: |
| </p> |
| <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> |
| <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> |
| </pre> |
| <p> |
| If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Moveable</span></code> |
| (both <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and |
| <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 |
| also <code class="computeroutput"><span class="identifier">Moveable</span></code> and additionally |
| can be constructed and assigned from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>. |
| </p> |
| <p> |
| 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> |
| is also <code class="computeroutput"><span class="identifier">Copyable</span></code> and additionally |
| can be constructed and assigned from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>. |
| </p> |
| <p> |
| <code class="computeroutput"><span class="identifier">T</span></code> <span class="emphasis"><em>is not</em></span> |
| 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>. |
| </p> |
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| 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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