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Added limited emplace() for older compilers
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Andrzej Krzemienski
@ -27,22 +27,73 @@
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<a name="boost_optional.tutorial.type_requirements"></a><a class="link" href="type_requirements.html" title="Type requirements">Type requirements</a>
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</h3></div></div></div>
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<p>
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At the very minimum for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
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to work with a minimum interface it is required that <code class="computeroutput"><span class="identifier">T</span></code>
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has a publicly accessible no-throw destructor. In that case you need to initialize
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the optional object with function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code> or use <span class="bold"><strong>InPlaceFactories</strong></span>.
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Additionally, if <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Moveable</span></code>, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
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is also <code class="computeroutput"><span class="identifier">Moveable</span></code> and can
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be easily initialized from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>
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and be passed by value. Additionally, if <code class="computeroutput"><span class="identifier">T</span></code>
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is <code class="computeroutput"><span class="identifier">Copyable</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
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also <code class="computeroutput"><span class="identifier">Copyable</span></code> and can be
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easily initialized from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
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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>
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is that <code class="computeroutput"><span class="identifier">T</span></code> is a complete type
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and that it has a publicly accessible destructor. <code class="computeroutput"><span class="identifier">T</span></code>
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doesn't even need to be constructible. You can use a very minimum interface:
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</p>
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<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>
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<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>
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<span class="identifier">assert</span><span class="special">(!</span><span class="identifier">o</span><span class="special">);</span> <span class="comment">//</span>
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<span class="identifier">o</span><span class="special">.</span><span class="identifier">value</span><span class="special">();</span> <span class="comment">// always throws</span>
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</pre>
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<p>
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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>
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to be able to do anything useful and offer all the spectrum of ways of accessing
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the contained value, <code class="computeroutput"><span class="identifier">T</span></code> needs
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to have at least one accessible constructor. In that case you need to initialize
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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,
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resort to <a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place
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Factories</a>:
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</p>
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<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>
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<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>
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</pre>
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<p>
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If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>,
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<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
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also <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and
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can be easily initialized from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>
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and be passed by value:
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</p>
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<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>
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<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>
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</pre>
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<p>
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If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">CopyConstructible</span></code>,
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<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
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also <code class="computeroutput"><span class="identifier">CopyConstructible</span></code> and
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can be easily initialized from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>:
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</p>
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<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>
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<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>
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<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>
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</pre>
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<p>
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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
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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>
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using function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>:
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</p>
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<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>
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<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>
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</pre>
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<p>
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If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Moveable</span></code>
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(both <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and
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<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
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also <code class="computeroutput"><span class="identifier">Moveable</span></code> and additionally
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can be constructed and assigned from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
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</p>
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<p>
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<code class="computeroutput"><span class="identifier">T</span></code> <span class="underline">is
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not</span> required to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top">Default
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Constructible</a>.
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Similarly, if <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Copyable</span></code> (both <code class="computeroutput"><span class="identifier">CopyConstructible</span></code>
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and <code class="computeroutput"><span class="identifier">CopyAssignable</span></code>) then
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<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
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also <code class="computeroutput"><span class="identifier">Copyable</span></code> and additionally
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can be constructed and assigned from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
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</p>
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<p>
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<code class="computeroutput"><span class="identifier">T</span></code> <span class="emphasis"><em>is not</em></span>
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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>.
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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