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Added performance notes to documentation
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Andrzej Krzemienski
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doc/html/boost_optional/tutorial/performance_considerations.html
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doc/html/boost_optional/tutorial/performance_considerations.html
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>Performance considerations</title>
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_optional.tutorial.performance_considerations"></a><a class="link" href="performance_considerations.html" title="Performance considerations">Performance
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considerations</a>
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</h3></div></div></div>
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<p>
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Technical details aside, the memory layout 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 more-less this:
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</p>
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<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
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<span class="keyword">class</span> <span class="identifier">optional</span>
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<span class="special">{</span>
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<span class="keyword">bool</span> <span class="identifier">_initialized</span><span class="special">;</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">aligned_storage_t</span><span class="special"><</span><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">t</span><span class="special">),</span> <span class="keyword">alignof</span><span class="special">(</span><span class="identifier">T</span><span class="special">)></span> <span class="identifier">_storage</span><span class="special">;</span>
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<span class="special">};</span>
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</pre>
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<p>
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But for the purpose of this analysis, considering memory layouts, we can
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think of it as:
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</p>
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<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
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<span class="keyword">class</span> <span class="identifier">optional</span>
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<span class="special">{</span>
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<span class="keyword">bool</span> <span class="identifier">_initialized</span><span class="special">;</span>
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<span class="identifier">T</span> <span class="identifier">_storage</span><span class="special">;</span>
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<span class="special">};</span>
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</pre>
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<p>
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Given type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>, and
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assuming that <code class="computeroutput"><span class="keyword">sizeof</span><span class="special">(</span><span class="keyword">int</span><span class="special">)</span> <span class="special">==</span>
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<span class="number">4</span></code>, we will get <code class="computeroutput"><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">>)</span>
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<span class="special">==</span> <span class="number">8</span></code>.
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This is so because of the alignment rules, for our two members we get the
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following alignment:
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</p>
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<p>
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<span class="inlinemediaobject"><img src="../../images/opt_align1.png" alt="opt_align1"></span>
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</p>
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<p>
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This means you can fit twice as many <code class="computeroutput"><span class="keyword">int</span></code>s
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as <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>s into
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the same space of memory. Therefore, if the size of the objects is critical
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for your application (e.g., because you want to utilize your CPU cache in
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order to gain performance) and you have determined you are willing to trade
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the code clarity, it is recommended that you simply go with type <code class="computeroutput"><span class="keyword">int</span></code> and use some 'magic value' to represent
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<span class="emphasis"><em>not-an-int</em></span>.
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</p>
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<p>
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Even if you cannot spare any value of <code class="computeroutput"><span class="keyword">int</span></code>
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to represent <span class="emphasis"><em>not-an-int</em></span> (e.g., because every value is
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useful, or you do want to signal <span class="emphasis"><em>not-an-int</em></span> explicitly),
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at least for <code class="computeroutput"><span class="identifier">Trivial</span></code> types
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you should consider storing the value and the <code class="computeroutput"><span class="keyword">bool</span></code>
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flag representing the <span class="emphasis"><em>null-state</em></span> separately. Consider
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the following class:
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</p>
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<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
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<span class="special">{</span>
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<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">_min</span><span class="special">;</span>
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<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">_max</span><span class="special">;</span>
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<span class="special">};</span>
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</pre>
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<p>
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Its memory layout can be depicted as follows:
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</p>
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<p>
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<span class="inlinemediaobject"><img src="../../images/opt_align2.png" alt="opt_align2"></span>
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</p>
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<p>
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This is exactly the same as if we had the following members:
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</p>
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<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
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<span class="special">{</span>
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<span class="keyword">bool</span> <span class="identifier">_has_min</span><span class="special">;</span>
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<span class="keyword">int</span> <span class="identifier">_min</span><span class="special">;</span>
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<span class="keyword">bool</span> <span class="identifier">_has_max</span><span class="special">;</span>
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<span class="keyword">int</span> <span class="identifier">_max</span><span class="special">;</span>
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<span class="special">};</span>
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</pre>
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<p>
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But when they are stored separately, we at least have an option to reorder
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them like this:
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</p>
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<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
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<span class="special">{</span>
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<span class="keyword">bool</span> <span class="identifier">_has_min</span><span class="special">;</span>
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<span class="keyword">bool</span> <span class="identifier">_has_max</span><span class="special">;</span>
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<span class="keyword">int</span> <span class="identifier">_min</span><span class="special">;</span>
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<span class="keyword">int</span> <span class="identifier">_max</span><span class="special">;</span>
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<span class="special">};</span>
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</pre>
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<p>
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Which gives us the following layout (and smaller total size):
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</p>
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<p>
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<span class="inlinemediaobject"><img src="../../images/opt_align3.png" alt="opt_align3"></span>
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</p>
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<p>
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Sometimes it requires detailed consideration what data we make optional.
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In our case above, if we determine that both minimum and maximum value can
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be provided or not provided together, but one is never provided without the
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other, we can make only one optional memebr:
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</p>
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<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Limits</span>
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<span class="special">{</span>
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<span class="keyword">int</span> <span class="identifier">_min</span><span class="special">;</span>
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<span class="keyword">int</span> <span class="identifier">_max</span><span class="special">;</span>
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<span class="special">};</span>
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<span class="keyword">struct</span> <span class="identifier">Record</span>
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<span class="special">{</span>
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<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Limits</span><span class="special">></span> <span class="identifier">_limits</span><span class="special">;</span>
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<span class="special">};</span>
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</pre>
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<p>
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This would give us the following layout:
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</p>
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<p>
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<span class="inlinemediaobject"><img src="../../images/opt_align4.png" alt="opt_align4"></span>
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</p>
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<h5>
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<a name="boost_optional.tutorial.performance_considerations.h0"></a>
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<span class="phrase"><a name="boost_optional.tutorial.performance_considerations.optional_function_parameters"></a></span><a class="link" href="performance_considerations.html#boost_optional.tutorial.performance_considerations.optional_function_parameters">Optional
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function parameters</a>
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</h5>
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<p>
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Having function parameters of type <code class="computeroutput"><span class="keyword">const</span>
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<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span></code>
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may incur certain unexpected run-time cost connected to copy construction
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of <code class="computeroutput"><span class="identifier">T</span></code>. Consider the following
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code.
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">fun</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">>&</span> <span class="identifier">v</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">v</span><span class="special">)</span> <span class="identifier">doSomethingWith</span><span class="special">(*</span><span class="identifier">v</span><span class="special">);</span>
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<span class="keyword">else</span> <span class="identifier">doSomethingElse</span><span class="special">();</span>
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<span class="special">}</span>
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<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">></span> <span class="identifier">ov</span><span class="special">;</span>
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<span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
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<span class="identifier">fun</span><span class="special">(</span><span class="identifier">none</span><span class="special">);</span>
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<span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// no copy</span>
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<span class="identifier">fun</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> <span class="comment">// copy constructor of Big</span>
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<span class="special">}</span>
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</pre>
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<p>
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No copy elision or move semantics can save us from copying type <code class="computeroutput"><span class="identifier">Big</span></code> here. Not that we need any copy, but
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this is how <code class="computeroutput"><span class="identifier">optional</span></code> works.
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In order to avoid copying in this case, one could provide second overload
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of <code class="computeroutput"><span class="identifier">fun</span></code>:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">fun</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Big</span><span class="special">&</span> <span class="identifier">v</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="identifier">doSomethingWith</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
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<span class="special">}</span>
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<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">></span> <span class="identifier">ov</span><span class="special">;</span>
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<span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
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<span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// no copy</span>
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<span class="identifier">fun</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> <span class="comment">// no copy: second overload selected</span>
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<span class="special">}</span>
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</pre>
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<p>
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Alternatively, you could consider using an optional reference instead:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">fun</span><span class="special">(</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="identifier">Big</span><span class="special">&></span> <span class="identifier">v</span><span class="special">)</span> <span class="comment">// note where the reference is</span>
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<span class="special">{</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">v</span><span class="special">)</span> <span class="identifier">doSomethingWith</span><span class="special">(*</span><span class="identifier">v</span><span class="special">);</span>
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<span class="keyword">else</span> <span class="identifier">doSomethingElse</span><span class="special">();</span>
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<span class="special">}</span>
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<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">></span> <span class="identifier">ov</span><span class="special">;</span>
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<span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
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<span class="identifier">fun</span><span class="special">(</span><span class="identifier">none</span><span class="special">);</span>
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<span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// doesn't compile</span>
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<span class="identifier">fun</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> <span class="comment">// no copy</span>
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<span class="special">}</span>
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</pre>
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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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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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</p>
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</div></td>
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</tr></table>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="type_requirements.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="../../optional/reference.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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@ -7,7 +7,7 @@
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<link rel="home" href="../../index.html" title="Boost.Optional">
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<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
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<hr>
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<div class="titlepage"><div><div><h3 class="title">
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@ -103,7 +103,7 @@
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