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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_optional.tutorial.performance_considerations"></a><a class="link" href="performance_considerations.html" title="Performance considerations">Performance
  28       considerations</a>
  29 </h3></div></div></div>
  30 <p>
  31         Technical details aside, the memory layout 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>
  32         is more-less this:
  33       </p>
  34 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
  35 <span class="keyword">class</span> <span class="identifier">optional</span>
  36 <span class="special">{</span>
  37   <span class="keyword">bool</span> <span class="identifier">_initialized</span><span class="special">;</span>
  38   <span class="identifier">std</span><span class="special">::</span><span class="identifier">aligned_storage_t</span><span class="special">&lt;</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">)&gt;</span> <span class="identifier">_storage</span><span class="special">;</span>
  39 <span class="special">};</span>
  40 </pre>
  41 <p>
  42         But for the purpose of this analysis, considering memory layouts, we can
  43         think of it as:
  44       </p>
  45 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
  46 <span class="keyword">class</span> <span class="identifier">optional</span>
  47 <span class="special">{</span>
  48   <span class="keyword">bool</span> <span class="identifier">_initialized</span><span class="special">;</span>
  49   <span class="identifier">T</span> <span class="identifier">_storage</span><span class="special">;</span>
  50 <span class="special">};</span>
  51 </pre>
  52 <p>
  53         Given type <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>, and
  54         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>
  55         <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">&lt;</span><span class="keyword">int</span><span class="special">&gt;)</span>
  56         <span class="special">==</span> <span class="number">8</span></code>.
  57         This is so because of the alignment rules, for our two members we get the
  58         following alignment:
  59       </p>
  60 <p>
  61         <span class="inlinemediaobject"><img src="../../images/opt_align1.png" alt="opt_align1"></span>
  62       </p>
  63 <p>
  64         This means you can fit twice as many <code class="computeroutput"><span class="keyword">int</span></code>s
  65         as <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>s into
  66         the same space of memory. Therefore, if the size of the objects is critical
  67         for your application (e.g., because you want to utilize your CPU cache in
  68         order to gain performance) and you have determined you are willing to trade
  69         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
  70         <span class="emphasis"><em>not-an-int</em></span>, or use something like <a href="https://github.com/akrzemi1/markable" target="_top"><code class="computeroutput"><span class="identifier">markable</span></code></a> library.
  71       </p>
  72 <p>
  73         Even if you cannot spare any value of <code class="computeroutput"><span class="keyword">int</span></code>
  74         to represent <span class="emphasis"><em>not-an-int</em></span> (e.g., because every value is
  75         useful, or you do want to signal <span class="emphasis"><em>not-an-int</em></span> explicitly),
  76         at least for <code class="computeroutput"><span class="identifier">Trivial</span></code> types
  77         you should consider storing the value and the <code class="computeroutput"><span class="keyword">bool</span></code>
  78         flag representing the <span class="emphasis"><em>null-state</em></span> separately. Consider
  79         the following class:
  80       </p>
  81 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
  82 <span class="special">{</span>
  83   <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">_min</span><span class="special">;</span>
  84   <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">_max</span><span class="special">;</span>
  85 <span class="special">};</span>
  86 </pre>
  87 <p>
  88         Its memory layout can be depicted as follows:
  89       </p>
  90 <p>
  91         <span class="inlinemediaobject"><img src="../../images/opt_align2.png" alt="opt_align2"></span>
  92       </p>
  93 <p>
  94         This is exactly the same as if we had the following members:
  95       </p>
  96 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
  97 <span class="special">{</span>
  98   <span class="keyword">bool</span> <span class="identifier">_has_min</span><span class="special">;</span>
  99   <span class="keyword">int</span>  <span class="identifier">_min</span><span class="special">;</span>
 100   <span class="keyword">bool</span> <span class="identifier">_has_max</span><span class="special">;</span>
 101   <span class="keyword">int</span>  <span class="identifier">_max</span><span class="special">;</span>
 102 <span class="special">};</span>
 103 </pre>
 104 <p>
 105         But when they are stored separately, we at least have an option to reorder
 106         them like this:
 107       </p>
 108 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
 109 <span class="special">{</span>
 110   <span class="keyword">bool</span> <span class="identifier">_has_min</span><span class="special">;</span>
 111   <span class="keyword">bool</span> <span class="identifier">_has_max</span><span class="special">;</span>
 112   <span class="keyword">int</span>  <span class="identifier">_min</span><span class="special">;</span>
 113   <span class="keyword">int</span>  <span class="identifier">_max</span><span class="special">;</span>
 114 <span class="special">};</span>
 115 </pre>
 116 <p>
 117         Which gives us the following layout (and smaller total size):
 118       </p>
 119 <p>
 120         <span class="inlinemediaobject"><img src="../../images/opt_align3.png" alt="opt_align3"></span>
 121       </p>
 122 <p>
 123         Sometimes it requires detailed consideration what data we make optional.
 124         In our case above, if we determine that both minimum and maximum value can
 125         be provided or not provided together, but one is never provided without the
 126         other, we can make only one optional memebr:
 127       </p>
 128 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Limits</span>
 129 <span class="special">{</span>
 130   <span class="keyword">int</span>  <span class="identifier">_min</span><span class="special">;</span>
 131   <span class="keyword">int</span>  <span class="identifier">_max</span><span class="special">;</span>
 132 <span class="special">};</span>
 133
 134 <span class="keyword">struct</span> <span class="identifier">Record</span>
 135 <span class="special">{</span>
 136   <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">Limits</span><span class="special">&gt;</span> <span class="identifier">_limits</span><span class="special">;</span>
 137 <span class="special">};</span>
 138 </pre>
 139 <p>
 140         This would give us the following layout:
 141       </p>
 142 <p>
 143         <span class="inlinemediaobject"><img src="../../images/opt_align4.png" alt="opt_align4"></span>
 144       </p>
 145 <h5>
 146 <a name="boost_optional.tutorial.performance_considerations.h0"></a>
 147         <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
 148         function parameters</a>
 149       </h5>
 150 <p>
 151         Having function parameters of type <code class="computeroutput"><span class="keyword">const</span>
 152         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span></code>
 153         may incur certain unexpected run-time cost connected to copy construction
 154         of <code class="computeroutput"><span class="identifier">T</span></code>. Consider the following
 155         code.
 156       </p>
 157 <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">&lt;</span><span class="identifier">Big</span><span class="special">&gt;&amp;</span> <span class="identifier">v</span><span class="special">)</span>
 158 <span class="special">{</span>
 159   <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>
 160   <span class="keyword">else</span>   <span class="identifier">doSomethingElse</span><span class="special">();</span>
 161 <span class="special">}</span>
 162
 163 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 164 <span class="special">{</span>
 165   <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">Big</span><span class="special">&gt;</span> <span class="identifier">ov</span><span class="special">;</span>
 166   <span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
 167   <span class="identifier">fun</span><span class="special">(</span><span class="identifier">none</span><span class="special">);</span>
 168   <span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// no copy</span>
 169   <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>
 170 <span class="special">}</span>
 171 </pre>
 172 <p>
 173         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
 174         this is how <code class="computeroutput"><span class="identifier">optional</span></code> works.
 175         In order to avoid copying in this case, one could provide second overload
 176         of <code class="computeroutput"><span class="identifier">fun</span></code>:
 177       </p>
 178 <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">&amp;</span> <span class="identifier">v</span><span class="special">)</span>
 179 <span class="special">{</span>
 180   <span class="identifier">doSomethingWith</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
 181 <span class="special">}</span>
 182
 183 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 184 <span class="special">{</span>
 185   <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">Big</span><span class="special">&gt;</span> <span class="identifier">ov</span><span class="special">;</span>
 186   <span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
 187   <span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// no copy</span>
 188   <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>
 189 <span class="special">}</span>
 190 </pre>
 191 <p>
 192         Alternatively, you could consider using an optional reference instead:
 193       </p>
 194 <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">&lt;</span><span class="keyword">const</span> <span class="identifier">Big</span><span class="special">&amp;&gt;</span> <span class="identifier">v</span><span class="special">)</span> <span class="comment">// note where the reference is</span>
 195 <span class="special">{</span>
 196   <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>
 197   <span class="keyword">else</span>   <span class="identifier">doSomethingElse</span><span class="special">();</span>
 198 <span class="special">}</span>
 199
 200 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 201 <span class="special">{</span>
 202   <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">Big</span><span class="special">&gt;</span> <span class="identifier">ov</span><span class="special">;</span>
 203   <span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
 204   <span class="identifier">fun</span><span class="special">(</span><span class="identifier">none</span><span class="special">);</span>
 205   <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>
 206   <span class="identifier">fun</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>  <span class="comment">// no copy</span>
 207 <span class="special">}</span>
 208 </pre>
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 211 <td align="left"></td>
 212 <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>
 213         Distributed under the Boost Software License, Version 1.0. (See accompanying
 214         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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