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<title>Obtaining the same types and reducing symbol length</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="intrusive.obtaining_same_type_reducing_space"></a><a class="link" href="obtaining_same_type_reducing_space.html" title="Obtaining the same types and reducing symbol length">Obtaining
the same types and reducing symbol length</a>
</h2></div></div></div>
<p>
The flexible option specification mechanism used by <span class="bold"><strong>Boost.Intrusive</strong></span>
for hooks and containers has a couple of downsides:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
If a user specifies the same options in different order or specifies some
options and leaves the rest as defaults, the type of the created container/hook
will be different. Sometimes this is annoying, because two programmers
specifying the same options might end up with incompatible types. For example,
the following two lists, although using the same options, do not have the
same type:
</li></ul></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive</span><span class="special">;</span>
<span class="comment">//Explicitly specify constant-time size and size type</span>
<span class="keyword">typedef</span> <span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">constant_time_size</span><span class="special"><</span><span class="keyword">true</span><span class="special">>,</span> <span class="identifier">size_type</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">></span> <span class="identifier">List1</span><span class="special">;</span>
<span class="comment">//Implicitly specify constant-time size and size type</span>
<span class="keyword">typedef</span> <span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">List2</span><span class="special">;</span>
</pre>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
Option specifiers lead to long template symbols for classes and functions.
Option specifiers themselves are verbose and without variadic templates,
several default template parameters are assigned for non-specified options.
Object and debugging information files can grow and compilation times may
suffer if long names are produced.
</li></ul></div>
<p>
To solve these issues <span class="bold"><strong>Boost.Intrusive</strong></span> offers
some helper metafunctions that reduce symbol lengths and create the same type
if the same options (either explicitly or implicitly) are used. These also
improve compilation times. All containers and hooks have their respective
<code class="computeroutput"><span class="identifier">make_xxx</span></code> versions. The previously
shown example can be rewritten like this to obtain the same list type:
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive</span><span class="special">;</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">intrusive</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive</span><span class="special">;</span>
<span class="comment">//Explicitly specify constant-time size and size type</span>
<span class="keyword">typedef</span> <span class="identifier">make_list</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">constant_time_size</span><span class="special"><</span><span class="keyword">true</span><span class="special">>,</span> <span class="identifier">size_type</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">List1</span><span class="special">;</span>
<span class="comment">//Implicitly specify constant-time size and size type</span>
<span class="keyword">typedef</span> <span class="identifier">make_list</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">List2</span><span class="special">;</span>
</pre>
<p>
Produced symbol lengths and compilation times will usually be shorter and object/debug
files smaller. If you are concerned with file sizes and compilation times,
this option is your best choice.
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2005 Olaf Krzikalla<br>Copyright © 2006-2011 Ion Gaztanaga<p>
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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