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<title>Containers of Incomplete Types</title>
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="container.containers_of_incomplete_types"></a><a class="link" href="containers_of_incomplete_types.html" title="Containers of Incomplete Types">Containers of
Incomplete Types</a>
</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="containers_of_incomplete_types.html#container.containers_of_incomplete_types.recursive_containers">Recursive
containers</a></span></dt>
<dt><span class="section"><a href="containers_of_incomplete_types.html#container.containers_of_incomplete_types.type_erasure">Type
Erasure</a></span></dt>
</dl></div>
<p>
Incomplete types allow <a href="http://en.wikipedia.org/wiki/Type_erasure" target="_top"><span class="bold"><strong>type erasure </strong></span></a> and <a href="http://en.wikipedia.org/wiki/Recursive_data_type" target="_top"><span class="bold"><strong>recursive data types</strong></span></a>, and C and C++ programmers
have been using it for years to build complex data structures, like tree structures
where a node may have an arbitrary number of children.
</p>
<p>
What about standard containers? Containers of incomplete types have been under
discussion for a long time, as explained in Matt Austern's great article (<a href="http://drdobbs.com/184403814" target="_top"><span class="bold"><strong>The Standard Librarian:
Containers of Incomplete Types</strong></span></a>):
</p>
<p>
<span class="quote">“<span class="quote"><span class="emphasis"><em>Unlike most of my columns, this one is about something you
can't do with the C++ Standard library: put incomplete types in one of the
standard containers. This column explains why you might want to do this, why
the standardization committee banned it even though they knew it was useful,
and what you might be able to do to get around the restriction.</em></span></span>”</span>
</p>
<p>
<span class="quote">“<span class="quote"><span class="emphasis"><em>In 1997, shortly before the C++ Standard was completed, the
standardization committee received a query: Is it possible to create standard
containers with incomplete types? It took a while for the committee to understand
the question. What would such a thing even mean, and why on earth would you
ever want to do it? The committee eventually worked it out and came up with
an answer to the question. (Just so you don't have to skip ahead to the end,
the answer is "no.") But the question is much more interesting than
the answer: it points to a useful, and insufficiently discussed, programming
technique. The standard library doesn't directly support that technique, but
the two can be made to coexist.</em></span></span>”</span>
</p>
<p>
<span class="quote">“<span class="quote"><span class="emphasis"><em>In a future revision of C++, it might make sense to relax
the restriction on instantiating standard library templates with incomplete
types. Clearly, the general prohibition should stay in place - instantiating
templates with incomplete types is a delicate business, and there are too many
classes in the standard library where it would make no sense. But perhaps it
should be relaxed on a case-by-case basis, and <code class="computeroutput"><span class="identifier">vector</span></code>
looks like a good candidate for such special-case treatment: it's the one standard
container class where there are good reasons to instantiate it with an incomplete
type and where Standard Library implementors want to make it work. As of today,
in fact, implementors would have to go out of their way to prohibit it!</em></span></span>”</span>
</p>
<p>
C++11 standard is also cautious about incomplete types and STL: <span class="quote">“<span class="quote"><span class="emphasis"><em>17.6.4.8
Other functions (...) 2. the effects are undefined in the following cases:
(...) In particular - if an incomplete type (3.9) is used as a template argument
when instantiating a template component, unless specifically allowed for that
component</em></span></span>”</span>. Fortunately <span class="bold"><strong>Boost.Container</strong></span>
containers are designed to support type erasure and recursive types, so let's
see some examples:
</p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="container.containers_of_incomplete_types.recursive_containers"></a><a class="link" href="containers_of_incomplete_types.html#container.containers_of_incomplete_types.recursive_containers" title="Recursive containers">Recursive
containers</a>
</h3></div></div></div>
<p>
All containers offered by <span class="bold"><strong>Boost.Container</strong></span>
can be used to define recursive containers:
</p>
<p>
</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">container</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</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">container</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="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">container</span><span class="special">/</span><span class="identifier">map</span><span class="special">.</span><span class="identifier">hpp</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">container</span><span class="special">/</span><span class="identifier">stable_vector</span><span class="special">.</span><span class="identifier">hpp</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">container</span><span class="special">/</span><span class="identifier">string</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">container</span><span class="special">;</span>
<span class="keyword">struct</span> <span class="identifier">data</span>
<span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">i_</span><span class="special">;</span>
<span class="comment">//A vector holding still undefined class 'data'</span>
<span class="identifier">vector</span><span class="special"><</span><span class="identifier">data</span><span class="special">></span> <span class="identifier">v_</span><span class="special">;</span>
<span class="comment">//A list holding still undefined 'data'</span>
<span class="identifier">list</span><span class="special"><</span><span class="identifier">data</span><span class="special">></span> <span class="identifier">l_</span><span class="special">;</span>
<span class="comment">//A map holding still undefined 'data'</span>
<span class="identifier">map</span><span class="special"><</span><span class="identifier">data</span><span class="special">,</span> <span class="identifier">data</span><span class="special">></span> <span class="identifier">m_</span><span class="special">;</span>
<span class="keyword">friend</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special"><(</span><span class="keyword">const</span> <span class="identifier">data</span> <span class="special">&</span><span class="identifier">l</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">data</span> <span class="special">&</span><span class="identifier">r</span><span class="special">)</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">l</span><span class="special">.</span><span class="identifier">i_</span> <span class="special"><</span> <span class="identifier">r</span><span class="special">.</span><span class="identifier">i_</span><span class="special">;</span> <span class="special">}</span>
<span class="special">};</span>
<span class="keyword">struct</span> <span class="identifier">tree_node</span>
<span class="special">{</span>
<span class="identifier">string</span> <span class="identifier">name</span><span class="special">;</span>
<span class="identifier">string</span> <span class="identifier">value</span><span class="special">;</span>
<span class="comment">//children nodes of this node</span>
<span class="identifier">list</span><span class="special"><</span><span class="identifier">tree_node</span><span class="special">></span> <span class="identifier">children_</span><span class="special">;</span>
<span class="special">};</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">//a container holding a recursive data type</span>
<span class="identifier">stable_vector</span><span class="special"><</span><span class="identifier">data</span><span class="special">></span> <span class="identifier">sv</span><span class="special">;</span>
<span class="identifier">sv</span><span class="special">.</span><span class="identifier">resize</span><span class="special">(</span><span class="number">100</span><span class="special">);</span>
<span class="comment">//Let's build a tree based in </span>
<span class="comment">//a recursive data type</span>
<span class="identifier">tree_node</span> <span class="identifier">root</span><span class="special">;</span>
<span class="identifier">root</span><span class="special">.</span><span class="identifier">name</span> <span class="special">=</span> <span class="string">"root"</span><span class="special">;</span>
<span class="identifier">root</span><span class="special">.</span><span class="identifier">value</span> <span class="special">=</span> <span class="string">"root_value"</span><span class="special">;</span>
<span class="identifier">root</span><span class="special">.</span><span class="identifier">children_</span><span class="special">.</span><span class="identifier">resize</span><span class="special">(</span><span class="number">7</span><span class="special">);</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="container.containers_of_incomplete_types.type_erasure"></a><a class="link" href="containers_of_incomplete_types.html#container.containers_of_incomplete_types.type_erasure" title="Type Erasure">Type
Erasure</a>
</h3></div></div></div>
<p>
Containers of incomplete types are useful to break header file dependencies
and improve compilation types. With Boost.Container, you can write a header
file defining a class with containers of incomplete types as data members,
if you carefully put all the implementation details that require knowing
the size of the <code class="computeroutput"><span class="identifier">value_type</span></code>
in your implementation file:
</p>
<p>
In this header file we define a class (<code class="computeroutput"><span class="identifier">MyClassHolder</span><span class="special">)</span></code> that holds a <code class="computeroutput"><span class="identifier">vector</span></code>
of an incomplete type (<code class="computeroutput"><span class="identifier">MyClass</span></code>)
that it's only forward declared.
</p>
<p>
</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">container</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="comment">//MyClassHolder.h</span>
<span class="comment">//We don't need to include "MyClass.h"</span>
<span class="comment">//to store vector<MyClass> </span>
<span class="keyword">class</span> <span class="identifier">MyClass</span><span class="special">;</span>
<span class="keyword">class</span> <span class="identifier">MyClassHolder</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="keyword">void</span> <span class="identifier">AddNewObject</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">MyClass</span> <span class="special">&</span><span class="identifier">o</span><span class="special">);</span>
<span class="keyword">const</span> <span class="identifier">MyClass</span> <span class="special">&</span> <span class="identifier">GetLastObject</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">container</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">MyClass</span><span class="special">></span> <span class="identifier">vector_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
Then we can define <code class="computeroutput"><span class="identifier">MyClass</span></code>
in its own header file.
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//MyClass.h</span>
<span class="keyword">class</span> <span class="identifier">MyClass</span>
<span class="special">{</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="keyword">int</span> <span class="identifier">value_</span><span class="special">;</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">MyClass</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">val</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">value_</span><span class="special">(</span><span class="identifier">val</span><span class="special">){}</span>
<span class="keyword">friend</span> <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">==(</span><span class="keyword">const</span> <span class="identifier">MyClass</span> <span class="special">&</span><span class="identifier">l</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">MyClass</span> <span class="special">&</span><span class="identifier">r</span><span class="special">)</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">l</span><span class="special">.</span><span class="identifier">value_</span> <span class="special">==</span> <span class="identifier">r</span><span class="special">.</span><span class="identifier">value_</span><span class="special">;</span> <span class="special">}</span>
<span class="comment">//...</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
And include it only in the implementation file of <code class="computeroutput"><span class="identifier">MyClassHolder</span></code>
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//MyClassHolder.cpp</span>
<span class="preprocessor">#include</span> <span class="string">"MyClassHolder.h"</span>
<span class="comment">//In the implementation MyClass must be a complete</span>
<span class="comment">//type so we include the appropriate header</span>
<span class="preprocessor">#include</span> <span class="string">"MyClass.h"</span>
<span class="keyword">void</span> <span class="identifier">MyClassHolder</span><span class="special">::</span><span class="identifier">AddNewObject</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">MyClass</span> <span class="special">&</span><span class="identifier">o</span><span class="special">)</span>
<span class="special">{</span> <span class="identifier">vector_</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">o</span><span class="special">);</span> <span class="special">}</span>
<span class="keyword">const</span> <span class="identifier">MyClass</span> <span class="special">&</span> <span class="identifier">MyClassHolder</span><span class="special">::</span><span class="identifier">GetLastObject</span><span class="special">()</span> <span class="keyword">const</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">vector_</span><span class="special">.</span><span class="identifier">back</span><span class="special">();</span> <span class="special">}</span>
</pre>
<p>
</p>
<p>
Finally, we can just compile, link, and run!
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//Main.cpp</span>
<span class="preprocessor">#include</span> <span class="string">"MyClassHolder.h"</span>
<span class="preprocessor">#include</span> <span class="string">"MyClass.h"</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">MyClass</span> <span class="identifier">mc</span><span class="special">(</span><span class="number">7</span><span class="special">);</span>
<span class="identifier">MyClassHolder</span> <span class="identifier">myclassholder</span><span class="special">;</span>
<span class="identifier">myclassholder</span><span class="special">.</span><span class="identifier">AddNewObject</span><span class="special">(</span><span class="identifier">mc</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">myclassholder</span><span class="special">.</span><span class="identifier">GetLastObject</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">mc</span> <span class="special">?</span> <span class="number">0</span> <span class="special">:</span> <span class="number">1</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2009-2011 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>)
</p>
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