Init derived class object using base class object
<p>I have the following types:</p>

<pre><code>typedef std::vector&lt;Node&gt; Nodes;
struct Top: public Nodes {};
struct Middle: public Nodes {};
struct Bottom: public Nodes {};
</code></pre>

<p>Now lets say that my test class constructs a <code>Nodes</code> object called <code>_standardNodes</code>:</p>

<pre><code>TestDifferentTypes(): _standardNodes(standardNodes(_model)) 
{
}
</code></pre>

<p>How do I create an instance of Top which is initialised with the data stored in nodes?</p>
