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    copied!<p>While rethinking on the question I just realized of another problem you have. Polymorphic behavior can only be achieved through pointers and/or references. Trying to do so with values produces what is called <em>slicing</em>, where the original derived object is converted (copied) into a base object loosing all the extra information.</p> <p>The common example of slicing is this one:</p> <pre><code>class Base { public: virtual void polymorphic() { std::cout &lt;&lt; "Base" &lt;&lt; std::endl; } }; class Derived : public Base { public: virtual void polymorphic() { std::cout &lt;&lt; "Derived" &lt;&lt; std::endl; } }; void f( Base b ) { b.polymorphic(); } int main() { Derived d; f( d ); // prints Base } </code></pre> <p>Now, the original question deals with conversion of pointer types and I answered <a href="https://stackoverflow.com/questions/1007390/is-it-possible-to-make-pointers-to-distinct-template-types-convertible/1008183#1008183">before</a>. I believe that your code presents a variant of the <em>slicing</em> problem. We can play a little with the <em>undefined behavior</em> of forcing pointer conversions from unrelated types:</p> <pre><code>// same Base and Derived definitions as above template &lt;typename T&gt; struct Holder { T data; }; int main() { Holder&lt;Derived&gt; derived; Holder&lt;Derived&gt;* derived_ptr = &amp;derived; Holder&lt;Base&gt;* base_ptr = reinterpret_cast&lt;Holder&lt;Base&gt;* &gt;( derived_ptr ); // undefined!!! std::cout &lt;&lt; "derived_ptr=" &lt;&lt; derived_ptr &lt;&lt; ", base_ptr=" &lt;&lt; base_ptr &lt;&lt; std::endl; base_ptr-&gt;data.polymorphic(); // prints Base } </code></pre> <p>First of all, this is undefined behavior according to the standard... but bear with me. We can try to work what a common compiler could implement. We have an object of type <em>Holder</em>, and a pointer to it (<em>derived_ptr</em>), now we reinterpret the pointer as a pointer to <em>Holder</em>. At this point <em>base_ptr</em> and <em>derived_ptr</em> have the same value, they point to the same address (check the output).</p> <p>In the last step we access the data field of the <em>Holder</em> stored at <em>&amp;derived</em>, and we call the virtual function * polymorphic()* on it... but in <em>Holder</em>, the <em>data</em> field is not a reference or pointer, it is an element of type <em>Base</em>, so the compiler can decide that it will call the <em>Base</em> version of it, and it so does in g++ 4.0.</p> <p>This is all <em>undefined behavior</em>, so try it with your compiler of choice, it may end up with the same or different outcomes.</p> <p>If you change the template definition to store a pointer, then you get the <em>expected</em> polymorphic behavior. That is, expected in this case, knowing the memory footprint of the objects at hand. Not that you can <em>expect</em> anything from the unexpected <em>undefined behavior</em>.</p>
 

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