Thread: copy assignment of polymorphic classes

I have noticed that when using the default copy assignment for a virtual polymorphic class, I do not get the behaviour I expect. Although copy assignment of the derived class works fine, copy assignment of the parent class does not. See below to a sketch of an example. Why is there a difference?

Code:

struct trace
    {
    virtual trace* copytype() = 0 ;
    ...
    } ;

struct intersection: public trace	// has no copy assignment operator
    {
    Polynomial<long double> P ;		// has copy assignment operator
    ...
    } ;

    ...
    trace* T = this ;
    trace* X = dynamic_cast<intersection*>( T->copytype() ) ;
    ...
    // below works - X.P is copied to T.P
    *this = *( dynamic_cast<intersection*>(X) ) ;
    // below fails - X.P is not properly copied to T.P
    *T = *X ;