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In article RAA08320@pythagoras.csc.ncsu.edu, bwmott@unity.ncsu.edu writes:
>
>I've got a question about forward declaring a class as a sub-class of
>another class.  Is it possible under the C++ draft standard to do
>something like:
>
>  class S;
>  class T : public S;

No, for several reasons.

You cannot mention class internals until the definition of the class has
been seen. You could otherwise in principle add things to a class
without modifying the class definition. (Some languages allow you to
do so, but C++ does not.)

Knowing only that T is derived from S is not any more helpful than knowing
only that T is a class.  For example, casting a T* to an S* or the reverse
in general requires the full class definitions.

Your example involved covariant return types. The compiler can't generate
the appropriate pointer adjustments without seeing the class definitions.
---
Steve Clamage, stephen.clamage@eng.sun.com
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