what was wrong with struct & class in C++?

[guslay]

Bill Baxter Wrote:

> Another problem is that it's far too easy for derived classes to forget 
> that they need to implement their own dup(), so you end up with what's 
> effectively another variation of the slicing problem.  You get an object 
> that's a Derived but for some reason all the Derived-specific members 
> are bogus.  I suppose it's not quite as bad as the real slicing problem 
> though, because once detected it can always be fixed at the source, 
> whereas with slicing, the line of code needing fixing could be anywhere.
> 
> And that problem exists with the split dup/copy solution too.
> 
> --bb


It would be nice to have an attribute to say "this method must be implemented or re-implemented in the most derived class". It's basically the opposite of final, it's a must-override. This would be very useful when implementing cloning, serialization.

For instance, if you derive class B from a concrete class A, but fail to provide a reimplementation of dup(), class B will be abstract.

There are other ways to do this (never derive from concrete class, always carry the parent's interface when inheriting, documentation), but they don't offer strong guarantees.