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   Re: XML Schema equivClass.

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  • From: Stefan Haustein <haustein@kimo.cs.uni-dortmund.de>
  • To: Steve Harris <sharris@primus.com>
  • Date: Fri, 04 Feb 2000 10:23:13 +0100

> > BTW, a similar problem exists in C++:
> >
> > consider class A, class B extends A
> >
> > you cannot assign an instance of B to an A variable because it may
> > consume more memory and thus not fit...
> If B extends A, then A was probably not designed to accept assignment
> from the then-unknown type B. You can slice B to cram it into A, but
> of course you shouldn't. I'm not sure of any scenario where you would
> want to assign a concrete B to a concrete A.

Can you please explain me then why Michael Anderson expected 
exactly this behaviour in XML Schema? If OOP is watered for 
efficency or legacy reasons in C++, that's OK to me. But when
creationg something new like XML Schema, it's probably a good 
chance to do it a bit more elegant. 

> We use the Standard C++ containers with pointers all the time. You can
> take a container of A*'s, put your A*'s and B*'s (and any other types
> derived from A) into it, and call polymorphic methods on the elements
> using the std::mem_fun* functors.

Ah, that sounds very elegant. Did you ever tell that a Smalltalk
programmer? He'll probably switch to C++ immediately <g>

Best regards



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