🚀 go-pugleaf

> Jean-Rene Bouvier <jr@isoit109.bbn.hp.com> writes:
>> 1. is my understanding correct (i.e. that a DerviedObject*& cannot be
>> passed in lieu of an Object*&) ?

martelli@cadlab.cadlab.it (Alex Martelli) writes:
> It would violate type-safety.  It may be easier to see with pointers to
> pointers:

> B b;
> void tricky(B ** pointer_to_pointer)
> {*pointer_to_pointer = &b;}
> 
> void ingenue()
> {D d;  D* pd = &d;  tricky(&pd);  pd->aMethodOnlyDhadAnd_NOT_B();}

> This is, in a way, related to of the old problem that a bag-of-apples
> is-NOT-a bag-of-fruit (even if an apple is-A fruit) *under mutation*
> (because if you passed it to a function treating it as bag-of-fruit, it
> could insert in it a banana, thus violating semantic constraints [class
> invariants] of bag-of-apples).  [_that_, in turn, can be more abstractly
> related to the problem that a circle is-NOT-an ellipse *under mutation*,
> because a rescale(mX,mY) would violate the semantic symmetry constraint of
> class circle...].  The presence of mutators makes is-A determinations far
> from intuitive sometimes...!

The fact that `D **' is-NOT-a `B **' is closely related to the fact that
`B **' is-NOT-a `const B **'.  Both pairs fail because the referent of the
latter item can be mutated in ways not allowed by the former item.
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