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From: Topher Cooper (topher_at_[hidden])
Date: 20060601 19:02:46
That's even worse. ISA relationships are about *specialization* not
generalization. If A is a subclass of B then the set of objects
which are A's should be special cases of objects which are B's, and
by *dint of those special circumstances* have additional
capabilities. To give an example from mathematical theory rather
than practical programming: a positive integer is a (ISA) number, but
it is a special kind of number that by dint of that specialness has
the special capability of it being meaningful to ask about its prime
factorization.
The practical test is whether under virtually all circumstances you
would be comfortable "receiving" a (signed) integer when your spec
requests an unsigned_integer. I rather doubt it. It is reasonable,
however, to receive an unsigned_integer in place of a signed integer
IF unsigned_integer is defined transparently  e.g., so that negate
returns the negative of the number (which would have to be a signed
integer if the input were a nonzero unsigned_integer).
If you wish to have an unsigned_integer class which is closed for
negation and subtraction, then either you should give it no formal
relationship to signed integer or have them both inherit from an
abstract class, call it abstract_integer, whose interface would
probably turn out very similar to that of unsigned_integer. Although
the spec may be identical the meaning is different. While neither
would have negate, or negate would be speced to perhaps throw an
exception, an abstract_integer might be negative but an
unsigned_integer never would.
Topher
At 04:03 PM 6/1/2006, you wrote:
>Topher Cooper wrote:
> > <>
> >
> > The problem seems to me to be the idea that it inherits from integer
> >  an almost classic example of misuse of inheritance. Pardon me if
> > I lost the thread somewhere but that seems to be what is being proposed.
> >
> > As others have said, inheritance should represent ISA both in
> > interface and conceptually. If unsigned_integer followed this
> > relationship than there would be no argument here  the negative of
> > an unsigned_integer would be clearly and unambiguously defined  it
> > just wouldn't happen to be an unsigned_integer. But that seems to
> > eliminate the point of having a separate unsigned_integer
> > type. Inheriting unsigned_integer from integer means that I can
> > never be sure that negating an integer (which might actually be an
> > unsigned_integer) is a reasonable thing to do. VERY BAD DESIGN.
> >
>Seems to me that, if the author is really set on using inheritance here,
>integer should inherit from unsigned_integer, since integer can do
>everything that an unsigned can do, but not viceversa. Integer extends
>unsigned with the ability to represent negative values and the result of
>subtraction.
>
> Marsh
>
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