Re: 7.0 wishlist?

Andreas Leitgeb wrote:
Joshua Cranmer <Pidgeot18@xxxxxxxxxxxxxxx> wrote:
Note, for example, that making a Square class extend a Rectangle (since a Square is-a Rectangle) does not actually work since it would fail to match the Rectangle's contract.

I may not understand your example in all its possible deepness, but
what type of rectangle-related contract do you have in mind, that is not met by a square? I hope it is not something like "width!=height
must be allowed": such types of contracts would thwart almost every "is-a" relation.

This is the example I've seen:

void assertContract(Rectangle r) {
int width = r.getWidth();
r.setHeight(5);
assert width == r.getWidth();
}

The idea that changing the height does not change the width is what is violated. A more general way of putting it is that it violates the contract that properties (most of them, in any case) are distinct and individually modifiable without affecting others.

--
Beware of bugs in the above code; I have only proved it correct, not tried it. -- Donald E. Knuth
.

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