bool operator== ( orange const& l, apple const& r ) { return l.weight == r.weight; } // comparing apples with oranges?!
The expression "like comparing apples and oranges" is not usually meant to inspire confidence in the act of comparison. Just because it's legal doesn't make it good. A definition of equality should support equational reasoning. If two values are equal, then you should be able to substitute one for the other and produce an equivalent program. I seriously doubt that substituting an apple in place of an orange would produce an equivalent program; it will probably result in type errors. Objects of different types cannot be equal (with some exceptions). Even if apples and oranges were structurally identical, your operator does not define equality. What about color? Could I substitute an orange of equal weight in the following program to produce a correct program? apple x = pick_fruit(tree); assert(x.color != orange); I suspect not. Your apples and may equivalent weights, but they are not equal. Equivalence is more general than equality and is not sufficient to guarantee substitutability. I suppose it doesn't really matter if you're just trying to make your program look neat, but if you have to formally verify your system, you'd better nail down the semantics. As far as I know there are no systems of logic that have, in their basis, equality defined for objects of different types. The correct way to write the comparison is: weight(a) == weight(o) Assuming that weights are numeric, I'm reasonably certain that there's an operator that conforms to the ideal. Even if the results of these operations are typed quantities (1kg vs 1/4lb), there's a principle that allows correct and semantically meaningful comparison: both can be converted to a common type and compared. Comparing apples and oranges remains as meaningless an operation as the idiom suggests.