Andrei Alexandrescu wrote:
Consider:
class Apple {}
class Orange {}
void main() {
writeln(new Apple == new Orange);
}
This program always prints "false". By and large, it is odd that one
would attempt comparison between unrelated classes. I was thinking, is
this ever legitimate, or we should just disallow it statically whenever
possible?
The comparison would still remain possible by casting to a parent class:
writeln(cast(Object) new Apple == cast(Object) new Orange);
I could think of rare cases in which one would want two sibling types to
compare equal. Consider:
class Matrix { ... }
// No state added, operations optimized with BLAS
class BLASMatrix : Matrix {}
// No state added, operations optimized with LAPACK
class LAPACKMatrix : Matrix {}
Since neither derived class adds any state, both act in comparisons just
like the base class Matrix, so it's valid to compare a BLASMatrix with a
LAPACKMatrix.
How do you think we should go about this? Stay error-prone for the
benefit of a few cases, or disallow sibling class comparisons statically?
Andrei
You can already explicitly do "(new Apple).opEquals(new Orange);" so why
not first resolve == to opEquals and then try to match the parameters,
walking through all known opEquals until a matching one is found.
