Beginner problem: casting in opCmp override

[Jonathan M Davis]

On Monday, July 08, 2013 16:48:05 Jonathan M Davis wrote:
> On Monday, July 08, 2013 16:38:16 H. S. Teoh wrote:
> > On Tue, Jul 09, 2013 at 12:35:32AM +0200, Ugbar Ikenaki wrote:
> > > Here's one more question:
> > > 
> > > Before I knew that opEquals existed, I tried overloading the
> > > equality expressions (== and !=) in opBinary using the code below.
> > > It worked. Why would the overloaded opBinary version get called if
> > > the equality expressions are held in opEquals?
> > 
> > [...]
> > 
> > I doubt opBinary was actually called. Probably what happened was that ==
> > defaulted to the built-in implementation of opEquals, which simply
> > performs a bitwise comparison of the struct/class, and it just so
> > happened that it was close to (or the same) as what you intended.
> > 
> > Basically, when you write x==y, the compiler looks for opEquals and
> > opCmp. If opEquals is found, then it's rewritten as x.opEquals(y);
> > otherwise, if opCmp is found, it's rewritten as x.opCmp(y)==0. If
> > neither are found, then the compiler generates a default implementation
> > of opEquals, which basically does a bitwise comparison of x and y.
> 
> Actually, what it's supposed to do isn't necessarily a bitwise comparison.
> It's supposed to do a recursive comparison of all of the members, where it
> calls == on each of the members. If a bitwise comparison will do that, then
> it may end up as a bitwise comparison for efficiency reasons, but it's not
> necessarily a bitwise comparison. It used to be that it was doing a bitwise
> comparison when it wasn't supposed to, but that was fixed fairly recently
> (though I don't recall if that fix has been released yet).
> 
> Basically, there's no reason to overload opEquals unless you have a member
> which you don't want to compare with == (e.g. pointers).

I should point out though (in case someone misunderstands) is that that 
applies specifically to structs. You always have to overload opEquals on 
classes if you want == to work properly (since Object's opEquals does a 
comparison of the references rather than the objects).

- Jonathan M Davis