Beginner problem: casting in opCmp override

[H. S. Teoh]

On Mon, Jul 08, 2013 at 05:09:14PM -0700, Jonathan M Davis wrote:
> On Monday, July 08, 2013 16:58:03 H. S. Teoh wrote:
> > On Mon, Jul 08, 2013 at 04:48:05PM -0700, Jonathan M Davis wrote:
> > > On Monday, July 08, 2013 16:38:16 H. S. Teoh wrote:
> > [...]
> > 
> > > > 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).
> > 
> > [...]
> > 
> > Unfortunately, this isn't true for opCmp. If you don't define opCmp,
> > the typeinfo of the struct will have a compare function that
> > basically does bitwise comparison. Proof:
> > 
> > 	struct S {
> > 		int[] data;
> > 	}
> > 	void main() {
> > 		auto s = S([1,2,3]);
> > 		auto t = S([1,2,3]);
> > 		auto u = S([1,2,4]);
> > 
> > 		assert(s == t);
> > 		assert(s != u);
> > 		assert(typeid(s).compare(&s, &t) == 0); // FAILS
> > 		assert(typeid(s).compare(&s, &u) != 0);
> > 	}
> > 
> > Should this be filed as a DMD bug?
> 
> Yes. opCmp's behavior should definitely match opEquals' behavior.
[...]

http://d.puremagic.com/issues/show_bug.cgi?id=10588


T

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