Why are opEquals/opCmp arguments not in or const for Objects?

[Peter Williams]

On 19/03/13 15:38, Jonathan M Davis wrote:
> On Tuesday, March 19, 2013 02:17:28 Peter Williams wrote:
>> Am I right in thinking that removal of these methods from Object
>> will mean that it will no longer be necessary for the the
>> argument to be of type Object and that the need for casting in
>> the implementation will go away?
>
> Well, IIRC, if when you override a function, and you give it a different
> parameter type, it's a new overload rather than actually overriding anything
> (even if the type of the parameter in the derived type's function is a derived
> type of the type of the parameter in the base class' function). So, if you
> don't use a common type for the parameter, you'll run into overload conflicts
> regardless. So, while you _could_ use something other than Object, you could
> run into overload conflicts if you do. That can be gotten around by aliasing
> base class functions into the scope of the derived class (though that may
> cause derived types to be compared as base types if they're referred to via
> references to the base type, so that may no be a good idea) or by creating
> overloads for comparing every base class, but it's arguably easier to just
> accept Object and cast - or accept whatever the base type is which first
> introduces opEquals into the hierarchy.
>
> So, there's a good chance that what you'll end up doing is using the base type
> in the hierarchy which introduces opEquals as the parameter for opEquals. This
> is better than using Object in that it'll avoid having completely unrelated
> types even be comparable (as their opEquals wouldn't accept each other), but
> it would still require derived types to use that type as the parameter type
> for opEquals for the reasons mentioned above.
>
> - Jonathan M Davis
>

Yes.  I didn't mean to imply that all casting would go away just the 
OBLIGATORY casting of Object.

Peter
PS I'd like to say that this is the first disappointment I've 
encountered in my mission to learn D.  Otherwise, I'm finding it a 
pleasure to use and find the compiler error messages very useful 
compared to those I'm used to with similar languages. Well done!!