On Wednesday, 23 July 2014 at 18:53:57 UTC, H. S. Teoh via
Digitalmars-d wrote:
On Wed, Jul 23, 2014 at 11:48:42AM -0700, Andrei Alexandrescu
via Digitalmars-d wrote:
On 7/23/14, 9:45 AM, H. S. Teoh via Digitalmars-d wrote:
>Why isn't "a==b" rewritten as "a.opCmp(b)==0"?? I'm pretty
>sure TDPL
>says this is the case (unfortunately I'm at work so I can't
>check my
>copy of TDPL).
>
>https://issues.dlang.org/show_bug.cgi?id=13179
>
>:-(
It's a good decision. There are types that are comparable for
equality
but not compared for ordering. -- Andrei
That's the wrong way round. I fully agree that we should not
autogenerate opCmp if the user defines opEquals, since not all
types
comparable with equality are orderable. However, surely all
orderable
types are equality-comparable! Therefore, if opCmp is defined
but
opEquals isn't, then we should autogenerate opEquals to be the
same as
a.opCmp(b)==0.
That would incur a silent performance hit. We should either force
the programmer to define opEquals (even if they just make it
return a.opCmp(b) == 0) or we should keep the normal, generated
one.
The best option though would be to provide some way for the
programmer to tell the compiler that they want to use the default
one so that they still have to declare opEquals when they define
opCmp (to make sure that the programmer didn't forget it), but
they're still able to use the built-in one rather than writing it
themselves. IIRC, C++11 has a way of doing that. Maybe we should
add something similar.
- Jonathan M Davis
