On Monday, 13 July 2015 at 09:13:06 UTC, Yuxuan Shui wrote:
On Monday, 13 July 2015 at 09:10:41 UTC, ketmar wrote:
On Mon, 13 Jul 2015 06:49:09 +0000, Yuxuan Shui wrote:
The main reason here is to use '==' in @safe code:
class A {
override @safe bool opEquals(Object o) {
return cast(A)o !is null;
}
}
@safe void main() {
A a = new A;
A b = new A;
a == b;
}
This doesn't compile in current D, because objects.opEquals
takes two Object, and Object.opEquals is not @safe.
Why can't we have a template objects.opEquals?
auto opEquals(T, S)(T a, S b) { ... }
'case templates can't be virtual functions, so the following
will not work:
bool cmp (Object a, Object b) { return a.opQeuals(b); }
MyObj a, b;
cmp(a, b);
What do you mean by virtual function? objects.opEquals is not a
member function.
Currently, Object has a function named opEquals which _is_ a
member function, and it is largely the reason, why opEquals on
classes cannot be @safe - or pure, or nothrow - or even
technically, const (though there's a hack in the free function,
opEquals which casts away const to make const Objects comparable,
which is a pretty dangerous thing to do in reality). Now, there
is a free function called opEquals that calls the opEquals on a
class object, and as I described in another post in this thread,
it's supposed to be templatized at some point, but templatizing
wouldn't allow you to have == be @safe, pure, or nothrow. For
that to work, opEquals on the class itself must have those
attributes, and to do that, opEquals needs to be removed from
Object (as I described in that other post).
- Jonathan M Davis
