On 7/20/21 11:49 AM, H. S. Teoh wrote:
> On Tue, Jul 20, 2021 at 11:32:26AM -0700, Ali Çehreli via
Digitalmars-d-learn wrote:
>> On 7/19/21 11:20 PM, Tejas wrote:
>>
>>> trying to create the spaceship operator of C++
>>
>> Just to make sure, D's opCmp returns an int. That new C++ operator was
>> added to provide the same semantics.
> [...]
>
> FYI, opCmp *may* return float, which may be useful sometimes for
> implementing partial orders (return float.nan when two elements are
> incomparable).
Thanks. For that matter, opCmp can return a UDT (and bool as well as
demonstarted below) but then the UDT cannot know what to do in its opCmp: :)
import std.stdio;
void main() {
S a, b;
auto result = a < b;
// The operator above is reduced to the following:
//
// a.opCmp(b) < 0;
//
// (It would be 'a.opCmp(b) >= 0' if we used the >= operator, etc.)
//
// But because the result of opCmp is an O object in this code
// (let's call it 'o'), the reduction is applied to 'o < 0' as well:
//
// o.opCmp(0);
//
// where, O.opCmp will receive a 0 'int' value but will never know
// operator context we are in. (<, >=, etc.)
}
struct S {
// Weirdly returns an O.
O opCmp(const S that) {
return O();
}
}
struct O {
bool opCmp(int result) {
// result will always be '0' in the case of reduction because it
// will be the 0 used in code reduction by dmd.
// We might be able to do something here but we don't know the
// operator context. Are we inside <, >=, etc?
// Extra credit: This is similar to being able to return float.nan
// but I can't decipher the semantics at this time. :)
return true;
}
}
Ali
