named arguments (C++) - Something D could learn from

[Simen Kjærås]

On Tuesday, 18 December 2018 at 18:23:20 UTC, aliak wrote:
> Woah... that's quite neat.
>
> I feel like this is how std.typecons.Flag should have been 
> made!!
>
> I played around with it for a bit, and if there was a way to 
> get some better error messages then that could be quite nice to 
> use! - i dunno, maybe through some static assert on the "string 
> names"s being passed in or something...

Thanks, but it has some serious drawbacks - like this:

void fun(Args!byte.foo foo) {}

unittest {
     fun(args.foo = 3); // function foo.fun(Arg!("foo", byte) foo) 
is not callable using argument types (Arg!("foo", int))
}

For a limited use case like std.typecons.Flag, it could work 
pretty well, though:

struct Flag {
     alias opDispatch(string name) = FlagImpl!name;
}

template FlagImpl(string name) {
     // mixin to embetter type name.
     mixin(`struct `~name~` {
         bool value;
         alias value this;

         static typeof(this) opAssign(bool value) {
             return typeof(this)(value);
         }
     }`);
     mixin("alias FlagImpl = "~name~";");
}

void fun(Flag.exclusive exclusive) {
     assert(exclusive);
}

unittest {
     fun(Flag.exclusive = true);
}

--
   Simen