A request: templated opCast() signature for class/struct

[Andrei Alexandrescu (See Website For Email)]

Walter Bright wrote:
> Pragma wrote:
>> The problem, for the uninitiated, is that D cannot overload by return 
>> type.
> 
> I think I understand the problem, and what you're trying to do. The 
> problem is that T opCast(T)() wouldn't operate like anything else in D, 
> it would be a total hack. I don't know what the right answer is here, 
> we'll just have to work on it.
> 
> There are currently no user defined implicit casts, though Andrei has 
> suggested adding them with something like opImplicitCast().

Probably the signature for opCast should be:

void opCast(out T destination);

which fortunately brings extra motivation to keep "out" in the language 
:o). "We're all a big family," inout said.

That wold allow defining multiple _explicit_ casts.

opImplicitCast should follow a similar signature, or probably the more 
flexible global:

void opImplicitCast(T from, out U to);

and follow the same exact rules are built-in conversions. In other 
words, in the graph I sent a while ago, the user would be able to put 
their own types and draw new arrows that add to the graph, 
indistinguishable from the current arrows. New arrows among built-in 
types would be possible, but otherwise any arrows having a user-defined 
type on one end should be allowed.

The problem with implicit casts is that they fight directly against 
modularity. I don't know how to make the global signature modular; the 
member signature is easier to tame.


Andrei