UDA magic?

[Rikki Cattermole]

On Thursday, 17 April 2014 at 09:39:01 UTC, evilrat wrote:
> Greetings,
>
> it is possible to put function as attribute, but is this 
> possible to get "subject" of UDA's? so combining both could we 
> should get something like this:
> ----------------
> // module scope or static struct member
> int[Variant] values; // should be typeinfo?
>
> void registerStructVal(alias val)()
> {
>   alias symbol = ... some pragma or trait to get struct S ... 
> // not any yet?
>   static if ( is(symbol == struct) )
>   {
>     values[val] = Variant(symbol);
>   }
> }
>
> @(registerStructVal!42())
> struct S {}
>
> @(registerClassVal!3())
> class C
> {
> int x;
> this(int val) { x = val; }
> }
>
>
> void main()
> {
> writeln(values); // 42 = S, 3 = C
> auto a = values[3](123); // ok for class there typeinfo.create, 
> but how to call specific ctor using it?
> writeln(a.x) // 123
> }
> ----------------
>
> it may be hacked some way like defining any specific UDA as 
> tag, use module ctor or template to inject code into structs 
> with that tag, or maybe other ways which would be intrusive. 
> what i'm looking for is to be able declare such 
> function/template non-intrusive way.

Theres no way right now to grab the type that a UDA is on.
The only way I know how that is 100% going to work is:

struct S(alias T) {
	alias T me;
}

@S!(x)
int x;

void main() {
	assert(x == 0);
	__traits(getAttributes, x)[0].me = 5;
	assert(x == 5);
}

There may be another way.. But its not nice. By far. And thats to
parse allMembers (not really reasonable).

Either way I say, we need a new trait for this. Perhaps given
that UDA's shouldn't be classes that we could repurpose parent
for this as it kinda make sense imho.