On Friday, 12 July 2013 at 04:42:28 UTC, Jonathan M Davis wrote:
On Friday, July 12, 2013 05:15:43 Roderick Gibson wrote:
I was recently looking up how to assign a unique ID based on
each
different implementation of a templated type (NOT per instance,
but per unique type). For example:
class MyTemplate(T) {
//the ? where the actual number would go
const int type_id = ?;
}
void main() {
auto a = new MyTemplate!(int)(); //a.type_id = 0
auto b = new MyTemplate!(string)(); //b.type = 1
auto c = new MyTemplate!(int)(); //c.type_id = 0
auto d = new MyTemplate!(double)();//d.type_id = 2
}
There's some solutions to this sort of thing at this
stackoverflow question in c++ which are possible at run-time:
http://stackoverflow.com/questions/8596490/counting-with-template-metaprogra
mming
BUT I was wondering with CTFE and/or mixins would it be
possible
to do this at compile time?
You can't have any kind of global mutable state in CTFE, so
there's no way for
different template instantiations to share data like that. You
could generate
IDs based on the types (e.g. a hash) and get different values
that way (though
it might be tricky to do that in a way that would guarantee no
collisions),
but if you want some kind of incrementing ID across them, I
think that you're
stuck with a runtime solution.
Now, it's probably possible to use static constructors to use
global mutable
state to intialize all of those IDs when the program starts up,
which would
initialize the IDs to incrementing values, so they could be
immutable if you
wanted, but that would still be at runtime.
- Jonathan M Davis
Hmm, I'll look into the static constructors, as that sounds like
a fairly ideal solution anyway. I was more just curious to see if
the problem was solvable in D at compile time (as it seems to be
a fairly common request in C++).
