Template unique IDs per type

[Roderick Gibson]

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++).