is expression for template structs/classes instances?

[Jonathan M Davis]

On Monday 20 December 2010 20:23:49 d coder wrote:
> Greetings
> 
> I want to find if a given struct type is instantiated from a
> particular template struct type. For example:
> 
> struct S (T)  {
>   alias T Type;
>   T t;
> }
> 
> And later I want to find out if a given type is of type S(*)
> (basically any type instantiated from template struct S). In fact I do
> not know the type value T used at the time of instantiating S!(T).
> 
> I was looking at "is ( Type Identifier : TypeSpecialization ,
> TemplateParameterList )" expression at
> http://www.digitalmars.com/d/2.0/expression.html#IsExpression .
> Thought there would be some way using that, but I could not find any.
> 
> Regards
> Cherry

Well, from the compiler's perspective S!int would have no relation to S!float, 
S!bool, or any other S!T. The template is instantiated with whatever types and 
values that it's given and then it's its own beast. So, really, there is no 
relation between the various instantiations of any particular template. I'm not 
sure that it would be impossible to have something in __traits or std.traits 
which tested whether a particular type was an instantiation of a particular 
template, but I'm not at all certain that it _is_ possible. Templates are used 
to generate code, but once generated, that code is essentially the same as it 
would have been had you typed it all yourself. So, my guess would be that you 
can't do what you're trying to do. I agree that it could be useful to be able to 
do it, but unfortunately, I don't think that it's possible.

If you knew enough about the type, you might be able to do some template voodoo 
to do it in a round-about manner, but it would be specific to the type in 
question. For instance, given your definiton of S, you could use 
_traits/std.traits to check that the type that you're testing has a member 
variable t. You could then check that S!(typeof(t)) was the same as the type 
that you were testing. So, if you get particularly cunning about it, I believe 
that it can be tested for in specific cases, but I don't believe that it can be 
done in any general way.

- Jonathan M Davis