Extracting template parameters

[Simen Kjaeraas]

On 2012-11-06, 16:20, Joseph Rushton Wakeling wrote:

> Suppose that I have two struct templates which take identical parameter  
> lists:
>
>      struct Foo(T1, T2, T3)
>      {
>          ...
>      }
>
>      struct Bar(T1, T2, T3)
>      {
>          ...
>      }
>
> Now suppose that I have a Foo which has been instantiated with a given  
> set of parameters.  Is there any way for me to say, "now instantiate a  
> Bar with the same parameters?"
>
> The use-case I'm thinking of is a function something like this (somewhat  
> pseudo-code-y):
>
>      auto fooToBar(FooInstance f)
>      {
>          Bar!(f.T1, f.T2, f.T3) b;
>          // set values etc.
>          return b;
>      }
>
> Of course the f.T1 notation is my fiction, but it gives the idea of what  
> is needed -- is there a means to extract and use template parameters in  
> this way? I assume something from std.traits but it's not entirely clear  
> what or how ...

In addition to Dan's answer, let me present a general solution:

template InstantiationInfo( T ) {
     static if ( is( T t == U!V, alias U, V... ) ) {
         alias U Template;
         alias V Parameters;
     } else {
         static assert(false, T.stringof ~ " is not a template type  
instantiation.");
     }
}

With this, you can extract the parameters to a template
(InstantiationInfo!Foo.Parameters) or the template used
(InstantiationInfo!Foo.Template).

-- 
Simen