Access template parameters?

[Ali Çehreli via Digitalmars-d-learn]

On 02/16/2016 10:34 AM, Gavin Maye wrote:
> Say you have
>
> class Foo(type1,type2)
> {
>     ....
> }
>
> And a concrete Foo is passed as a parameter to another template, is
> there a way to get type1 and type2 from Foo so you can use them in the
> new template... For example..
>
> class Bar(FooType)
> {
>      FooType.type1 DoSomething() { ... }
> }
>
> or Even something like
>
> class Bar(FooType) : Baz!(FooType.type1)
> {
>      FooType.type1 DoSomething() { ... }
> }
>
>

std.traits.TemplateArgsOf:

   http://dlang.org/phobos/std_traits.html#TemplateArgsOf

import std.traits;

class Foo(type1,type2)
{}

class Bar(FooType)
{
     // pragma(msg, TemplateArgsOf!FooType);

     alias ReturnType = TemplateArgsOf!FooType[0];

     ReturnType DoSomething() {
         return ReturnType.init;
     }
}

void main() {
     auto b = new Bar!(Foo!(string, double));
}

However, there are other ways of achieving the same thing at least when 
returning from a function, e.g.:

     auto DoSomething() {}

(Check out typeof(return) which may be useful in that case.)

Ali