Is there other way to do that?

[Jack via Digitalmars-d-learn]

I need to check if an instance is of a specific type derived from 
my base class but this class has template parameter and this type 
isn't available at time I'm checking it. Something like:

class B { }
class A(T) : B { }
class X : B { }
class Z : B { }
auto c = can be any derived class from B
bool isX = cast(A!???) c !is null;
assert(isX);

an interface would work:

interface IA { }
class A(T) : B, IA { }

bool isX = cast(IA) c !is null;
assert(isX);

but I would have create one just for that. It feels a bit hacky? 
also, does an empty interface like that increase the A class 
memory size at all?