Having a template parameter depend on a parameter list

Question

I have defined the class

template <typename... Ts> struct Bar {
   using inner_type = /* whatever */;
};

Now, I need to define a templated class Foo whose template parameters are some parameter pack, and a value of type Bar::inner_type instantiated for that parameter pack. Unfortunately I can't seem to be able to do it. If I define it this way:

template <Bar<Ts...>::inner_type SomeValue, typename... Ts> struct Foo { };

the compiler doesn't recognize Ts when it's used, since it hasn't see the parameter pack yet; but if I define it this way:

template <typename... Ts, Bar<Ts...>::inner_type SomeValue> struct Foo { };

the compiler sneers at my attempt to use a parameter pack before other template parameters.

So how can I do this?

Note: In case it matters, this failed for me with GCC 4.9.3.

Answer 1

You can partially specialize your struct:

template<typename...>
struct Bar { using inner_type = int; };

template <typename T, typename T::inner_type T>
struct Foo;

template <typename... Ts, typename Bar<Ts...>::inner_type SomeValue>
struct Foo<Bar<Ts...>, SomeValue> { };

int main() {
    Foo<Bar<int>, 3> foo;
}

This way Ts parameter pack is deduced and Foo expects the second template parameter to be of type Bar<Ts...>::inner_type.