部分专门化具有不同模板参数的模板的模板类

Question

How can I get a class template to accept another class template that could have one of two different parameter lists? Namely, a non-type parameter or a type and non-type parameter:

template <int X>
struct Foo1{};

template <typename T, int X>
struct Foo2{};

I want to be able to pass either of these templates to my template (plus future templates that follow in their footsteps). I hope this illustrates what I'm after, although the syntax is all wrong:

template <typename T, int X, class>
struct Magic; //Don't accept non template parameters

template <typename T, int X, class <int> class C>
struct Magic<T, X, C> {}; //Template non-type

template <typename T, int X, class <class, int> class C>
struct Magic<T, X, C> {}; //Template type and non-type

I can't think of a way to write a specialization for these. If it isn't possible, I can just have Foo1 and all templates like it have a template type parameter that doesn't do anything ( template <typename, int X> Foo1{}; ) and write Magic with that in mind, but I was hoping for a more elegant solution.

Answer 1

One solution you could apply is to introduce wrappers for each distinct class template's declaration, and specialize the magical structure based on what the wrapper wraps. Eventually the only thing you need to know is which wrapper is associated with which class template.

template <int X>
struct Foo1{};

template <typename T, int X>
struct Foo2{};

template <template <int> class C> struct W1;

template <template <class, int> class C> struct W2;

template <typename T, int X, class>
struct Magic; //Don't accept non template parameters

template <typename T, int X, template <int> class C>
struct Magic<T, X, W1<C> > {}; //Template non-type

template <typename T, int X, template <class, int> class C>
struct Magic<T, X, W2<C> > {}; //Template type and non-type

int main()
{
    Magic<int, 1, W1<Foo1> > m1;
    Magic<int, 2, W2<Foo2> > m2;
}

DEMO