Struct containing unusual templated function in C++ code

In the following C++ code (from a Microsoft COM header file), what is the part that starts with template<class Q>...?

I am thoroughly confused for other reasons as well, as although a struct is in use, it has class-like elements; for example, the public keyword.

extern "C++" {
    struct IUnknown {
    public:
        virtual HRESULT WINAPI QueryInterface(REFIID riid,void **ppvObject) = 0;
        virtual ULONG WINAPI AddRef(void) = 0;
        virtual ULONG WINAPI Release(void) = 0;
        template<class Q> HRESULT WINAPI QueryInterface(Q **pp) { return QueryInterface(__uuidof(*pp),(void **)pp); }
    };
  }

The part that starts with template<class Q> HRESULT WINAPI QueryInterface is a template member function. In other words, it's a function template that's a member of a class (or struct, in this case).

Being a template means you can pass any interface type as its parameter, and the compiler will generate a function to query an object for an interface of that type:

IFoo *x;
IBar *y;

if (foo.QueryInterface(&x) != S_OK) {
    // use x->whatever to invoke members of IFoo
}

if (foo.QueryInterface(&y) != S_OK) {
    // use y->whatever to invoke members of IBar
}

Since it's a function template, the compiler deduces the type for Q from the type of the parameter you pass so when you pass an IFoo **, Q has the type IFoo, and when you pass an IBar **, Q has the type IBar.

In C++, the only difference between a class and struct is that member visibility in a class defaults to private, but in a struct defaults to public (so the public: tag isn't accomplishing anything in this case).