Determining the type of the template parameter method argument

Question

There are many IEnumXXXX type COM interfaces that have a pure virtual method Next, like this:

IEnumString : IUnknown {
...
virtual HRESULT Next(ULONG, LPOLESTR*, ULONG*) = 0;
...
};

IEnumGUID : IUnknown {
...
virtual HRESULT Next(ULONG, GUID*, ULONG*) = 0;
...
};

Need a template, like this:

enum_value_type<IEnumString>::type // LPOLESTR
enum_value_type<IEnumGUID>::type   // GUID

Answer 1

Something along these lines:

template <typename T> struct ExtractArgType;

template <typename C, typename T>
struct ExtractArgType<HRESULT (C::*)(ULONG, T*, ULONG*)>{
    using type = T;
};

template <typename IEnum>
struct enum_value_type {
    using type = typename ExtractArgType<decltype(&IEnum::Next)>::type;
};

Demo