如何使用变量模板类方法将函数指针作为参数与从函数模板派生的类型？

Question

Sorry the title is a mouthful. I'm working on an array class similar to the one discussed here . I want to define a "map" function that takes a user-defined function and applies it to each element of the array. For the purposes of type-checking, I'd like to define it such that the user-specified function must take the same number of arguments as are passed to the map function, so that

double f(double a, double b) { return a + b; }
Array<double,2> x, y, z; x.map(f, y, z);

will compile but

double g(double a, double b, double c) { return a + b + c; }
Array<double,2> x, y, z;. x.map(g, y, z);

won't, becuase g takes the wrong number of arguments based on what was passed to the map function.

I've tried a syntax like:

template<typename T, size_t ... Ns> class Array
{
    template<class ... Args> inline const Array<T, Ns...>
        map(T (*fn)(decltype(Args, double)...), Args...)
    {
        // doesn't compile
    }
}

I think this is close, but obviously wrong, since it doesn't compile. I'd be grateful to learn the correct syntax for an operation like this.

Answer 1

template <typename T, std::size_t ... Ns>
struct Array
{
    template <typename>
    using arg_type = T;

    template <class ... Args>
    Array<T, Ns...> map(T (*fn)(arg_type<Args>...), Args...)
    {
        return {};
    }
};

DEMO