C++ template <R(*)(T)> meaning

Question

What does the following template do in C++?

template <class R, class T>
struct my_type<R(*)(T)> { typedef T type; };

Specifically, what does the * stand for in this context?

Answer 1

In this context it makes the template specialization refer to a pointer to a function, which is declared to accept a single argument of type T, and return a value of type R. The specialization extracts the argument type and makes it available as a typedef.

Example (demo):

#include <iostream>
#include <type_traits>

int foo(float);

template <typename T>
struct my_type;

template <class R, class T>
struct my_type<R(*)(T)> { typedef T type; };

int main() {
    std::cout

        << std::is_same<
            float,
            typename my_type<decltype(&foo)>::type
        >::value

        << "\n";

    return 0;
}

This outputs 1 because float and typename my_type<decltype(&foo)>::type both refer to the same type -- decltype(&foo) is the type int (*)(float), which matches the specialization R(*)(T).