Use return type and parameter type of function as template types [duplicate]

Question

Is it possible to figure out the return type and parameter type of a function and use them as template types? Consider the following example:

template <typename ret, typename in>
class Bar {
  // Some code
}

int foo(float x) {
  return 0;
}

int main() {
  Bar<int, float> b; // Can this be done automatically by inspection of foo at compile time?
}

Can I use the function signature of foo to set the template types of Bar?

Answer 1

Yessir.

template <class Function>
struct BarFor_;

template <class Ret, class In>
struct BarFor_<Ret(*)(In)> {
    using type = Bar<Ret, In>;
};

template <auto function>
using BarFor = typename BarFor_<decltype(function)>::type;

Now you can get your type via:

 BarFor<foo> b;

See it live on Coliru