Question

I’m now using this code:

    size_t argc(std::function<Foo()>)
    { return 0; }

    size_t argc(std::function<Foo(Bar)>)
    { return 1; }

    size_t argc(std::function<Foo(Bar, Bar)>)
    { return 2; }

    size_t argc(std::function<Foo(Bar, Bar, Bar)>)
    { return 3; }

    // ...

But it is kinda ugly and limited (the user can’t call argc with a function with any number of arguments.) Is there a better way to do it?

Note: the return type and the argument type are always the same. I know I can use templates to accept any type, but I don’t need it.

Answer 1

Cleaner version of @Paolo’s answer, usable with actual objects:

template<class R, class... Args>
constexpr unsigned arity(std::function<R(Args...)> const&){
  return sizeof...(Args);
}