Troubles with 'bind' and 'function'

Question

A while back, I ran into the question of how to pass a callable returned by bind (call it A) to another function (call it B) which expects a parameter which is a pointer to a function of the from A. I discovered that a callable A returned by bind has a very complicated type and so gave up on my approach.

Then I learned about 'function' in the functional header which sounded as if it would solve my problem. However, after a few attempts, I was again thwarted! Perhaps you can help? Here is some code that will not work:

#include <iostream>
#include <functional>  //Library for "bind" and "function"                                                                                                                                     
#include <cmath>
using namespace std;
using namespace std::placeholders;  //for _1, _2, _3 ...                                                                                                                                       

//praboloid function                                                                                                                                                                           
int parab(int x, int y)  {
  return pow(x,2) + pow(y,2);
}

//Take an integer and a pointer to a function                                                                                                                                                  
int funk(int a, int (*f)(int) )  {
  return (*f)(a);
}

int main()  {
  auto f = bind(parab, _1, 0); // Bind the second value to 0                                                                                                                                  
  function<int (int)> fp = f;

  //Any of the following lines creates and error                                                                                                                                               

  function<int (*)(int)> fp = f; //Error!                                                                                                                                                      
  funk(-5, f);  //Error!                                                                                                                                                                       
  funk(-5, &f); //Error!                                                                                                                                                                       
  funk(-5, fp); //Error!
  funk(-5 &fp); //Error!                                                                                                                                                                   

  return 0;
}

All I wish to do is first create a new callable using bind and then send that bound function to another function as a parameter. If possible, I don't want to have to deal with the messy type returned by bind.

I am open to alternate approaches to solving this problem.

Answer 1

Try auto f = [](int n) -> int { return parab(n, 0); };

That should work with funk(-5, f) (because non-capturing closures are convertible to function pointers).

bind and function have their place, but they're not always the best choice.

Answer 2

f is not int (*f)(int). It's functor of unspecified type. But it can be used as a function. So just get any tpe in funk

template<typename F>
int funk(int a, F f)  {
  return f(a);
}