Passing an inherited method to another method - c++

Question

I am trying to build a class that has a member function with a method as argument. The methods are defined in inherited classes. I build a minimal example:
#include <iostream>
struct base
{
base() {}
int number(int (*f)(int))
{
return f(1);
}
};
struct option1 : base
{
int timesTwo(int i){return 2*i;}
option1()
{
std::cout << number(timesTwo);
}
};
struct option2 : base
{
int timesThree(int i){return 3*i;}
int timesFour (int i){return 4*i;}
option2()
{
std::cout << number(timesThree);
}
};
int main()
{
option1 a; //I would expect this to print "2"
}
The current syntax in the function number is for a general function, but I cannot get it to work for a method of any inherited classes.

Answer 1

The problem here is that you're passing a pointer to a member function, which is completely different from a pointer to a non-member function (which is what your number function takes as an argument).
You could use std::function and std::bind:
int number(std::function<int(int)> f)
{
return f(1);
}
...
number(std::bind(&option1::timesTwo, this, _1));
You could also use templates, and extra arguments, like
template<typename T>
int number(T* object, int(T::*f)(int))
{
return (object->*f)(1);
}
...
number(this, &option1::timesTwo);
Or the simple (but not always correct, depending on situation and use case): Make the callback-function static:
static int timesTwo(int i){return 2*i;}
My recommendation is that you look over the solution using std::function, because then it's easy to call the number function with any type of callable object, like a lambda:
number([](int x){ return x * 2; });

Answer 2

The given error says :
error: reference to non-static member function must be called
You can just add static before your method members.
And I would suggest you to use std::function instead of pointer functions.
A working code :
#include <iostream>
#include <functional>
struct base
{
base() {}
int number(std::function<int(int)> f)
{
return f(1);
}
};
struct option1 : base
{
static int timesTwo(int i){return 2*i;}
option1()
{
std::cout << number(timesTwo);
}
};
struct option2 : base
{
static int timesThree(int i){return 3*i;}
static int timesFour (int i){return 4*i;}
option2()
{
std::cout << number(timesThree);
}
};
int main()
{
option1 a; // now it works
}