Do functions need to be declared anywhere else besides in the class and before the program?

I've been trying to convince myself that that objects of the same type have access to each others private data members. I wrote some code that I thought would help me better understand what is going on, but now I am getting an error from XCODE7 (just 1), that says that I am using the undeclared identifier "combination."

If someone could help me understand where I have gone awry with my code, I would love to learn.

My code should simply print false, if running correctly.

#include <iostream>
using std::cout;
using std::endl;

class Shared {
public:
    bool combination(Shared& a, Shared& b);
private:
    int useless{ 0 };
    int limitless{ 1 };
};

bool Shared::combination(Shared& a,Shared& b){
    return (a.useless > b.limitless);
}

int main() {

    Shared sharedObj1;
    Shared sharedObj2;

    cout << combination(sharedObj1, sharedObj2) << endl;

    return 0; 
}