Constant instances

When we make a constant instance of a class, any operations on its objects are forbidden. If we want to access one of the methods of the constant instance, we can write const after its declaration, which will make it available.

#include <iostream>
using namespace std;

class Example {
    int x = 0;
    public:
        Example(){}
        ~Example(){}
        int getX() const {
            return x;
        }
};


int main() {
    const Example x; // remember that "Example" acts as a data type here
    cout << x.getX() << endl;
    return 0;
}
                                    

Friend functions

Friend functions are functions from outside the class that can operate on its private objects. We can befriend a function by writing its declaration in the class (preceded by the friend keyword).

#include <iostream>
using namespace std;

class Example {
    int x = 0;
    public:
        Example(){}
        ~Example(){}
        int getX() {
            return x;
        }
        friend void setX(Example &, int);
};

void setX(Example &object, int x) { // the reference is there so that the function edits the object, not its local version
    object.x = x;
}

int main() {
    Example x;
    cout << x.getX() << endl;
    setX(x, 10);
    cout << x.getX() << endl;
    return 0;
}
                                    

Friend classes

Classes also can be befriended. We can accomplish this just like with functions (friend class B;). It doesn't matter if we write this line in the public, protected, or private mode. If class B is a friend of class A, it doesn't mean that class A is a friend of class B.

#include <iostream>
using namespace std;

class A {
    friend class B;
    int x = 0;
    public:
        A(){}
        ~A(){}
        int getX() {
            return x;
        }
};

class B {
    public:
        B(){}
        ~B(){}
        void setX(A &object, int x) {
            object.x = x;
        }
};

int main() {
    A objA;
    B objB;
    objB.setX(objA, 10);
    cout << objA.getX() << endl;
}