Why don't I have access to the private values of a class through a friend function?-CodePudding

I create a class, and in the class I declare a friend function so that I can later change a private value with an if..else statement, though I can't even change it without the if..else.

    #include <iostream>
    using namespace std;

    class A {
        
        private:

            float money;
            friend void _setMoney(A a, float i);

        public: 

        void setMoney(float i) {
            money = i;
        };

        float getMoney() {
            return money;
        };

        A(float i) {
            i = money;
        };

    };

    void _setMoney(A a, float i) {
        a.setMoney(i);
    };

    int main(){

        A a(0);

        cout << a.getMoney() << endl;

        a.setMoney(10);

        cout << a.getMoney() << endl;

        _setMoney(a, 20);

        cout << a.getMoney() << endl;
    }

After executing this in VS Code, I get 0, 10, 10 instead of 0, 10, 20.

CodePudding user response：

The problem is not with _setMoney() being a friend or not. If that were the issue, your code would not even compile.

The real issue is that you are passing the a object in main() by value to _setmoney(), so you are passing in a copy of the object, and are then modifying the copy rather than the original object.

Simply pass the object by reference instead:

void _setMoney(A& a, float i) {
    a.setMoney(i);
};

That being said, A::setMoney() is public, so _setMoney() does not need to be a friend of A in order to call it. Only if _setMoney() wanted to access A::money directly, eg:

void _setMoney(A& a, float i) {
    a.setMoney(i); // <-- friend not required for this
    a.money = i; // <-- friend required for this
};

CodePudding user response：

#include <iostream>
using namespace std;

class A {
private:
    float money;
//////////////////////////////////////////////////////////
    friend void _setMoney(A& a, float i);
//////////////////////////////////////////////////////////

public:
    void setMoney(float i) {
//////////////////////////////////////////////////////////
        money = i;
//////////////////////////////////////////////////////////
    }

    float getMoney() {
        return money;
    }

    A(float i) {
        money = i;
    }

};

//////////////////////////////////////////////////////////
void _setMoney(A& a, float i) {
    a.money = i; // friend privilege
}
//////////////////////////////////////////////////////////

int main(){
    A a(0);
    cout << a.getMoney() << endl;
    a.setMoney(10);
    cout << a.getMoney() << endl;
    _setMoney(a, 20);
    cout << a.getMoney() << endl;
    return 0;
}