Conflict between C++ friend functions and access control

In C, friend functions and access control may conflict. To access private members, you can declare the member as protected or use a proxy function. For example, the Student class has private members name and score, and the friend functions printName and printScore can print these members respectively.

C Conflict between friend functions and access control

In C, friend functions can access private members of a class Non-member function. However, when it comes to access control, friend functions may encounter conflicts with class member access control.

Access Control

C provides three access control levels:

• public: Available from the class Access anywhere.
• protected: Can only be accessed from the class and its derived classes.
• private: Can only be accessed from within the class.

Friend functions

Friend functions are declared through the friend keyword. Friend functions have access to all members of a class, regardless of access control level. However, friend functions do not become part of the class.

Access control violation

An access control violation occurs when a friend function attempts to access a private member. For example:

class MyClass {
private:
    int x;

friend void printX(MyClass& obj) {
    std::cout << obj.x << std::endl;
}
};

In this example, the printX function is a friend of the class, but it is trying to access the private member x. This can cause compiler errors.

Resolving conflicts

To resolve access control conflicts, you can use the following methods:

1. Use protected members

Declare private members as protected members. This allows derived classes and friend functions to access the member.

class MyClass {
protected:
    int x;

friend void printX(MyClass& obj) {
    std::cout << obj.x << std::endl;
}
};

2. Use proxy function

Create another class member function as a proxy for private members. This proxy function is publicly accessible for use by friend functions.

class MyClass {
private:
    int x;

public:
    int getX() const { return x; }

friend void printX(MyClass& obj) {
    std::cout << obj.getX() << std::endl;
}
};

Practical case

In the following actual case, we have a Student class, which has name and scoreTwo private members. We want to create two friend functions printName() and printScore() to print these private members respectively.

class Student {
private:
    std::string name;
    int score;

friend void printName(Student& obj) {
    std::cout << "Name: " << obj.name << std::endl;
}

friend void printScore(Student& obj) {
    std::cout << "Score: " << obj.score << std::endl;
}
};

Using friend functions, we can easily print students' names and grades even though they are private members.

int main() {
    Student student;
    student.name = "John Doe";
    student.score = 95;

    printName(student);
    printScore(student);

    return 0;
}

Output:

Name: John Doe
Score: 95

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