What type members can C++ friend functions access?

In C, friend functions can access all public, protected, and private members of a class, but access to private members is restricted and can only be done when a member function of the class is called.

#C What type members can a friend function access?

What is a friend function?

A friend function is a special function in C that can access private and protected members of a class. Friend functions must be declared outside the class definition.

Type members that friend functions can access

Friend functions can access the following type members:

• public members: Friend functions can access all public members of the class.
• protected members: Friend functions can access all protected members of the class.
• Private members: Friend functions can access private members of a class, but only when the friend function is called by a member function of the class.

Practical case

Consider the following code:

class MyClass {
private:
    int m_num;

public:
    MyClass(int num) : m_num(num) {}

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

int main() {
    MyClass obj(42);
    print_num(obj);  // 友元函数访问私有成员
    return 0;
}

Output result:

42

In this example, print_num () is a friend function of MyClass. It can access the private member m_num even though it is not a member function of MyClass.

Restricted Access

It should be noted that the access of friend functions to private members is restricted. Friend functions can only access private members when a member function of the class is called. In other words, friend functions cannot directly access private members from the outside.

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