How do C++ friend functions access private members?

There are two ways for friend functions in C to access private members: declare the friend function within the class. Declare a class as a friend class so that all member functions in the class can access the private members of another class.

C Friend function’s method of accessing private members

A friend function is defined outside the class but can be accessed Functions of private members of the class. There are two ways to implement friend functions' access to private members:

1. Declare a friend function

Declare a friend function within the class, the syntax is as follows:

class ClassName {
public:
  // 类成员...

  // 声明友元函数
  friend void friend_function();
};

In this way, the declared friend function can access the private members of the class.

2. Declare a friend class

Declare a class as a friend class, and all member functions in the class can access the private members of another class. The syntax is as follows:

class ClassName1 {
public:
  // 类成员...

  // 声明友元类
  friend class ClassName2;
};

All member functions declared in ClassName2 can access the private members of ClassName1.

Practical case

Consider the following C code:

class Person {
private:
  int age;
  string name;

public:
  // 友元函数
  friend void print_person_info(const Person& person);

  // 访问私有成员的友元函数
  void print_info() const {
    cout << "Name: " << name << endl;
    cout << "Age: " << age << endl;
  }
};

// 友元函数外部分类的定义
void print_person_info(const Person& person) {
  cout << "Name: " << person.name << endl;
  cout << "Age: " << person.age << endl;
}

int main() {
  Person person;
  person.name = "John";
  person.age = 30;

  person.print_info();
  print_person_info(person);

  return 0;
}

In this example, the print_person_info function is a friend function , which has access to private members of the Person class. In the Person class, the print_info function also accesses private members, which uses a friend function declaration.

Running the above code will output:

Name: John
Age: 30
Name: John
Age: 30

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