Detailed explanation of the friend mechanism of C++ functions

C The friend mechanism allows non-member functions or classes to access private or protected members of other classes to achieve the following purposes: Allow non-member functions to access private members Allow member functions to access private members of other classes Allow class member functions to access another class Private members of a class

Detailed explanation of the friend mechanism of C functions

The friend mechanism is a method that allows functions or classes to access other Characteristics of private or protected members of a class or function. In C, the friend mechanism can achieve the following purposes:

• Allow non-member functions to access private members of a class
• Allow member functions of a class to access private members of another class

Syntax

• Global function friend declaration:

  friend 返回值类型 函数名(参数列表);

• Class friend declaration:

  friend class 类名;

• Class member function friend declaration:

  friend 返回值类型 类名::成员函数名(参数列表);

Practical case:

Problem: Design a Point class, which has private members x and y , and there is a print() function that prints all private members. Now, we want an additional printInfo() function that can access the private members of the Point class and print them.

Implementation:

// Point 类
class Point {
private:
    int x;
    int y;

public:
    // 友元函数，可以访问 Point 类的私有成员
    friend void printInfo(Point& point);

    // Point 类的成员函数
    void print() {
        std::cout << "x: " << x << ", y: " << y << std::endl;
    }
};

// 全局友元函数，可以访问 Point 类的私有成员
void printInfo(Point& point) {
    std::cout << "x: " << point.x << ", y: " << point.y << std::endl;
}

int main() {
    Point point{10, 20};
    point.print();  // 输出：x: 10, y: 20
    printInfo(point);  // 输出：x: 10, y: 20
    return 0;
}

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