28 - Yugen's Embarrassing Ability

28 - Yugen's Embarrassing Ability

Friendship concept

• A relation in C++ of friend
• Friendships occur between functions and classes or between classes and classes
• Friendships are single-item and cannot be passed

Friend usage

• Declare friends with the friend keyword in a class
• Friends of a class can be other classes or concrete functions
• Friends are not part of the class
• Friends are not restricted by the access level in the class
• Friends have direct access to all members of a concrete class

friend syntax

• Declare a function or class with the friend keyword in a class

class Point {
	double x;
	double y;
	friend void func(Point& p);
};
void func(Point& p) {

}

[Example code] A preliminary study on the use of friends

#include <stdio.h>
#include <math.h>

class Point {
    double x;
    double y;
public:
    Point(double x, double y) {
        this->x = x;
        this->y = y;
    }
    double getX() {
        return x;
    }
    double getY () {
        return y;
    }
    friend double func(Point& p1, Point& p2);
};
double func(Point& p1, Point& p2) {
    double ret = 0;
    ret = (p2.y - p1.y) * (p2.y - p1.y) +
          (p2.x - p1.x) * (p2.x - p1.x);
    ret = sqrt (ret);
    return ret;
}
int main() {
    Point p1(1, 2);
    Point p2(10, 20);
    printf("p1(%f, %f)\n", p1.getX(), p1.getY());
    printf("p2(%f, %f)\n", p2.getX(), p2.getY());
    printf("|(p1, p2)| = %f\n", func(p1, p2));
    return 0;
}

Friend's embarrassment

• Friend was born to take into account the efficiency of the C language
• Friends directly destroy the object-oriented encapsulation
• Youyuan's high efficiency in actual products is not worth the loss
• Friendship has been gradually abandoned in modern software engineering

Precautions:

• Friendship is not transitive
• Friends of a class can be member functions of other classes
• A friend of a class can be a complete class, so all member functions are friends

[Example code] In-depth analysis of friends

#include <stdio.h>

class ClassC {
    const char* n;
public:
    ClassC(const char* n) {
        this->n = n;
    }
    friend class ClassB;
};
class ClassB {
    const char* n;
public:
    ClassB(const char* n) {
        this->n = n;
    }
    void getClassCName(ClassC& c) {
        printf("c.n = %s\n", c.n);
    }
    friend class ClassA;
};
class ClassA {
    const char* n;
public:
    ClassA(const char* n) {
        this->n = n;
    }
    void getClassBName(ClassB& b) {
        printf("b.n = %s\n", b.n);
    }
    /*
    void getClassCName(ClassC& c)
    {
        printf("c.n = %s\n", c.n);
    }
    */
};
int main() {
    ClassA A("A");
    ClassB B("B");
    ClassC C("C");
    A.getClassBName(B);
    B.getClassCName(C);
    return 0;
}