# The concept of friend

What is friendship?

• Friend is a relationship in C + +
• Friendships occur between functions and classes or between classes
• The relationship of friends is single and cannot be transferred # Usage of friends

• Declare friends in a class with the friend keyword
• A friend of a class can be another class or a concrete function
• Friend is not part of class
• Friends are not restricted by the access level in the class
• Friends have direct access to all members of a specific class

# Grammar of friends

• To declare a function or class in a class with the friend keyword

``````class Point
{
double x;
double y;

friend void func(Point& p);
};

void func(Point& p)
{
}``````

## Programming experiment: the use of friend

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

class Point
{
private:
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_2(Point& p1, Point& p2);
};

// Multiple function calls, slightly inefficient
double func_1(Point& p1, Point& p2)
{
double ret = 0;

ret = (p2.getY() - p1.getY()) * (p2.getY() - p1.getY()) +
(p2.getX() - p1.getX()) * (p2.getX() - p1.getX());

ret = sqrt(ret);

return ret;
}

double func_2(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("func_1 : |(p1, p2)| = %f\n", func_1(p1, p2));
printf("func_2 : |(p1, p2)| = %f\n", func_2(p1, p2));

return 0;
}``````
``````Output:
p1(1.000000, 2.000000)
p2(10.000000, 20.000000)
func_1 : |(p1, p2)| = 20.124612
func_2 : |(p1, p2)| = 20.124612
``````

# Friend's embarrassment

• Friend is born to give consideration to the efficiency of C language
• Friend directly destroys the encapsulation of object-oriented
• Friend's high efficiency in actual products is not worth the loss
• Friend has been gradually abandoned in modern software engineering

# Matters needing attention

• Friendship is not transitive
• A friend of a class can be a member function of another class
• A friend of a class can be a complete class

• All member functions are friends ## Programming experiment: in-depth analysis of friend

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

class ClassC
{
private:
const char* n;
public:
ClassC(const char* n)
{
this->n = n;
}

friend class ClassB;
};

class ClassB
{
private:
const char* n;
public:
ClassB(const char* n)
{
this->n = n;
}

void getClassCName(ClassC& c)        // Pay attention here!
{
printf("c.n = %s\n", c.n);
}

friend class ClassA;
};

class ClassA
{
private:
const char* n;
public:
ClassA(const char* n)
{
this->n = n;
}

void getClassBName(ClassB& b)        // Pay attention here!
{
printf("b.n = %s\n", b.n);
}
};

int main()
{
ClassA A("A");
ClassB B("B");
ClassC C("C");

A.getClassBName(B);
B.getClassCName(C);

return 0;
}``````
``````Output:
b.n = B
c.n = C
``````

# Summary

• Friend is born to give consideration to the efficiency of C language
• Friend directly destroys the encapsulation of object-oriented
• Friendship is not transitive
• A friend of a class can be a member function of another class
• A friend of a class can be a complete class

The above contents refer to the series courses of Ditai Software Institute, please protect the original!

Keywords: C++ Programming C

Added by Jaspal on Fri, 06 Dec 2019 02:13:34 +0200