C++ friend functions

In the declaration, it can be placed in any part of private, public, and protect in the class.
You can declare a top-level function as a friend, you can also declare a function as a friend of another class, and you can declare a class as a friend of another class. The following code case analysis (friend of a class)!

#include <iostream>
using namespace std;

class A{
private:
    int a;
    friend class B;
};
class B{
private:
    int b;
    public:void f(){A a;a.a=10;b=20;cout<<"a.a+b="<<a.a+b<<endl;};
};

int main()
{
    B b;
    b.f();
    return 0;
}

• The relationship of friends is one-way, not two-way. If A is declared to be B's friend, it does not mean that B is A's friend.
• Friend relationships are not transitive.
• The friend function is not a member function of the class, but it can access the private and protected members of the class. In a strict sense, this does not conform to the object-oriented principle.
• Therefore, the use of the friend function is controversial and is easily discarded.

Let's take an example of using operator overloading and friend together to implement +&- for complex numbers, see the following code.

#include <iostream>
using namespace std;
//用friend和运算符重载实现复数的加减运算。
//class Complex
class Complex
{
private:
        float Real,Image;
public:
        //构造函数
        Complex(float r=0,float i=0)

        {Real=r;Image=i;};
        //成员函数show
        void Show(int i)
        {cout<<"c"<<i<<"="<<Real<<"+"<<Image<<"i"<<endl;}
            //friend的声明
            friend Complex operator+(Complex &,Complex &);
            friend Complex operator-(Complex &,Complex &);
            friend Complex operator+(Complex &,float);

};
//函数的定义和操作符的重载
Complex operator+(Complex &c1,Complex &c2)
{
    Complex t;
    t.Real=c1.Real+c2.Real;
    t.Image=c1.Image+c2.Image;
    return t;
}

Complex operator-(Complex &c1,Complex &c2)
{
    Complex t;
    t.Real=c1.Real-c2.Real;
    t.Image=c1.Image-c2.Image;
    return t;
}

int main(void)
{
   //创建对象 并且初始化
    Complex c1(25,50),c2(110,200),c3,c4;

    c1.Show(1);
    c2.Show(2);

    c3=c1+c2;
    c3.Show(3);

    c4=c2-c1;
    c4.Show(4);

    return 0;
}