friend class in C++

A friend class is a class that can access the private and protected members of other class in which it is declared as friend. It is sometimes useful to allow a particular class to access the private and protected members of other class.
```
#include <iostream>
using namespace std;

class A {
private:
    int a;
public:
    A():a(0) {}
    friend class B; // Friend Class
};

class B {
public:
    void showA(A& a){
        cout << "A::a=" << a.a;
    }
};

int main() {
    A a;
    B b;
    b.showA(a);
}
```