Concept of Friend Class in OPP with C++

#include <iostream>
using namespace std;
class B;
class A
{int a;
public:
   void set_val (int x ){
a=x;  }
     friend class B;};
class B{
int b;
public:
void set_val (int y ){
b=y;}
void  show (A)
{  cout << "I am In Class B"  << endl;}};
int main()
{A obj;
    B o;
 o.show(obj);
 return 0;
}

Output :