友元类用发 ，看到网上的友元类错误百出（把调用的类当做私有函数用）不好

#include <iostream>


using namespace std;
class first{
private :
      int a;
      int b;
public:
  first():a(0),b(0){}
  first(int x,int y):a(x),b(y){}
  int get_a(){return a;}
  int get_b(){return b;}
  friend class second;
};
class second{


  public:
     void set_a(first &b){b.a=12;}
};
int main()
{
    first a(19,19);
    cout<<a.get_a()<<endl;
    second b;
    b.set_a(a);
    cout<<a.get_a()<<endl;


    cout << "Hello world!" << endl;
    return 0;
}