Simple Example of the Friend keyword in C++. Not much practicality but its different than using it for ostream.

friend.cpp

// THIS CODE WILL ERROR
#include <iostream>
#include <string>
#include <vector>

using namespace std;

class A {
private:
  int private_part;

public:
  A() : private_part{rand() % 100} {}

  friend class B;
};

class B {
private:
  int some_value;
  A a;

public:
  B() : some_value{rand() % 100} {}

  void PrintPrivateParts(A privates) {
    cout << "A's private_part:" << privates.private_part << endl;
  }
};

class Grades {
private:
  vector<int> grades;

public:
  Grades(){};
  Grades(vector<int> grades) : grades{grades} {}
  void addGrade(int g) { grades.push_back(g); }
  friend class Printer;
  int size() { return grades.size(); }
};

struct Printer {
  void printGrades(Grades g) {
    for (auto g : g.grades) {
      cout << g << " ";
    }
  }
};

// Driver program
int main() {
  A a;
  B b;

  b.PrintPrivateParts(a);

  Grades G({77, 88, 99, 78, 98, 68});

  Printer P;

  P.printGrades(G);

  return 0;
}