C++ friend functions

C++ allows a non-member function to access all private and protected members of a class. This is possible through the use of the keyword friend.

Example 1

Let us develop a program to implement the concept of friend function.

#include <iostream>
using namespace std;

class Price {
  private:
    int value;

  public:
    // constructor
    Price() : value(10) { } 

    // friend function
    friend int addTen(Price); 
};

// friend function definition
int addTen(Price p) {
   // accessing private members from friend function
   p.value += 10;
   return p.value;
}

int main() {
   Price P;
   cout << "Price: " << addTen(P);
   return 0;
}

Output:

Price: 20

Example 2

Let us see another program to demonstrate the working of friend function.

#include <iostream>
using namespace std;

class B; // forward declaration

class A { 
  int x; 
  public: 
    void setData(int i) { 
      x = i; 
    } 
    friend void max(A, B); // friend function 
};

class B { 
  int y; 
  public: 
    void setData(int i) { 
      y = i; 
    } 
    friend void max(A, B); // friend function 
};

void max(A a, B b) { 
  if(a.x >= b.y) 
    cout << "Maximum: " << a.x << endl; 
  else 
    cout << "Maximum: " << b.y << endl; 
}

int main() { 
  A a; 
  B b; 
  a.setData(30); 
  b.setData(20); 
  max(a, b); 
  return 0; 
}

Output:

Maximum: 20

friend class

The concept of friend class is quite similar to that friend function. A friend class can access private and protected members of other class in which it is declared as friend. However, the reverse is not true. See the following example.

Example 3

Let us develop a program to implement the concept of friend function.

#include <iostream>
using namespace std;

// forward declaration
class ClassB;

class ClassA {
  private:
    int numA;

    // friend class declaration
    friend class ClassB;

    public:
      // constructor to initialize numA to 13
      ClassA() : numA(13) { }
};

class ClassB {
  private:
    int numB;

  public:
    // constructor to initialize numB to 12
    ClassB() : numB(12) { }

   // member function to add numA of ClassA and numB of ClassB
   int add() {
      ClassA objectA;
      return objectA.numA + numB;
   }
};

int main() {
   ClassB objectB;
   cout << "Sum: " << objectB.add();
   return 0;
}

Output:

Sum: 25