friend Functions and friend Classes

• friend function
  • Defined outside class's scope
  • Right to access non-public members
• Declaring friends
  • Function; Precede function prototype with keyword friend
  • All member functions of class ClassTwo as friends of class ClassOne
    • Place declaration of form; friend class ClassTwo;
    • in ClassOne definition
• Properties of friendship
  • Friendship granted, not taken
    • Class B friend of class A; Class A must explicitly declare class B friend
• Not symmetric
  • Class B friend of class A
  • Class A not necessarily friend of class B
• Not transitive
  • Class A friend of class B
  • Class B friend of class C
  • Class A not necessarily friend of Class C

The program of Figs. 2.15-2.16 (top) defines friend function setX to set the private data member x of class Count. Friend declaration can appear anywhere in the class. The program of Figs. 2.16 (bottom) -2.17 demonstrates the error messages produced by the compiler when nonfriend function cannotSetX is called to modify private data member x.

Figure 2.15: Friends can access private members of the class.

Figure 2.16: Nonfriend/nonmember functions cannot access private members. (part 1 of 2)

Figure 2.17: Nonfriend/nonmember functions cannot access private members. (part 2 of 2)