C++ Friend Functions and Friend Classes

C++ allows you to declare another class to be a “friend” of the current class to make it easier to access variables. OOP purists have criticized this feature as weakening the principles of encapsulation and information hiding. You can do everything you might want to do without using the “friend” feature. For example, Java is not friendly. There is no friend feature. However, this feature can make some code easier to write (at least in the short term; arguably not in the long term). You may also see other code that uses friends, so it is good to know what it means.

Friend Functions

• It has been emphasized that the private members cannot be accessed from outside the class.
• That is, a non-member function cannot have an access to the private data of a class.
• However, there could be a situation where we would like two classes to share a particular function.
• Ex: Consider a case where two classes, manager and scientist, would like to use a common function income_tax() to operate on the objects of both the classes.
• C++ allows the common function to be made friendly with both the classes, thereby allowing the function to have access to the private data of these classes.
• Such a function need not be a member of any of these classes.

Friend Non-member Function

• A friend function is a non-member function that has special rights to access private data members of any object of the class of whom it is a friend.
• In this section, we will study only those friend functions that are not member functions of some other class(i,e, outside function).
• To make an outside function “friendly” to a class, we have to simply declare this function as a friend of the class as shown below:

Class abc

{

….

….

public:

….

….

Friend void xyz(void); // declaration

};

• The function declaration should be preceded by the keyword friend.
• The function is defined elsewhere in the program like a normal C++ function.
• The function definition does not use either the keyword friend or the scope resolution operator ( :: ).
• The functions that are declared with the keyword friend are known as friend functions.
• A function can be declared as a friend in any number of classes.
• A friend function, although not a member function, has full access rights to the private members of the class.
• Friend functions are not called with respect to an object.

Characteristics of Friend Functions

• It is not in the scope of the class to which it has been declared as friend.
• Since it is not in the scope of the class, it cannot be called using object of that class.
• It can be invoked like a normal function without the help of any object.
• Unlike member functions, it cannot access the member names directly and has to use an object name and dot membership operator with each member name. (Ex: A.x)
• It can be declared either in the public or the private part of a class without affecting its meaning.
• Usually it has the objects as arguments.
• Ex: Friend1 program

Friend Member Functions

• Member functions of one class can be friend functions of another class.
• In such cases they are defined using the scope resolution operator.

class X

{

– – – – –

 – – – – –

int fun1();

 – – – – –

};

class Y

{

– – – – –

– – – – –

friend int X :: fun1(); //fun1() of X is friend of Y

– – – – –

};

• The function fun1() is a member of class X and a friend of class Y.

Friend Classes

• We can also declare all the member functions of one class as the friend functions of another class.
• In such cases, a class is called as a friend class.
• That is, a class can be a friend of another class.
• Member functions of a friend class can access private data members of objects of the class of which it is a friend.
• Ex: If class B is to be made a friend of class A, then the statement

friend class B;

• Should be written within the definition of class A.

Ex:

class B; //Forward declaration.

class A

{

friend class B; //rest of the class A

};

• It does not matter whether the statement declaring class B as a friend is mentioned within the private or the public section of class A.
• Now, member functions of class B can access the private data members of objects of class A.
• Forward declaration: Necessary because definition of class B is after the statement that declares the class B a friend of class A.

NOTE:

• Friendship is not transitive.
• That is, If class A is friend with class B, and class B is friend with class C. This doesn’t mean that class A is friend with class C.