This presentation is for Friend function in C++. Topics included: Friend Function Friend Class.

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Friend Function
CE 142: Object Oriented
Programming with C++
Kamlesh Makvana

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Example: Rectangle
• calcArea(): Public
Member function of
class Rectangle
• this: to access data
member
• What if calcArea() is not
a member function of
class Rectangle?

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What if calcArea() is not a member
function of class Rectangle?
 Two Possibilities
 calcArea(): Member function of some other
class
 calcArea(): A Normal User Defined function

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calcArea(): A Normal User Defined
function
• No. of Errors:4
• In calcArea():3
• area, width and height
was not declared in the
scope
• In main():1
• class Rectangle has no

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Solution?
• Call calcArea() as
normal function
• Access data member of
r1 in calcArea()
• No. of Errors: 1
• In calcArea()

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Solution?
• Solution
• Pass Object as
Function parameters
• Error: 6
• area, width, height is
private!!!!!

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Solution
 Make calcArea() as friend of class
Rectangle
 What it means?
 If any function is friend of class then that
function can access Private/Protected
members of that class.
 How to make any function as friend of
class?
 Syntax:
 Write prototype of function in class
with prefixed friend!!!!

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Solution?
• Solution
• Make calcArea() as
friend of class
Rectangle
• Output?
• Garbage value!!!!!
• r1 passed by value

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Solution?
• Solution
• Passs r1 as reference.
• Output?
• 7 Error!!!!
• Prototype Does not
matched in friend
function declarations

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Solution?
• Solution
• Function prototype
and Function
definition must
matched
• Output?
• 200

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What if calcArea() is not a member
function of class Rectangle?
 Two Possibilities
 calcArea(): Member function of some
other class
 calcArea(): A Normal User Defined
function

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calcArea(): Member function
of some other class
• Error: Rectangle has not
declared in the scope.
• Solution?
• Forward Declaration of
Rectangle class.

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Forward Declaration of class.
 What is forward declaration?
 To instruct the compiler that declared
identifier is class that has been defined
within a program.
 How to forward declare a class
 Syntax:
 class class_name;
 Use this forward declaration before the use
of that class in program.

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calcArea(): Member function
of some other class
• Error: Incomplete Data
Type (class)
• Solution?
• Make it Complete!!!!

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Incomplete Data Type
 When incomplete data type error in case
of class occurs in program?
 Class is user-defined data type
 No memory gets allocated to class but it’s
instance (Object)
 Required memory blocks to assign to
objects depends on data member of the
class.
 So when you create object before defining
the class leads to incomplete data type
error.

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Incomplete Data Type
 How to resolve error of incomplete data
type? (How to make it complete?!!!!)
 Define class before it’s instance created.

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calcArea(): Member function
of some other class
• Error: Incomplete Data
Type (class)
• Solution?
• Make it Complete!!!!

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Friend class

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Important Point to Note:
 Friends should be used only for limited purpose.
too many functions or external classes are
declared as friends of a class with protected or
private data, it lessens the value of encapsulation
of separate classes in object-oriented
programming.
 Declaration can be in either part of function either
public or private.
 Friendship is not mutual. If a class A is friend of B,
then B doesn’t become friend of A automatically.
 Concept of friend is not in java
 Friendship is not inherited