The document discusses the concept of friend functions in C++ within the context of class design, specifically focusing on the member function 'calcarea()' of the 'rectangle' class. It explains the implications of making a function a friend, the necessity of forward declarations to avoid scope errors, and the resolution of incomplete data type errors. Additionally, it highlights the cautious use of friend functions to maintain encapsulation in object-oriented programming.

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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