Chapter 15: Operator Overloading

1
C Programming From Problem Analysis to Program
Design, Fifth Edition

• Chapter 15 Operator Overloading

2
Objectives

• In this chapter, you will
• Learn about overloading
• Become aware of the restrictions on operator
  overloading
• Examine the pointer this
• Learn about friend functions

3
Objectives (cont'd.)

• Explore the members and nonmembers of a class
• Discover how to overload various operators
• Learn about templates
• Explore how to construct function templates and
  class templates

4
Why Operator Overloading Is Needed

• Consider the following statements
• Which of the following would you prefer?

5
Why Operator Overloading is Needed (cont'd.)

• Operator overloading extend definition of an
  operator to work with a user-defined data type
• The only built-in operations on classes are
  assignment and member selection
• Other operators cannot be applied directly to
  class objects
• C allows you to extend the definitions of most
  of the operators to work with classes

6
Operator Overloading

• Can overload most C operators
• Cannot create new operators
• Most existing operators can be overloaded to
  manipulate class objects
• Write an operator function to overload an
  operator
• Use reserved word operator
• Example write a function called operatorgt

7
Syntax for Operator Functions

• The syntax of an operator function heading
• The operator function is value-returning
• operator is a reserved word
• To overload an operator for a class
• Include operator function in the class definition
• Write the definition of the operator function

8
Overloading an Operator Some Restrictions

• Cannot change precedence or associativity
• Default arguments cannot be used
• Cannot change number of arguments
• Cannot create new operators
• Cannot overload . . ? sizeof
• How operator works with built-in types remains
  the same
• Can overload for user-defined objects or for a
  combination of user-defined and built-in objects

9
Pointer this

• Every object of a class maintains a (hidden)
  pointer to itself called this
• When an object invokes a member function
• this is referenced by the member function

10
Friend Functions of Classes

• Friend function (of a class) nonmember function
  of the class that has access to all the members
  of the class
• To make a function friend to a class
• Reserved word friend precedes the function
  prototype in the class definition

11
Definition of a friend Function

• "friend" doesnt appear in function definition
• When writing the friend function definition
• The name of the class and the scope resolution
  operator are not used

12
Operator Functions as Member Functions and
Nonmember Functions

• To overload (), , -gt, or for a class,
  function must be a member of the class
• If op is overloaded for opOverClass
• If the leftmost operand of op is an object of a
  different type, the overloading function must be
  a nonmember (friend) of the class
• If the overloading function for op is a member of
  opOverClass, then when applying op on objects of
  type opOverClass, the leftmost operand must be of
  type opOverClass

13
Operator Functions as Member Functions and
Nonmember Functions (contd.)
14
Operator Functions as Member Functions and
Nonmember Functions (contd.)
15
Overloading Binary Operators

• If represents a binary operator (e.g., or )
  that is to be overloaded for rectangleType
• Operator can be overloaded as either a member
  function of the class or as a friend function

16
Overloading the Binary Operators as Member
Functions
17
Overloading the Binary Operators (Arithmetic or
Relational) as Nonmember Functions
18
Overloading the Stream Insertion (ltlt) and
Extraction (gtgt) Operators

• Consider the expression
• cout ltlt myRectangle
• The leftmost operand of ltlt is an ostream object,
  not an object of type rectangleType
• Thus, the operator function that overloads ltlt for
  rectangleType must be a nonmember function of the
  class
• The same applies to the function that overloads gtgt

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Overloading the Stream Insertion Operator (ltlt)
20
Overloading the Stream Extraction Operator (gtgt)
21
Overloading the Assignment Operator ()
22
Overloading Unary Operators

• To overload a unary operator for a class
• If the operator function is a member of the
  class, it has no parameters
• If the operator function is a nonmember (i.e., it
  is a friend function), it has one parameter

23
Overloading the Increment () and Decrement (--)
Operators

• General syntax to overload the pre-increment
  operator as a member function

24
Overloading the Increment () and Decrement (--)
Operators (cont'd.)

• General syntax to overload the pre-increment
  operator as a nonmember function

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Overloading the Increment () and Decrement (--)
Operators (cont'd.)

• General syntax to overload the post-increment
  operator as a member function

26
Overloading the Increment () and Decrement (--)
Operators (cont'd.)

• Syntax to overload the post-increment operator
  as a nonmember function

27
Operator Overloading Member versus Nonmember

• Certain operators must be overloaded as member
  functions and some must be overloaded as
  nonmember (friend) functions
• The binary arithmetic operator can be
  overloaded either way
• Overload as a member function
• Operator has direct access to data members of
  one of the objects
• Need to pass only one object as a parameter

28
Operator Overloading Member versus Nonmember
(cont'd.)

• Overload as a nonmember function
• Must pass both objects as parameters
• Could require additional memory and time to make
  a local copy of the data
• For efficiency purposes, overload operators as
  member functions

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Classes and Pointer Member Variables (Revisited)

• Classes with pointer member variables must
• Explicitly overload the assignment operator
• Include the copy constructor
• Include the destructor

30
Operator Overloading One Final Word

• Suppose that an operator op is overloaded for a
  classsay, rectangleType
• Whenever we use op on objects of type
  rectangleType, the body of the function that
  overloads the operator op for the class
  rectangleType executes
• Therefore, whatever code you put in the body of
  the function executes

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Programming Example Complex Numbers

• Complex number number of the form a ib, in
  which i2 -1 and a and b are real numbers
• Addition and multiplication of complex numbers
  are defined by the following rules
• (a ib) (c id) (a c) i(b d )
• (a ib) (c id) (ac - bd) i(ad bc)
• C has no built-in data type that allows us to
  manipulate complex numbers
• Construct a data type, complexType, that can be
  used to process complex numbers

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Programming Example Complex Numbers (contd.)

• Overload
• Stream insertion
• Stream extraction

33
Programming Example Complex Numbers (contd.)
34
Programming Example Complex Numbers (contd.)
35
Programming Example Complex Numbers (contd.)
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Programming Example Complex Numbers (contd.)

• Output a complex number in the form (a, b)
• Output the left parenthesis, (
• Output the real part
• Output the comma and a space
• Output the imaginary part
• Output the right parenthesis, )

37
Programming Example Complex Numbers (contd.)
38
Programming Example Complex Numbers (contd.)

• The input is of the form (3, 5)
• Read this complex number
• Read and discard the left parenthesis
• Read and store the real part
• Read and discard the comma
• Read and store the imaginary part
• Read and discard the right parenthesis

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Programming Example Complex Numbers (contd.)
40
Overloading the Array Index (Subscript) Operator
()

• Syntax to declare operator as a member of a
  class for nonconstant arrays
• Syntax to declare operator as a member of a
  class for constant arrays

41
Function Overloading

• Overloading a function several functions with
  the same name, but different parameters
• Parameter types determine which function will
  execute
• Must provide the definition of each function

42
Templates

• Templates a single code body for a set of
  related functions (called function template) and
  related classes (called class template)
• Syntax
• where Type is the type of the data and
  declaration is either a function declaration or a
  class declaration

43
Templates (cont'd.)

• The word class in the heading refers to any
  user-defined type or built-in type
• Type is called a formal parameter to the template
• Just as variables are parameters to functions
• Data types are parameters to templates

44
Function Templates

• The syntax of the function template is
• where Type is called a formal parameter of the
  template
• Type
• Specifies type of parameters to the function
• Specifies return type of the function
• Declares variables within the function

45
Class Templates

• Class templates a single code segment represents
  a set of related classes
• Called parameterized types
• Syntax
• A template instantiation can be created with
  either a built-in or user-defined type
• The function members of a class template are
  considered function templates

46
Header File and Implementation File of a Class
Template

• Passing a parameter to a function takes effect at
  run time
• Passing a parameter to a class template takes
  effect at compile time
• Cannot compile the implementation file
  independently of the client code
• Can put class definition and definitions of the
  function templates directly in the client code
• Can put class definition and the definitions of
  the function templates in the same header file

47
Header File and Implementation File of a Class
Template (cont'd.)

• Another alternative put class definition and
  function definitions in separate files
• However, include directive to implementation file
  at the end of header file
• In either case, function definitions and client
  code are compiled together
• We will put the class definition and the function
  definitions in the same header file

48
Summary

• An operator that has different meanings with
  different data types is said to be overloaded
• Any function that overloads an operator is called
  an operator function
• operator is a reserved word
• Operator functions are value-returning
• Operator overloading provides the same concise
  notation for user-defined data types as for
  built-in data types

49
Summary (cont'd.)

• Only existing operators can be overloaded
• The pointer this refers to the object
• A friend function is a nonmember of a class
• If an operator function is a member of a class
• The leftmost operand of the operator must be a
  class object (or a reference to a class object)
  of that operators class

50
Summary (cont'd.)

• Every instance of an overloaded function has
  different sets of parameters
• Templates
• Function template a single code segment for a
  set of related functions
• Class template a single code segment for a set
  of related classes
• Called parameterized types