Chapter 14: Overloading and Templates

1
C ProgrammingProgram Design IncludingData
Structures, Fifth Edition

• Chapter 14 Overloading and Templates

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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

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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

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Why Operator Overloading Is Needed

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

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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

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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

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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

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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

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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

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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

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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

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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

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Operator Functions as Member Functions and
Nonmember Functions (contd.)
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Operator Functions as Member Functions and
Nonmember Functions (contd.)
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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

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Overloading the Binary Operators as Member
Functions
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Overloading the Binary Operators (Arithmetic or
Relational) as Nonmember Functions
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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)
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Overloading the Stream Extraction Operator (gtgt)
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Overloading the Assignment Operator ()
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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

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

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

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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

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

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

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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

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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

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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

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Programming Example Complex Numbers (contd.)
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Programming Example Complex Numbers (contd.)
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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, )

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Programming Example Complex Numbers (contd.)
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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.)
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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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