13'5 Operator Overloading

1
13.5 Operator Overloading

• Operators such as , , and others can be
  redefined when used with objects of a class
• The name of the function for the overloaded
  operator is operator followed by the operator
  symbol, e.g.,
• operator to overload the operator, and
• operator to overload the operator
• Prototype for the overloaded operator goes in the
  declaration of the class that is overloading it
• Overloaded operator function definition goes with
  other member functions

2
13.5 Operator Overloading

• Although copy constructors solve the
  initialization problems with objects containing
  pointer members, they do not work with simple
  assignment statements.
• Person person1, person2
• Person2 person1
• In order to change the way the assignment
  operator works, it must be overloaded
• C allows you to have special member functions
  called operator functions which permit you to
  redefine an existing operators behavior when
  used with a class object.

3
Operator Overloading

• Prototype
• void operator(const SomeClass rval)
• Operator is called via object on left side

parameter for object on right side of operator
return type
function name
4
Invoking an Overloaded Operator

• Operator can be invoked as a member function
• object1.operator(object2)
• It can also be used in more conventional manner
• object1 object2
• Note that the parameter, Right, was declared as a
  reference for efficiency purposes the reference
  prevents the compiler form making a copy of the
  object being passed into the function.
• Also note that the parameter was declared
  constant so the function will not accidentally
  change the contents of the argument.

5
Program 1

• include ltiostreamgt
• include ltstringgt
• using namespace std
• class Person
• private
• char name
• int age
• public
• Person(char , int) // 2 argumument
  constructor
• Person(const Person ) // copy constructor
• // overloaded assignment operator
• void operator(const Person )
• void display()
• Person() // destructor

6
Program 1

• PersonPerson() delete name
• PersonPerson(const Person obj)
• name new charstrlen(obj.name) 1
• strcpy(name, obj.name)
• age obj.age
• void Personoperator (const Person right)
• delete name
• name new charstrlen(right.name) 1
• strcpy(name, right.name)
• age right.age
• void Persondisplay()
• cout ltlt "Name " ltlt name ltlt "\tAge " ltlt age ltlt
  endl
• PersonPerson(char n,int a)
• name new charstrlen(n) 1
• strcpy(name, n)

7
Program 1

• int main()
• Person object1("David", 21)
• Person object2 object1 // will call copy
  constructor
• Person object3("Mark", 19)
• cout ltlt "Object 1" ltlt endl
• object1.display()
• cout ltlt "Object 2" ltlt endl
• object2.display()
• cout ltlt "Object 3" ltlt endl
• object3.display()
• object3 object2 // will call overloaded
  assignment operator
• cout ltlt "Object 3 After assignment " ltlt endl
• object3.display()
• return 0

8
Program 1 Output

• Object 1
• Name David Age 21
• Object 2
• Name David Age 21
• Object 3
• Name Mark Age 19
• Object 3 After assignment
• Name David Age 21

9
The Operators Return Value

• If the operator function returns a void, as in
  the above example, then multiple assignment
  statements wont work.
• To enable a statement such as
• person3 person2 person1
• You must have the following prototype
• Person operator(const Person right)

10
The this Pointer

• this is a special built in pointer that is
  available in any member function. this contains
  the address of the object that called the member
  function.
• The this pointer is passed as a hidden argument
  to all non-static member functions.

11
Updated overloaded assignment operator

• Person Personoperator (const Person right)
• delete name
• name new charstrlen(right.name) 1
• strcpy(name, right.name)
• age right.age
• return this
  
• Another problem
• person1 person1

12
Solution Final implementation

• Person Personoperator (const Person right)
• if(this right)
• return this
• delete name
• name new charstrlen(right.name) 1
• strcpy(name, right.name)
• age right.age
• return this
  

13
Program 2 Class having private pointer variable

• include ltiostreamgt
• include ltstringgt
• using namespace std
• class Overloading
• private
• int num
• public
• Overloading()
• Overloading(int )
• Overloading(const Overloading )
• Overloading operator(const Overloading )
• void Display(int)
• Overloading operator(const Overloading )
• Overloading()

14
Program 2

• OverloadingOverloading()
• cout ltlt "Inside Default Constructor" ltlt endl
• num new int
• OverloadingOverloading(int value)
• cout ltlt "Inside One Argument Constructor" ltlt
  endl
• num new int
• num value
• OverloadingOverloading(const Overloading
  right)
• cout ltlt "Inside Copy Constructor" ltlt endl
• num new int
• num (right.num)

15
Program 2

• Overloading Overloadingoperator(const
  Overloading right)
• cout ltlt "Inside Overloaded Assignment Operator"
  ltlt endl
• if(this right)
• return this
• delete num
• num new int
• num (right.num)
• return this
• Overloading Overloadingoperator(const
  Overloading right)
• cout ltlt "Inside Overloaded addition operator" ltlt
  endl
• Overloading temp
• (temp.num) num (right.num)
• return temp

16
Program 2

• void OverloadingDisplay(int objNum)
• cout ltlt "Object " ltlt objNum ltlt " num " ltlt num
  ltlt endl
• OverloadingOverloading()
• delete num
• int main()
• Overloading object1
• Overloading object2(10)
• object1 object2
• Overloading object3 object1
• object1 object2 object3
• object1.Display(1)
• object2.Display(2)

17
Program 2 Output

• Inside Default Constructor
• Inside One Argument Constructor
• Inside Overloaded Assignment Operator
• Inside Copy Constructor
• Inside Overloaded addition operator
• Inside Default Constructor
• Inside Copy Constructor
• Inside Overloaded Assignment Operator
• Object 1 num 20
• Object 2 num 10
• Object 3 num 10

18
Program 3 Class having no private pointer
variable

• include ltiostreamgt
• using namespace std
• class Overloading
• private
• int num
• public
• Overloading()
• Overloading(int )
• Overloading(const Overloading )
• Overloading operator(const Overloading )
• void Display(int)
• Overloading operator(const Overloading )

19
Program 3

• OverloadingOverloading()
• cout ltlt "Inside Default Constructor" ltlt endl
• num 0
• OverloadingOverloading(int value)
• cout ltlt "Inside One Argument Constructor" ltlt
  endl
• num value
• OverloadingOverloading(const Overloading
  right)
• cout ltlt "Inside Copy Constructor" ltlt endl
• num right.num

20
Program 3

• Overloading Overloadingoperator(const
  Overloading right)
• cout ltlt "Inside Overloaded Assignment Operator"
  ltlt endl
• if(this right)
• return this
• num right.num
• return this
• Overloading Overloadingoperator(const
  Overloading right)
• cout ltlt "Inside Overloaded addition operator" ltlt
  endl
• Overloading temp
• temp.num num right.num
• return temp
• void OverloadingDisplay(int objNum)

21
Program 3

• int main()
• Overloading object1
• Overloading object2(10)
• object1 object2
• Overloading object3 object1
• object1 object2 object3
• object1.Display(1)
• object2.Display(2)
• object3.Display(3)
• return 0

22
Program 3 Output

• Inside Default Constructor
• Inside One Argument Constructor
• Inside Overloaded Assignment Operator
• Inside Copy Constructor
• Inside Overloaded addition operator
• Inside Default Constructor
• Inside Copy Constructor
• Inside Overloaded Assignment Operator
• Object 1 num 20
• Object 2 num 10
• Object 3 num 10

23
Notes on Overloaded Operators

• Can change meaning of an operator
• Cannot change the number of operands of the
  operator
• Only certain operators can be overloaded. Cannot
  overload the following operators
• ? . . sizeof

24
13.7 Object Composition

• Object composition a class is a member of a
  class
• Supports the modeling of has a relationship
  between classes enclosing class has a
  enclosed class
• Same notation as for structures within structures

25
13.7 Object Composition

• class Customer
• private
• string name
• string address
• Account savings
• Account checking
• The relationship between the different objects
  that make up this class can be described as
  follows
• the Customer has a name
• the Customer has an address
• the Customer has a savings account
• the Customer has a checking account