Prepared by

1
Object-Oriented Programming(Part 2)
Programming Language Principles Lecture 31

• Prepared by
• Manuel E. Bermúdez, Ph.D.
• Associate Professor
• University of Florida

2
The this Pointer

• To invoke a class member function, use the
  ''dot'' operator (structure field selection)
• ltobject namegt . ltmember function namegt
• The this pointer an implicit parameter to each
  class member function, of type (Object ).
• Its value is the address of the current object,
  through which the member function has been
  invoked.
• It can be used inside member functions. Examples
  coming up.

3
The const Qualifier

• Three places where the const qualifier can be
  used
• in a member function declaration
• class A
• ...
• const X f(const Y y) const
• // 1. Can't assign to object returned
• // 2. Can't change parameter y
• // 3. Can't change this.

4
The const Qualifier (contd)

• A a
• ...
• a.f(y)
• // f is a member of class A.
• // a is an object of class A.
• // a.f(y) can't be assigned a value.
• // a.f(y) cannot change y.
• // a.f(y) cannot change a.

5
Constructors

• class cell
• cell(int i, cell n) infoi nextn
• int info
• cell next
• cell a(1,0)
• //declare a and call cell(1,0)
• cell b(2,a)
• //declare b and call cell(2,a)
• These dont just declare a and b of type cell.
• They also call the constructor function cell.

6
Overloading Constructors

• class cell
• cell (int i) infoi nextthis
• cell (int i, cell n)
• infoi nextn
• int info
• cell next
• cell c(7)

7
Friend Classes and Information Hiding

• Class members are private by default.
• A friend declaration in a class allows functions
  from another class to access members of the
  current one.
• class cell
• friend class circlist
• cell (int i) infoi nextthis
• cell (int i, cell n) infoi nextn
• int info
• cell next
• cell has no public interface. Its an
  'auxiliary' data type, solely for use with
  circlist.

8
Friend Classes and Information Hiding

• class circlist
• private
• cell rear
• public
• circlist() rearnew cell(0)
• circlist()
• int empty() const return rearrear-gtnext
• int top() const return rear-gtnext-gtinfo
• void push (int)
• int pop()
• void enter(int)
• The class circlist has a total of seven member
• functions, four of which still need to be defined.

9
Friend Classes and Information Hiding

• circlistcirclist()
• cell tmp1rear, tmp2rear-gtnext
• while (tmp2!rear)
• delete tmp1
• tmp1tmp2
• tmp2tmp1-gtnext
• delete tmp1
• void circlistpush(int x)
• rear-gtnext new cell (x, rear-gtnext)

10
Friend Classes and Information Hiding

• void circlistenter(int x)
• rear-gtinfo x
• rear rear-gtnext new cell(0,rear-gtnext)
• int circlistpop()
• if ( empty() ) return 0
• cell front rear-gtnext
• rear-gtnext front-gtnext
• int x front-gtinfo
• delete front return x

11
Friend Classes and Information Hiding

• Class circlist hides its internal details.
• Objects of type circlist are manipulated
  exclusively through the public interface of
  circlist.
• Information hidden inside circlist circlist
  based on cell, only rear is stored.
• Should the implementation of circlist ever
  change, users of circlist will be COMPLETELY
  UNAFFECTED.
• Three principles of Object-Oriented Programming
• Abstraction, Encapsulation, Information Hiding.

12
Derived Classes

• Humans tend to abstract on two dimensions
• A part-of B, (a.k.a. has_a)
• A kind-of B, (a.k.a. is_a).
• ADT programming focuses on the has_a relation.
• OOP also includes the is_a abstraction.
• Support for the is_a abstraction implies
• if A is_a B, and B has_a X, then A has_a X.
• This is inheritance !!

13
Derived Classes (contd)

• In general,
• class ltderived-classgt public ltbase-classgt
• ltmember-declarationsgt
• All members of ltbase-classgt are also members of
  ltderived-classgt.
• The ltderived-classgt may have additional members.
• public clause derived members retain their
  attributes (public, private, or protected).
• private clause derived members will be private.

14
The circlist class, revisited

• class circlist
• public
• int empty () const return rearrear-gtnext
• int top() const return rear-gtnext-gtinfo
• // 'inspectors'
• protected
• circlist () rear new cell(0)
• circlist()
• void push(int)
• int pop()
• void enter(int) // 'mutators'
• private
• cell rear

15
The circlist class, revisited

• Protected members behave as if they were private,
  except that
• they are visible to members (and friends) of
  derived classes.
• Some of the circlist functions are now protected
  so they'll be inherited by the new class queue.

16
Derived class queue

• class queue private circlist
• public
• queue()
• queue()
• void enqueue (int x) enter(x)
• int dequeue () return pop()
• circlistempty
• circlisttop

17
Complete list of members of queue

• Public Functions
• queue new constructor
• queue new destructor
• enqueue new
• dequeue new
• empty, top inherited, explicitly made public
• Private Functions
• push inherited
• pop inherited
• enter inherited
• Private Variables (accessible to new functions of
  queue)
• none
• Private Variables (accessible only by inherited
  functions)
• rear inherited

18
Another class derived from circlist

• class stack private circlist
• public
• stack ()
• stack ()
• void push (int x) circlistpush(x)
• int pop() return circlistpop()
• circlistempty
• circlisttop

19
Sample Use of these Classes

• main ()
• stack s
• queue q
• s.push(1) s.push(2) s.push(3)
• q.enqueue(7) q.enqueue(8) q.enqueue(9)
• cout ltlt "Stack Top " ltlt s.top() ltlt endl
• cout ltlt "Queue Top " ltlt q.top() ltlt endl
• cout ltlt s.pop() ltlt " "
• cout ltlt s.pop() ltlt " "
• cout ltlt s.pop() ltlt endl
• cout ltlt q.dequeue() ltlt " "
• cout ltlt q.dequeue() ltlt " "
• cout ltlt q.dequeue() ltlt endl

Results Stack Top 3 Queue Top 7 3 2 1 7 8 9
20
Class Hierarchies

• class employee / . . . /
• class manager
• public employee / . . . /
• class director
• public manager / . . . /
• class temporary / . . . /
• class secretary
• public employee / . . . /
• class tsec
• public temporary, //MULTIPLE INHERITANCE !!
• public secretary / . . . /
• class consultant
• public temporary,
• public manager / . . . /

21
Class Hierarchy is a DAG
22
Type Identification

• In our example, given a pointer of type
  employee, it can point to
• employee, secretary, tsec, manager, or director.
• Three solutions, in general, to this problem
• Only use pointers to classes that have no derived
  classes. Sure ...
• Use a variable to determine the type of the
  object. Very easy to forget to check the type
  variable.
• Use virtual functions.

23
Virtual Functions

• Allow print in both employee and manager, with
  different
• definitions. C will do the right thing,
  based on the actual
• object class.

24
Virtual Functions (contd)
25
Virtual Functions (contd)
26
Virtual Functions (contd)
27
Operator Overloading

• Allow intrinsic operators in C to be applied to
  objects of new types.
• Overloading is achieved by defining functions
  named operatorXXX, where XXX is one of
• Cant overload these
• Cant change precedence, arity or associativity.
  Cant add new operators, either.?

28
Operator Overloading (contd)

• Example the subscript operator, which returns
• a reference. First, without operator overloading

29
Operator Overloading (contd)

• Now, simply replace elem with operator

30
Overloading Operators ltlt and gtgt

• We wish to use ltlt and gtgt for user-defined
  objects, the same way they are (normally) used
  for "built-in" objects (e.g. int, char, etc.).

31
Overloading Operators ltlt and gtgt

• Input is similar. The signature is
• To use them

32
The Orthodox Canonical Form

• An idiom (pattern). The OCF is characterized
• by the presence of
• A default constructor
• XX()
• A copy constructor
• XX(const X)
• An assignment operator
• X operator(const X)
• A destructor
• XX()

33
Object-Oriented Programming (Part 2)
Programming Language Principles Lecture 31

• Prepared by
• Manuel E. Bermúdez, Ph.D.
• Associate Professor
• University of Florida