Constructors

1
Constructors Destructors

• Controlling initialization destruction

2
Operator Overloading

• Most operators in C can be overloaded
• Two ways
• Member function of user defined class
• Non-member "stand alone" function
• Any operator overloading has to involve a
  user-defined class object
• Has to be at least one operand that is not
  built-in
• Most operators are binary (i.e. the have a left
  and a right-hand side), and the expression

3
Operator Overloading

• x op y can be defined either
• Stand-alone function call
• operatorop(x,y)
• Member-function call for x
• x.operatorop(y)
• If define overloaded operator as member function,
  it must be member function of left-hand-side
  object

4
Interface Implementation

• We want to limit access to data members
• Done with private keyword
• We want to separate interface from implementation
• Constructors automatically called when new object
  created
• Destructors automatically called when object
  destroyed

5
Creation

• Objects are created in a few ways
• double x
• Creates an object of type double
• C provides mechanism to control what happens
  when user defined classes are created in
  statement like above
• Constructor

6
Constructors

• Same name as class
• No return type (not even void!)
• Can be overloaded
• Same name, differ by arguments they take
• One with no arguments is default constructor

7
Example
int main() ToD x, y return 0
class ToD private int h, m bool
PM public ToD() cout ltlt "ToD
object created!" ltlt endl
Whats on the screen when the program runs?
8
Use?(besides cheesy messages)

• Initialization
• Sometimes objects need to have values or perform
  some operation before they can be used

9
Isolating Implementation

• User cant manipulate values
• User doesnt need to do anything to initialize
• User cant put object in invalid state

10
Destructors

• Only one per class
• Class name with in front
• Doesnt take any arguments
• Controls what happens when object destroyed
• Called automatically

11
Destructor Example
class Silly private string name
public Silly() cout ltlt"A silly object
is born!"ltlt endl Silly() cout
ltlt"Silly object "ltltnameltlt" dies!"ltlt endl

int main() Silly p if (1gt0) Silly
first first.name Tom" p new
Silly2 p0.name "John" p1.name
Sara" Silly last last.name Tom
Jr" delete p return 0
12
Use?

• When is destructor useful?
• Executed when object destroyed
• Can do anything, but interesting when deallocate
  memory
• Want to delete items created using new to free up
  memory

13
Destructor Example
/ DEFINITION OF CLASS NODE / class Node
public int data Node next
Node(int val, Node p) data val
next p
ListList() while(head ! 0) Node
p head head head-gtnext delete p

14
Letting the User Define

• What about letting the user choose initialization
  values?
• Example
• Point A, B, CA.x 2.4A.y -0.5B.x
  0.0B.y 3.3C.x (A.x B.x)/2C.y (A.y
  B.y)/2

15
Wouldnt It Be Nice?

• Point A(2.4,-0.5), B(0.0,3.3)Point C((A.x
  B.x)/2,(A.y B.y)/2)
• Constructors make this possible

16
To the point (class)

• Class Point public double x, y Point()
  x y 0 Point(double a) x a y
  0 Point(double a, double b) x a y
  b

17
Calling Constructors

• Called three ways
• Previously mentioned
• ex. Point p
• Using new Operator
• ex. Point p new Point(3.3,7.1)
• As function that returns object
• Point(3.3,7.1).magnitude()

Class Pointpublic double x, y
Point(double a, double b) x a y
b double magnitude() return
sqrt(xx yy)
18
Default Arguments

• In any function prototype, can follow name of
  parameter with default value

Class Point public double x, y
Point(double a 0, double b 0) x
a y b