1. COMS 261 Computer Science I
Title: Classes
Date: October 28, 2005
Lecture Number: 24

2. Announcements
Project 2
Due 11/2/05
Exam 2
11/02/05

3. Review
I/O manipulators

4. Outline
Classes

5. Class Types Class construct
Allows programmers to define new data types for representing information
Class type objects can have both attribute components and behavior components
Data members
Member functions
Provides the object-oriented programming in C++

6. Terminology Client Object behaviors Object attributes
Program using a class
Object behaviors
Realized in C++ via member functions (methods)
Object attributes
Are known as data members in C++

7. Member Functions
Provide an interface to data members, object access, and manipulation
Create objects of the class
Inspect, mutate, and manipulate object of the class
Can be used to keep data members in a correct state
setSize()
setColor()
draw()

8. Member Functions Constructors Inspectors or Accessors
Member functions that initialize an object during its definition
Inspectors or Accessors
Member functions that act as a messenger that returns the value of an attribute
Mutators or Accessors
Changes the value of an attribute

9. Member Functions Facilitators
Causes an object to perform some action or service

10. Vector Class Definition of a class to represent a 2D vector, (x, y)
Definition, not an implementation!!
Class definitions are stored in header files
vec.h
Contain
Constructor
Accessors
Facilitators

11. Vec Class Definition Defines the characteristics of an object
Defines the data members
Defines the member functions
public interface
Sort of a mold
Does not create an object
Instantiation

12. Point Class Definition class VEC {
Allows public access to what follows
public:
Allows access by members of the vec class to what follows
private: };

13. Public access public Declares public members of the class Typically
Member function declarations
Interface of the class
Allows access to the object’s public members (interface) outside the class

14. Private access private Declares private members of the class Typically
Data members
Some members function
Allows access to an object’s private members inside the class
Inside member functions

15. Access Specifiers Public access Private access
All clients and class members have access to the public members
Private access
Only class members have access to the private members

16. vec Class Definition class VEC { public: VEC();
// default constructor (no arguments)
private: };

17. Default constructor No parameter constructor VEC myVec;
Function overloading allows us to define more than one constructor
Only one is the default constructor
Other constructors must have a different number and/or type parameters
Used when creating an object of the class when no parameters are specified
VEC myVec;

18. VEC Class Definition: goes in vec.h class VEC { public: VEC();
// default constructor (no arguments)
private:
float x;
// x and y components of the vector
float y; };

19. Default constructor
Implementation of member functions are put in the vec.cpp file
We say
vec.h: definition file
vec.cpp: implementation file
Standard include statements
Also need to include “vec.h”
The definition of the class

20. Default constructor Implementation in the VEC.cpp file
Member functions prefaced with the scope resolution operator
VEC::VEC() {
x = 0.0f;
y = 0.0f; }

21. Application
Now we have enough to write an application program that creates an object of the VEC class
Instantiate a VEC object
VEC v1; // defines an object of type VEC
Of course the class is rather limited
We can add some functionality
Print member function

22. Implementation
Member functions prefaced with the scope resolution operator
void VEC::print () {
cout << “X: “ << x << “ “;
cout << “Y: “ << y << endl; }

23. Application
Objects of the VEC class can now call the member function print
VEC v1;
v1.print();
Run CodeWarrior

24. Accessors
Applications read and write the an objects data (private members) through accessor functions
Read accessors
getBlah();
Write accessors
setBlah(val);

25. VEC Class Definition: goes in vec.h class VEC { public: VEC();};
public:
VEC();
// default constructor (no arguments)
void print();
// output values in object
float getX();
void setX(float val);
private:
float x;
// x and y components of the vector
float y;

26. Accessor Implementation
float VEC::getX (){
return x; }
void VEC::setX (float val) {
x = val;
Run CodeWarrior

27. VEC Class
It may be useful to add a constructor that takes two parameters
An initial value for x and y
Allows us to construct VEC objects as:
VEC v1(1.2, 3.4);
Called and initial value constructor

28. VEC(float xval, float yval);
VEC Class
Definition: goes in vec.h
class VEC { };
public:
VEC();
VEC(float xval, float yval);
void print();
float getX();
float getY();
void setX(float val);
void setY(float val);
private:
float x;
// x and y components of the vector
float y;

29. Implementation Definition: goes in vec.cpp
VEC(float xval, float yval) {
x = xval;
y = yval; }
Run CodeWarrior

30. VEC Class
It may be useful to add a constructor that takes one parameter
An initial value for x
Allows us to construct VEC objects as:
VEC v1(1.2);
Also called and initial value constructor

31. VEC(float xval, float yval);
VEC Class
Definition: goes in vec.h
class VEC { };
public:
VEC();
VEC(float xval, float yval);
VEC(float xval);
void print();
float getX();
float getY();
void setX(float val);
void setY(float val);
private:
float x;
// x and y components of the vector
float y;

32. Implementation Definition: goes in vec.cpp VEC(float xval) { x = xval;
y = 0.0; }
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33. VEC Class
It may be more useful to add a constructor that takes one parameter and optionally takes another parameter
Default parameter
An initial value for x
Optional parameter for y
Allows us to construct VEC objects as:
VEC v1(1.2);
Also called and initial value constructor

34. VEC Class Definition: goes in vec.h class VEC { public:};
public:
VEC();
VEC(float xval, float yval);
VEC(float xval, float yval = 0.0f);
void print();
float getX();
float getY();
void setX(float val);
void setY(float val);
private:
float x;
// x and y components of the vector
float y;

35. Implementation Definition: goes in vec.cpp
VEC(float xval, float yval) {
x = xval;
y = yval; }
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