Classes

1

• Classes
• (Chapter 10)

Classes are derived types. They are
combination of data and functions defined
together to form a type .
2
Motivation

• Types such as int, double, and char are simple
  objects.
• They can only answer one question What value do
  you contain?

3!
3
Motivation

• Classes allow you to build smart objects that
  can answer many questions (and perform various
  actions).
• For example
• What is your temperature?
• What is your temperature in Fahrenheit?
• Print your temperature in Kelvin.

4
Temperature Example

• Write a program that, given a temperature in
  Fahrenheit, Celsius, or Kelvin, will display the
  equivalent temperature in each of the scales.
• double degree 0.0 // needs 2 items!
• char scale 'F'
• To apply a function f() to a temperature, we must
  specify both degree and scale
• f(degree, scale)
• Also to display a temperature
• cout ltlt degree ltlt scale

5
Temperature Example

• Is there a way to build a user-defined data type
  that combines degree and scale into one object?
• Can this object automatically convert between
  different scales, and know how to print itself
  out? (Can we construct a smart object?)
• Answer
• Yes, by using the class construct.

6
Object-Oriented Solution

• Design and build a class to represent the
  temperature object
• Identify
• 1) data required (data members), and
• 2) operations that this object can perform
  (member functions)
• class Temperature
• public
• // member functions
• private
• double degree // data members
• char scale

7
Access Rights

• Control the level of information hiding
• Accessible to member functions within the class
  only (private)
• Accessible to any functions (public)
• protected we will explain it in next chapter

8
Access Specifiers
Figure 10-2
direct access
external
9
Temperature Conversion Program

• include ltiostreamgt
• // definition of Temperature class goes here
• void main()
• Temperature temp
• // Temperature is a class type
• cout ltlt "Enter temperature (e.g., 98.6 F) "
• temp.read() cout ltlt "--gt"
• temp.Fahrenheit()
• temp.print() cout ltlt " "
• temp.Celsius()
• temp.print() cout ltlt " "
• temp.Kelvin()
• temp.print()

10
Smart Temperature Object

• A smart object should carry within itself the
  ability to perform its operations
• Operations of Temperature object
• initialize degree and scale with default values
• read a temperature from the user and store it
• compute the corresponding Fahrenheit temperature
• compute the corresponding Celsius temperature
• compute the corresponding Kelvin temperature
• display the degrees and scale to the user

11
Temperature Class Declaration

• class Temperature
• public
• void read()
• void print()
• void Fahrenheit()
• void Celsius()
• void Kelvin()
• private
• double degree
• char scale

12
Class Instantiation

• void main()
• Temperature temp1
• Temperature temp2
• . . .
• temp1 temp2
• degree degree
• scale scale

13
Function Printing Temperature

• A programmer can write
• temp1.print()
• temp2.print()
• Smart object interpretation
• temp1 Receives print() message and displays
  values stored in degree and scale
• temp2 Receives print() message and displays
  values stored in degree and scale

14
Associating Functions with Classes

• scope resolution operator
• Syntax
• ltclass_namegt ltmember_namegt
• Allows same member_name under different
  class_name
• Example
• void Temperatureprint() (Example01.cpp)
• void Fractionprint() (p.493 of text)

15
Function Printing Temperature

• void Temperatureprint()
• cout ltlt degree ltlt " " ltlt scale
• Remark
• Member functions of a class can access the
  private data members of their class, but normal
  functions cannot.

16
Manager Functions (Section 10-3 of text)

• Special Functions
• Constructors they are called when an instance
  of a class is created.
• Copy constructors they are called when a copy
  of an existing instance of a class needs to be
  created.
• Destructors they are the opposite of
  constructors, i.e. when a class (object) dies.

17
Constructors

• Constructors are member functions of a class
• A constructor of an object is invoked (called)
  once upon the creation of the object
• Constructor name and class name are the same
• Syntax
• className(parameter list) // Declaration
• classNameclassName(parameter list) //
  Definition
• . . .
• // className construction function

18
Default Constructor

• Example
• A constructor function initializes the data
  members when a Temperature object is declared.
• TemperatureTemperature()
• degree 0.0
• scale 'C'
• void main()
• Temperature temp1
• temp1.print()

19
Temperature Class

• class Temperature
• public
• Temperature() // default constructor
• void read()
• void print()
• void Fahrenheit()
• void Celsius()
• void Kelvin()
• private
• double degree
• char scale

20
Default Constructor

• Remarks
• Constructor functions have no return type (not
  even void!)
• Example Temperature()
• The constructor function is automatically called
  whenever a Temperature class object is declared.
• Example
• Temperature temp1

21
Explicit-Value Constructor

• An explicit-value constructor initializes the
  data members when a Temperature object is
  declared with parameters
• Temperature temp3(98.6, 'F')
• Explicit Value Constructor definition
• TemperatureTemperature(double d, char s)
• degree d
• scale s
• if(scale!'C' scale!'F' scale!'K')
• cout ltlt "Bad Temperature scale " ltlt scale ltlt
  endl

22
Temperature Class

• class Temperature
• public
• Temperature() // default constructor
• // explicit value constructor
• Temperature(double idegree, char iscale)
• void read()
• void print()
• void Fahrenheit()
• void Celsius()
• void Kelvin()
• private
• double degree
• char scale

23
Weird C Syntax

• The default constructor is a function with no
  parameters so you might think that it should
  actually be called using
• Temperature temp1()
• the same way as any other function without
  parameters.
• This is not correct. A default constructor should
  be called as
• Temperature temp1
• without using the ().

24
Reading Temperature

• Using the read() member function
• Temperature temp1
• cout ltlt "Enter temperature (e.g., 98.6 F) "
• temp1.read()
• // process the temperature in temp1
• When temp1 receives the read() message, it gets
  values from cin into degree and scale.
• void Temperatureread()
• cin gtgt degree gtgt scale
• if(scale!'C' scale!'F' scale!'K')
• cout ltlt "Bad Temperature scale " ltlt scale ltlt
  endl

25
Conversion Functions

• The member function Fahrenheit() changes the
  degree and scale of the member data to
  Fahrenheit.
• void TemperatureFahrenheit()
• if(scale 'C')
• degree degree1.832.0
• else if(scale 'K')
• degree (degree-273.15)1.8 32.0
• scale 'F'

26
Conversion Functions

• The functions Celsius() and Kelvin() are similar.
• void TemperatureCelsius()
• if(scale 'F')
• degree (degree-32.0)/1.8
• else if(scale 'K')
• degree degree - 273.15
• scale 'C'
• void TemperatureKelvin()
• if(scale 'F')
• degree (degree-32.0)/1.8 273.15
• else if(scale 'C')
• degree degree 273.15
• scale 'K'

27
Conversion Functions

• Calling the Fahrenheit() member function
• Temperature temp1 // default value 0 C
• temp1.Fahrenheit()
• temp1.print() // prints 32 F
• When temp1 receives the Fahrenheit() message, it
  converts to the Fahrenheit temperature 32 F.

28
Calling Member Functions from Member Functions

• We can simplify the main() program by building a
  member function printAll() to print the
  temperature in all scales
• void main()
• Temperature temp
• cout ltlt "Enter temperature (e.g., 98.6 F) "
• temp.read()
• temp.printAll() // prints temperature in F, C,
  K

29
Calling Member Functions from Member Functions

• This member function calls the other member
  functions
• void TemperatureprintAll()
• // prints temperature in all scales
• cout ltlt "--gt"
• // calls member function Fahrenheit()
• Fahrenheit()
• print() cout ltlt " "
• Celsius()
• print() cout ltlt " "
• Kelvin()
• print()

30
Temperature Class

• class Temperature
• public
• Temperature()
• Temperature(double idegree, char iscale)
• void read()
• void print()
• void printAll()
• void Fahrenheit()
• void Celsius()
• void Kelvin()
• private
• double degree
• char scale

31
Copy Constructor

• Syntax
• Declaration
• className (const className variable)
• Definition
• classNameclassName(const className variable)
• . . .
• // className copy constructor
• Example
• void main()
• // explicit value constructor
• Temperature t1(100, F)
• Temperature t2(t1) // t2 gets the value of t1
• . . .

32
Bitwise Logical Copy
Figure 10-9
33
Destructor

• The destructor of an object is called upon for
  its deletion. It is usually used to release
  memory allocated to that object.
• Syntax
• Declaration - className( )
• Definition className className()
• . . .

34
Example of destructor

• class MyArray // class definition
• private
• int size
• int a
• public
• MyArray(int aSize)
• MyArray()
• MyArrayMyArray(int aSize)
• // default constructor
• a new intaSize
• MyArrayMyArray()
• // destructor
• delete a

35
Summary

• Classes/Objects
• Allow you to combine data and functions together
  (to create smart objects)
• Member data, member functions of an object
• Scope operator
• Used to denote a member function is part of a
  classes
• Special Manager functions
• Constructors
• Called when object is created
• Copy Constructors
• Called when a copy of an existing instance needs
  to be created
• Destructor
• Called when object is destroyed