Building Classes

1
Building Classes

• Chapter 11

2
Objectives

• See OCD applied where no predefined types exist
  for objects
• Learn how classes are designed, built
• Study encapsulation, information hiding
• Implement attributes as instance variables
• Describe roles of
• Constructors, accessors, mutators
• Converter and utility methods
• Illustrate a class to model temperatures
• See operator overloading (including I/O
  operators)
• An overview of artificial intelligence

3
Introductory ExampleModeling Temperatures

• We seek a program that, given a temperature in
  Fahrenheit, Celsius, or Kelvin will
• Display the equivalent temperature
• In each of the scales
• We have two attributes
• Number of degrees
• The scale
• Operations would require both attributes to be
• Printed
• Read in
• Passed to functions
• etc.

This problem would be even worse for objects with
multiple attributes such as an IRS W-2 Form.
4
Extending Object-Centered Design

• Previously we used OCD where an operation for an
  object was not predefined.For each operation not
  predefined
• Define a function to perform the operation
• If reusable, store it in a library
• Also, we found that for an object with multiple
  attributes we define a class
• Now we combine those two concepts
• Declare class for multiple attributes
• Add methods to the class for needed operations

5
Behavior
For temperature conversionEnter scale FEnter
temperature 99.9 Celsius equivalent
99.9Kelvin equivalent 99.9
6
Objects
7
Operations

• Display a string on the screen
• Read a Temperature from an istream
• Determine the Fahrenheit equivalent of a
  Temperature
• Determine the Celsius equivalent of a Temperature
• Determine the Kelvin equivalent of a Temperature
• Display a Temperature to an ostream

8
Algorithm

• Via cout, display a prompt for a temperature
• From cin, read a temperature into theTemperature
• Via cout display
• The Fahrenheit equivalent of theTemperature
• The Celsius equivalent of theTemperature
• The Kelvin equivalent of theTemperature
• View source code, Figure 11.2
• Sample runs

9
Designing a Class

• Two phases
• Design phase plan the class
• Implementation phase encode the design in C
  language
• Design is identifying
• Behavior the operations that can be applied to
  a class object
• Its attributes the data that must be stored to
  characterize class objects

10
External and Internal Principles

• External view
• An observer looking from outside the program at
  its details
• Internal perspective
• Object autonomy
• When you design a class, think of being the
  object

11
Temperature Behavior

• Define myself implicitly, default initial values
• Read a temp from an istream, store in my data
  members
• Compute Fahrenheit, Celsius, Kelvin equivalents
  of me
• Display my degrees and scale using an ostream
• Define myself explicitly, initializing with
  specified values

12
Temperature Behavior

• Identify my number of degrees
• Identify my scale
• Compute my temp raised/lowered by a given number
  of degrees
• Compare myself to other Temperature objects using
  any of six relational operators
• Assign another Temperature value to me using the
  assignment operator

13
Temperature Attributes

• Two data attributes
• My degrees
• My scale
• Implemented asdouble myDegreeschar myScale
  // 'F', 'C', or 'K' only

14
Encapsulation

• Wrap a class declaration around the
  attributesclass Temperature public //
  to be filled in later private double
  myDegrees char myScale
• We declare Temperature temp1, temp 2

15
Information Hiding

• Preventing a program which uses a Temperature
  object from directly accessing private
  variablesclass Temperature public .
  . . private double myDegrees
  char myScale

User programs can access private attributes only
by using public methods.
16
Class Invariants

• Restrictions placed on values of the instance
  variables
• We have specified myScale can only be'F', 'C',
  or 'K'
• We will also saymyScale 'F' myDegrees gt
  -459.67 myScale 'C' myDegrees gt
  -273.15 myScale 'K' myDegrees gt 0.0

17
Conditional Compilation

• Preprocessor directivesinclude Filename
• If file is included by multiple files in a
  program project, it can generate a duplicate
  identifier error message
• We specify another directiveifndef
  TEMPERATUREdefine TEMPERATURE class
  Temperature . . .endif

Then the actual class declaration is only
processed once by the compiler
18
Temperature Output

• We want any Temperature object to be able to
  print itselftemp1.print(cout)
• Tells object temp1 to print itself, using cout

inline void Temperatureprint (ostream out)
const out ltlt myDegrees ltlt ' ' ltlt myScale
19
The Default-Value Constructor

• When we declareTemperature temp1, temp 2
• We must assume "garbage" initial values
• We prefer to have a constructor which assigns
  default values

20
The Default-Value Constructor

• Constructor has the same name as the class.
• inline TemperatureTemperature()myDegrees
  0.0myScale 'C'
• A constructor has no return type

21
Explicit Value Constructors

• The constructor must
• Receive potential values for the attributes
• Validate those values using the invariantsNote
  isvalid() utility function, Figure 11.5
• Assign values to the attributes
• Constructor source code, Figure 11.6
• Now we can declare Temperature temp1 (98.6,
  'F'), temp2What do the objects look like now?

22
Accessor Methods

• Allows some attribute of the class to be read
• But not modified
• Programmer sends message to an object"Get your
  degrees" temp1.getDegrees()inline double
  TemperaturegetDegrees() const return
  myDegrees

The getScale() method would be a similar
definition.
23
Temperature Input

• We should be able to tell a Temperature object to
  get attribute values from an istreamtemp2.read(ci
  n)
• The method should also validate the incoming
  attributes
• Will use isValid() utility method
• View the read() function, Figure 11.8

24
Conversion Methods

• We need methods that enable a Temperature object
  to return another Temperature object
• With a new value for myScale
• With the appropriate value for myDegrees
• The method will
• Check the current scale
• Perform the proper mathematical conversion
• View source code, Figure 11.9

25
Overloading Operators

• For some operations it is convenient to define an
  operator for a class
• We wish to be able to compare two Temperature
  objectsif (temp1 lt temp2)
• In general, we overload some operator with symbol
  ? by defining a method with the name operator?

26
Overloading Operators

• What would complicate the following
  comparison?Temperature(0,'C') lt
  Temperature(32,'F')
• Note source code, Figure 11.10
• Source code for operator, Figure 11.11
• Class declaration, Figure 11.12

The operatorlt method must account for the two
different scales!
?
27
Case StudyRetrieving Student Information

• We need an information retrieval system for a
  university registrar.
• A data file contains lines of data in pairs of
  lines
• 1st line studentNumber firstName lastName
• 2nd line studentYear credits gradePointAverage
• The program must
• Allow the user to enter a student number
• Then retrieve and display information for that
  student

28
Objects
29
Operations

• Read a sequence of students from the input file
• Display a prompt
• Read a long integer from keyboard
• Search a sequence of students for one with a
  particular ID number
• Compare two Student objects using lt
• Compare two Student objects using
• Display a student
• Repeat steps ii v an arbitrary number of times

30
Algorithm

• Read a sequence of students from INPUT_FILE into
  studentVec
• Repeatedly do the following
• Prompt for and read studentID
• Search studentVec for a match, returning its
  position
• If search successful Display student at
  position Otherwise Display an error message

31
Building a Student Class

• Behaviors
• Initialize myself with default values
• Initialize myself with explicitly supplied values
• Read my attributes from an istream and store them
  in me
• Display my attributes using an ostream
• Compare myself and another Student using the lt
  and the relational operators (also the other
  relational operators for other applications)
• Access any of my attributes

32
Coding

• Note the class declaration in the header file,
  Figure 11.13
• When we declare vectorltStudentgt studentVec
  the effect can be visualized as follows

33
Coding

• Partial implementation of the program (to be
  finished by the student), Figure 11.14
• Input file, students.txt
• Sample run.
• Check your version of the program against what
  you see here

34
OBJECTive ThinkingOperator Overloading and
Friends

• Recall the print() and read() functions from
  Temperature
• Useful but don't fit with normal iostream
  operators
• Would be nice to use ltlt and gtgt
• Not possible to define operatorltlt as a member of
  a class
• object ? operand is interpreted
  asobject.operator?(operand)
• cout ltlt temp1 // temp1 is on wrong side

35
Overloading

• We need to define a function which is not a class
  member
• operand ? operator is then interpreted
  asoperator? (object, operand)
• Now cout ltlt temp1 is interpreted asoperatorltlt
  (cout, temp1)
• Declarationinline ostream operatorltlt(ostream
  out, const Temperature
  theTemp) theTemp.print(out) return out

36
Overloading

• The return out enables chaining of operatorltlt
  callscout ltlt "The temp " ltlt temp1 ltlt endl

Value returned by this function call is a copy of
the ostream used in the call
37
Using ltlt and gtgt

• Declaration of operatorgtgt which would call the
  read() method is similar
• Note temperature conversion program using the ltlt
  and gtgt overloaded operators, Figure 11.19

38
Friend Functions

• Consider the need to create an operatorltlt
  function without the availability of a print()
  function
• What would be wrong?

inline ostream operatorltlt(ostream out,
const Temperature theTemp) out
ltlt theTemp.myDegrees ltlt ' ' ltlt
theTemp.myScale return out
39
Friend Functions

• This can be made legal
• Place a prototype of the function in the class
  declaration
• Specify it as a friend function
• class Temperature public . . .
  friend ostream operatorltlt (ostream
  out, const Temperature theTemp) . . .

A similar friend declaration can be made for the
operatorgtgt function
40
Artificial Intelligence

• Computers have successfully played chess against
  skilled players
• This is part of an area of computer science known
  as artificial intelligence or AI
• AI is an attempt to program computersto perform
  intelligenttasks

41
Artificial Intelligence

• A precise definition of artificial intelligence
  is hard to do
• Reasons
• Intelligent behavior is complex, hard to define
• Styles of programming used are diverse
• Check the website for the text
• Read Professor Vander Linden's introduction
• View some of the AI topics
• Try out code for a dice game called Not-One