Lecture 17 Classes and Objects

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Lecture 17Classes and Objects

• ENGR17 Engineering Programming
• Section 1
• Fall 2001

11/21/01
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Quote
This method is, to define as the number of a
class the class of all classes similar to the
given class. Bertrand Russell Principles of
Mathematics, Part II, Chapter 11, Section iii,
1903.
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Outline

• Create a class
• Use the public members of a class to manipulate
  the private members
• Create an object from a programmer-defined class
• Create a constructor function
• Include a value-returning function in a class

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OOP Concepts

• C is considered a hybrid language
• Procedure-oriented programs (what weve studied
  so far)
• Object-oriented programs (what well study now)
• Object-oriented programming creates reusable
  objects
• Every object in C is created from a class
• Built-in classes
• User-defined classes
• Structures allow us to package a set of related
  data items
• Classes allow us to package a set of related
  data items AND the functions that use the data

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OOP Concepts and Terms

• Class
• Is a pattern for creating one or more instances
  of the classone or more objects
• Encapsulates all of an objects attributes and
  behaviors
• Attributes
• Are characteristics that describe the object
• Behaviors
• Are operations (actions) that the object either
  can perform or have performed on it
• Objects created from a class are referred to as
  instances of the class

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OOP Concepts and Terms

• Abstraction (data hiding)
• Refers to hiding a classs internal details from
  user
• Helps prevent user from making inadvertent
  changes to the object
• Attributes and behaviors that are not hidden are
  said to be exposed to the user
• Limited to attributes and behaviors needed by
  user to use the object
• Public section
• Contains the attributes and behaviors you want to
  expose
• Allows program to access the private members
• Private section
• Contains the attributes (variables) and behaviors
  (functions) that you want to hide from the user

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Defining a Class in C

• A class must be defined for an object before you
  can create an object in C
• Use the class statement to define a class

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Defining a Class in C

• Data members
• Are the attributes included in the class
• Are typically listed in the private section of
  the class
• Member functions
• Are the functions included in the class
• Are typically listed in the public section of the
  class

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Defining a date Class

• Determine the attributes and behaviors
• Use the class statement to create a date class

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Creating a Header File

• Enter the class statement in a programmer-defined
  header file
• Choose File ? New ? C/C Header File
• Give the header file the same name as the class
• Be sure to put a include your_class.h at the
  above your main function
• Example of how to create a header file

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Creating a Header File
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Defining the setDate Function
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Defining the displayDate Function
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Using a Class to Create an Object
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Using a Class to Create an Object
Sample screen output
Enter the month 7 Enter the day 3 Enter the
year 99 The employee was hired on 7/3/99. Press
any key to continue
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Constructor Function

• Member function that C calls automatically each
  time the class instantiates (creates) and object
• Has the sole purpose of initializing the class
  variables
• Has the same name as the class
• Does not have a data type because it cannot
  return a value

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Constructor Function
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Constructor Function
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Creating the rectangle Class

• Identify the objects attributes and behaviors in
  a class

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Creating the rectangle Class

• Enter the class definition into a header file

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Creating the rectangle Class

• Enter the function definitions for the member
  functions
• A member function is needed for each behavior
  listed in the class
• Member functions can be either void or
  value-returning functions

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Creating the rectangle Class
Constructor
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Creating the rectangle Class
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Example Program

• Write a program using the rectangle class to
  calculate the area of a yard and the cost to
  install sod
• Input
• Length of rectangle (in feet)
• Width of rectangle (in feet)
• Price of square yard of sod ()
• Output
• Area (in square yards)
• Total Price ()

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Example Program
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Example Program
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Summary

• Create a programmer-defined class
• Use the class to instantiate (create) an object
• Create a constructor function that automatically
  initializes the variables in a class
• Use value-returning functions in a class

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Example

• Write a program using the rectangle class to
  calculate the perimeter of a yard and the cost to
  install a fence around it.
• Input
• Length of rectangle (in feet)
• Width of rectangle (in feet)
• Cost of fence per foot ()
• Output
• Perimeter (in feet)
• Total Price ()

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Code (1)
Continued
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Code (2)
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Example

• Write the class statistics that calculates the
  statistics of an array of integers.
• Behaviors
• Constructor Initialize array to all zeros
• AddMember Add a new value to end of array
• Average Return the average of the array
• High Return the max element of the array
• Attributes???

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Code(1)

• // statistics.h
• class statistics
• public
• statistics()
• void AddMember(int mem)
• int Average()
• int High()
• private
• int nums10
• int size
• statisticsstatistics()
• int i
• size 0
• for (i0 ilt10 i)
• numsi 0

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Code(2)

• void statisticsAddMember(int mem)
• numssize mem
• size
• int statisticsAverage()
• int i
• int total0
• for (i0 iltsize i)
• total numsi
• return total/i
• int statisticsHigh()
• int i
• int high