CLASSES AND DATA ABSTRACTION

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Chapter 13

• CLASSES AND DATA ABSTRACTION

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Objectives

• In this chapter you will
• Learn about classes
• Learn about private, protected, and public
  members of a class
• Explore how classes are implemented
• Examine constructors and destructors
• Learn about the abstract data type (ADT)
• Explore how classes are used to implement ADT
• Learn about information hiding
• Explore how information hiding is implemented in
  C

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Classes

• A class is a collection of a fixed number of
  components
• The components of a class are called members of
  the class
• The general syntax of defining a class is
• class classIdentifier
• classMemberList
• A member of a class can either be a variable
  (that is, to store some data) or a function

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Classes

• If a member of a class is a variable, it is
  declared just like any other variable
• In the definition of the class, you cannot
  initialize a variable when you declare it
• If a member of a class is a function, typically
  the function prototype is used to define that
  member
• A missing semicolon, therefore, will result in a
  syntax error

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Classes

• If a member of a class is a function,
• it can (directly) access any member of the
  classdata members and function members
• when you write the definition of the member
  function, you can directly access any data member
  of the class without passing it as a parameter
• the only obvious condition is that you must
  declare an identifier before you can use it
• Class is a reserved word and it only defines a
  data type, no memory is allocated
• The semicolon after the right brace is part of
  the syntax

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Classes

• The members of a class are classified into three
  categories
• private
• public
• protected
• Following are some facts about public and private
  members of a class
• By default, all members of a class are private
• If a member of a class is private, you cannot
  access it outside the class

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Classes

• A public member is accessible outside the class
• To make a member of a class public, you use the
  label public with a colon
• In C, private, protected, and public are
  reserved words

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Variable (Object) Declaration

• Once a class is defined, you can declare
  variables of that type
• In C terminology, a class variable is called a
  class object or class instance
• To become familiar with this terminology, we will
  use the term class object, or simply object, for
  a class variable
• The syntax for declaring a class object is the
  same as that for declaring any other variable
• clockType myClock
• clockType yourClock

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Accessing Class Members

• Once an object is declared, it can access the
  public members of a class
• The general syntax to access the member of a
  class is
• classVariableName.memberName
• In C, the dot, . (period), is an operator
  called the member access operator

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Built-in Operations on Classes

• Most of Cs built-in operations do not apply to
  classes
• Arithmetic operators cannot be used to perform
  arithmetic operations on class objects unless
  they are overloaded
• Also, you cannot use relational operators to
  compare two class objects for equality
• The two built-in operations that are valid for
  class objects are member access (.) and
  assignment ()

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Class Scope

• A class object has the same scope as other
  variables
• A member of a class is local to the class
• We access a class member outside the class by
  using the class object name and the member access
  operator (.)

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Functions and Classes

• Class objects can be passed as parameters to
  functions and returned as function values
• As parameters to functions, classes can be passed
  either by value or by reference
• If a class object is passed by value, the
  contents of the data members of the actual
  parameter are copied into the corresponding data
  members of the formal parameter

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Reference Parameters Variables

• As a parameter, a class object can be passed by
  value
• If a variable is passed by value, the
  corresponding formal parameter receives a copy of
  the data of the variable
• This operation might require, in addition to a
  large amount of storage space, a considerable
  amount of computer time to copy the value of the
  actual parameter into the formal parameter

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Reference Parameters Variables

• If a variable is passed by reference, the formal
  parameter receives only the address of the actual
  parameter
• Therefore, an efficient way to pass a variable as
  a parameter is by reference
• If a variable is passed by reference, then when
  the formal parameter changes, the actual
  parameter also changes
• You can pass a variable by reference and still
  prevent the function from changing its value, by
  using the keyword const in the formal parameter
  declaration

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Constructors

• Constructors
• We can guarantee the initialization of the data
  members of a class by using constructors
• There are two types of constructors
• with parameters
• without parameters
• The constructor without parameters is called the
  default constructor
• A constructor that has no parameters, or has all
  default parameters, is called the default
  constructor

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Constructors

• 1. The name of a constructor is the same as the
  name of the class
• 2. A constructor
• even though it is a function, has no type
• it is neither a value-returning function nor a
  void function
• 3. A class can have more than one constructor,
  however, all constructors of a class have the
  same name
• 4. If a class has more than one constructor, they
  must have different sets of parameters

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Constructors

• 5. Constructors
• are automatically executed when a class variable
  enters its scope
• since they have no types, they cannot be called
  like other functions
• 6. Which constructor executes depends on the type
  of values passed to the class variable when the
  class variable is declared

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Invoking a Constructor

• A constructor is automatically executed when a
  class variable is declared
• A class might have more than one constructor,
  including the default constructor

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Invoking the Default Constructor

• The syntax to invoke the default constructor is
• className classVariableName
• The statement
• clockType yourClock
• declares yourClock to be a variable of the type
  clockType
• In this case, the default constructor is executed
  and the data members of yourClock will be
  initialized to zero

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Constructor with Parameters

• The syntax to invoke a constructor with
  parameters is
• className classVariableName(argument1,
• argument2,. . . )
• where argument1, argument2, etc. is either a
  variable or an expression

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Constructor with Parameters

• 1. The number of arguments and their type should
  match the formal parameters (in the order given)
  of one of the constructors
• 2. If the type of the arguments do not match the
  formal parameters of any constructor (in the
  order given), C will use type conversion and
  look for the best match
• For example,
• an integer value might be converted to a
  floating-point value with zero decimal part
• an ambiguity will result in a compile-time error

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Arrays

• If a class has constructors and you declare an
  array of class objects, the class must have the
  default constructor
• The default constructor is used to initialize
  each (array) class object
• For example
• clockType clocks100

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Destructors

• Like constructors, destructors are also functions
• The name of a destructor is the character ''
  followed by the name of the class
• The name of the destructor for the class
  clockType is
• clockType()
• A class can have only one destructor and it has
  no parameters
• The destructor is automatically executed when the
  class object goes out of scope

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Abstract Data Types

• The data type clockType has three data members
  and the following basic operations
• 1. Set the time
• 2. Return the time
• 3. Print the time
• 4. Increment the time by one second
• 5. Increment the time by one minute
• 6. Increment the time by one hour
• 7. Compare the two times to see whether they are
  equal

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Abstract Data Types

• Abstract Data Type (ADT) - a data type that
  specifies the logical properties without the
  implementation details
• An ADT has three things associated with it
• The name of the ADT, called type name
• The set of values belonging to the ADT, called
  domain
• The set of operations on the data

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A Struct Versus a Class

• By default the members of a struct are public
• By default the members of a class are private
• The member access specifier private can be used
  in a struct to make a member private
• Both C classes and structs have the same
  capabilities
• Most programmers restrict their use of structures
  to adhere to their C-like structure form
• If all of the data members of a class are public
  and the class has no member functions, typically
  a struct is used to group these members

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Information Hiding

• The header file has an extension h
• The implementation file has an extension cpp
• The implementation file must include the header
  file via the include statement
• In an include statement, user-defined header
  files are enclosed in double quotes while
  system-provided header files are enclosed between
  angular brackets

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Files

• Visual C, C Builder, and CodeWarrior put the
  editor, compiler, and linker all into one package
  
• With one command, the program is compiled and
  linked with the other necessary files
• These systems also manage multiple file programs
  in the form of a project
• A project consists of several files, called the
  project files

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Files

• These systems usually have a command, called
  build, rebuild, or make
• When the build, rebuild, or make command is
  applied to a project, the system automatically
  compiles and links all files required to create
  the executable code
• When one or more files in the project change, you
  can use these commands to recompile and relink
  the files

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Programming Example

• A common place to buy candy is the candy machine
• A new candy machine is bought for the gym, but it
  is not working properly
• This candy machine currently sells candies,
  chips, gum, and cookies
• You have been asked to write a program for this
  candy machine so it can be put into operation

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Programming Example

• The program should
• 1. Show the customer the different products sold
  by the candy machine
• 2. Let the customer make the selection
• 3. Show the customer the cost of the item
  selected
• 4. Accept money from the customer
• 5. Release the item
• Input - item selection and cost of the item
• Output - selected item

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Problem Analysis

• A candy machine has two main components
• a built in cash register
• several dispensers to hold and release the
  product

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Main Program

• When the program executes, it must
• 1. Show the different products sold by the candy
  machine
• 2. Show how to select a particular product
• 3. Show how to terminate the program
• These instructions must be displayed after
  processing each selection, except exiting the
  program
• Once the user has made the appropriate selection,
  the candy machine must act accordingly

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Main Program

• If the user has selected to a buy a product and
  if the product is available, the candy machine
  should show the cost of the product and ask the
  user to deposit the money
• If the money deposited is at least the cost of
  the item, the candy machine should sell the item
  and display an appropriate message

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Menu

• 1. Show the selection to the customer
• 2. Get selection
• 3. If selection is valid and the dispenser
  corresponding to the selection is not empty, sell
  the product

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Menu

• The menu looks like the following
• showSelection
• a. Welcome to Shelly's Candy Shop "
• b. To select an item, enter
• c. 1 for Candy
• d. 2 for Chips
• e. 3 for Gum
• f. 4 for Cookies
• g. 9 to exit

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sellProduct

• If the dispenser is nonempty
• show and prompt the customer to enter the cost of
  the item
• get the amount entered by the customer
• if the amount entered by the customer is less
  than the cost of the product
• show and prompt the customer to enter the
  additional amount
• calculate the total amount entered by the
  customer

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sellProduct

• if the amount entered by the customer is at least
  the cost of the product
• update the amount in the cash register
• sell the product, that is, decrement the number
  of items in the dispenser by 1, display an
  appropriate message
• if the amount entered by the user is less than
  the cost of the item, ask the user to deposit
  additional money
• If the dispenser is empty, tell the user that
  this product is sold out

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The Function Main

• 1. Create the cash register, which means that we
  declare a variable of the type cashRegister
• 2. Create four dispensers, which means that we
  declare four objects of the type dispenserType
  and also initialize these objects
• For example,
• the statement dispenserType candy(100, 50)
  creates a dispenser object, candy, to hold
  candies
• the number of items in the dispenser is 100 and
  the cost of an item is 50 cents

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The Function Main

• 3. Declare additional variables as necessary
• 4. Show selection, call the function
  showSelection
• 5. Get the selection
• 6. While not done (a selection of 9 exists the
  program)
• a. Sell product, call the function sellProduct
• b. Show selection, call the function
  showSelection
• c. Get selection