CSci 152: Programming II Fall 2004

1
CSci 152 Programming IIFall 2004

• Classes and Data Abstraciton

2

• In this lecture, you will
• Learn about classes
• Learn about private, protected, and public
  members of a class

3

• 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.

4

• If a member of a class is a variable, you declare
  it just like any other variable. Also, in the
  definition of the class, you cannot initialize a
  variable when you declare it.
• If a member of a class is a function, you
  typically use the function prototype to define
  that member.
• If a member of a class is a function, it can
  (directly) access any member of the classdata
  members and function members. That is, 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.
• In C, 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.
• A missing semicolon, therefore, will result in a
  syntax error.

5

• The members of a class are classified into three
  categories private, public, and protected. This
  chapter mainly discusses the first two typesthat
  is, private and public.
• 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.
• 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. (member access specifiers)

6

• Define a class, clockType, to implement the time
  of day in a program.
• Time is represented as a set of three integers
  one to represent the hours, one to represent the
  minutes, and one to represent the seconds.
• Perform the following operations on the time
• 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 for equality.

7
(No Transcript)
8

• Some members of the class clockType will be
  private others will be public.
• Any member that needs to be accessed outside the
  class is declared public any member that should
  not be accessed directly by the user should be
  declared private.
• The user should be able to set the time and print
  the time. Therefore, the members that set the
  time and print the time should be declared
  public.

9

• class clockType
• public
• void setTime(int, int, int)
• void getTime(int, int, int)
• void printTime() const
• void incrementSeconds()
• void incrementMinutes()
• void incrementHours()
• bool equalTime(const clockType otherTime)
  const
• private
• int hr
• int min
• int sec

10

• The class clockType has seven function members
  setTime, getTime, printTime, incrementSeconds,
  incrementMinutes, incrementHours, and equalTime.
• It has three data members hr, min, and sec.
• The three data membershr, min, and sec, are
  private to the class and cannot be accessed
  outside the class.
• The seven function memberssetTime, getTime,
  printTime, incrementSeconds, incrementMinutes,
  incrementHours, and equalTimecan directly access
  the data members (hr, min, and sec).
• In the function equalTime, the parameter
  otherTime is a constant reference parameter. That
  is, in a call to the function equalTime, the
  parameter otherTime receives the address of the
  actual parameter, but otherTime cannot modify the
  value of the actual parameter.
• The word const at the end of the member functions
  printTime and equalTime specifies that these
  functions cannot modify the data members of a
  variable of the type clockType.

11

• 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

12
(No Transcript)
13

• 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.

14

• Example 13-1
• myClock.setTime(5,2,30)
• myClock.printTime()
• yourClock.setTime(x,y,z) //Assume x, y, and
• //z are variables of the type int
• if(myClock.equalTime(yourClock))
• .
• .
• .
• myClock.hr 10 //illegal
• myClock.min yourClock.min //illegal

15

• Built-in Operations on Classes
• Most of Cs built-in operations do not apply to
  classes.
• You cannot use arithmetic operators to perform
  arithmetic operations on class objects (unless
  they are overloaded).
• For example, you cannot use the operator to add
  two class objects of, say, the type clockType.
• 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 ().

16

• The Assignment Operator and Classes
• Suppose that myClock and yourClock are variables
  of the type clockType.

17

• The statement
• myClock yourClock
• copies the value of yourClock into myClock
• a. the value of yourClock.hr is copied into
  myClock.hr,
• b. the value of yourClock.min is copied into
  myClock.min
• c. the value of yourClock.sec is copied into
  myClock.sec

18

• Class Scope
• A class variable 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 class variable name and the member
  selection operator (.).

19

• 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.

20

• Reference Parameters and Class Objects
  (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.
• 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.
• In C, 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.

21

• void testTime(const clockType otherClock)
• clockType dClock
• ...