Classes and Data Abstraction
Andrew Davison
Noppadon Kamolvilassatian
Department of Computer Engineering
Prince of Songkla University
Contents
1. What is Data Abstraction?
2. C++: Classes and Data Abstraction
3. Implementing a User-Defined Type Time with a
Struct
4. Implementing a Time Abstract Data Type with
a Class
5. Classes as User-Defined Types
6. Using Constructors
2
1. What is Data Abstraction?
Abstract Data Types (ADTs)
type implementation & operations
hidden implementation
types are central to problem solving
a weapon against complexity
built-in and user-defined types are ADTs
3
Clients and Manufacturers
client
Interface
ADT
Implementation
client
use
client
manufacturers
responsibility
4
Benefits
Manufacturer Benefits:
easy to modify, maintain
profitable
reusable
Client Benefits:
simple to use, understand
familiar
cheap
component-based
5
How Well are ADTs Supported in C?
Does C enforce the use of the ADTs interface and
the hiding of its implementation?
No
C++ (Chapters 15-21)
C++ is a superset of C, which has added
features to support object-oriented programmi
ng.
C++ supports classes.
things very like ADTs
2. C++: Classes and Data Abstraction
C++ supports Object-Oriented Programming
(OOP).
OOP models real-world objects with software
counterparts.
OOP encapsulates data (attributes) and functions
(behavior) into packages called objects.
Objects have the property of information hiding.
8
Objects communicate with one another across
interfaces.
The interdependencies between the classes are
identified
makes use of
a part of
a specialisation of
a generalisation of
etc
C and C++
C programmers concentrate on writing functions.
C++ programmers concentrate on creating their
own user-defined types called classes.
Classes in C++ are a natural evolution of the C
notion of struct.
10
3. Implementing a User-Defined Type
Time with a Struct
// FIG16_1.CPP
// Create a structure, set its members, and print
it.
#include <iostream.h>
struct
int
int
int
};
Time {
hour;
minute;
second;
//
//
//
//
structure definition
0-23
0-59
0-59
void printMilitary(const Time &);
void printStandard(const Time &);
// prototype
// prototype
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main()
{
Time dinnerTime;
// set members to
dinnerTime.hour =
dinnerTime.minute
dinnerTime.second
// variable of new type Time
valid values
18;
= 30;
= 0;
cout << "Dinner will be held at ";
printMilitary(dinnerTime);
cout << " military time,\nwhich is ";
printStandard(dinnerTime);
cout << " standard time." << endl;
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// set members to
dinnerTime.hour =
dinnerTime.minute
dinnerTime.second
invalid values
29;
= 73;
= 103;
cout << "\nTime with invalid values: ";
printMilitary(dinnerTime);
cout << endl;
return 0;
}
13
// Print the time in military format
void printMilitary(const Time &t)
{
cout << (t.hour < 10 ? "0" : "") << t.hour << ":"
<< (t.minute < 10 ? "0" : "") << t.minute << ":"
<< (t.second < 10 ? "0" : "") << t.second;
}
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// Print the time in standard format
void printStandard(const Time &t)
{
cout << ((t.hour == 0 || t.hour == 12) ? 12 :
t.hour % 12)
<< ":" << (t.minute < 10 ? "0" : "") << t.minute
<< ":" << (t.second < 10 ? "0" : "") << t.second
<< (t.hour < 12 ? " AM" : " PM");
}
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Comments
Initialization is not required --> can cause
problems.
A program can assign bad values to members of
Time.
If the implementation of the struct is changed,
all the programs that use the struct must be chan
ged. [No interface]
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4. Implementing a Time Abstract Data
Type with a Class
#include <iostream.h>
// Time abstract data type (ADT) definition
class Time {
public:
Time();
// default constructor
void setTime(int, int, int);
void printMilitary();
void printStandard();
private:
int hour;
// 0 - 23
int minute;
// 0 - 59
int second;
// 0 - 59
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};
// Time constructor initializes each data member
to zero.
// Ensures all Time objects start in a consistent
state.
Time::Time() { hour = minute = second = 0; }
// Set a new Time value using military time.
// Perform validity checks on the data values.
// Set invalid values to zero (consistent state)
void Time::setTime(int h, int m, int s)
{
hour = (h >= 0 && h < 24) ? h : 0;
minute = (m >= 0 && m < 60) ? m : 0;
second = (s >= 0 && s < 60) ? s : 0;
}
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// Print Time in military format
void Time::printMilitary()
{
cout << (hour < 10 ? "0" : "") << hour << ":"
<< (minute < 10 ? "0" : "") << minute << ":"
<< (second < 10 ? "0" : "") << second;
}
// Print time in standard format
void Time::printStandard()
{
cout << ((hour == 0 || hour == 12) ? 12 : hour %
12)
<< ":" << (minute < 10 ? "0" : "") << minute
<< ":" << (second < 10 ? "0" : "") << second
<< (hour < 12 ? " AM" : " PM");
}
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// Driver to test simple class Time
main()
{
Time t; // instantiate object t of class Time
cout << "The initial military time is ";
t.printMilitary();
cout << "\nThe initial standard time is ";
t.printStandard();
t.setTime(13, 27, 6);
cout << "\n\nMilitary time after setTime is ";
t.printMilitary();
cout << "\nStandard time after setTime is ";
t.printStandard();
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t.setTime(99, 99, 99);
// attempt invalid settings
cout << "\n\nAfter attempting invalid settings:
\n"
<< "Military time: ";
t.printMilitary();
cout << "\nStandard time: ";
t.printStandard();
cout << endl;
return 0;
}
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Output
The initial military time is 00:00:00
The initial standard time is 12:00:00 AM
Military time after setTime is 13:27:06
Standard time after setTime is 1:27:06 PM
After attempting invalid settings:
Military time: 00:00:00
Standard time: 12:00:00 AM
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Comments
hour, minute, and second are private data
members. They are normally not accessible outsid
e the class. [Information Hiding]
Use a constructor to initiailize the data members.
This ensures that the object is in a consistate state
when created.
Outside functions set the values of data members
by calling the setTime method, which provides err
or checking.
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5. Classes as User-Defined Types
Once the class has been defined, it can be used as
a type in declarations as follows:
Time sunset,
arrayOfTimes[5],
*pointerToTime,
//object of type Time
//array of Time objects
//pointer to a Time object
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6. Using Constructors
Constructors can be overloaded, providing several
methods to initialize a class.
Time example:
Interface
Time();
Time(int hr, int min, int sec);
Implementation
Time::Time()
{ hour = minute = second = 0; }
Time::Time(int hr, int min, int sec)
{ setTime(hr, min, sec); }
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Use:
Time t1;
Time t2(08,15,04);
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