lecture10 introduction to Classes and Objects.ppt

Introduction
• Object-oriented programming (OOP)
– Encapsulation: encapsulates data (attributes) and
functions (behavior) into packages called classes
– Information hiding : implementation details are
hidden within the classes themselves
• Classes
– Classes are the standard unit of programming
– A class is like a blueprint – reusable
– Objects are instantiated (created) from the class
– For example, a house is an instance of a “blueprint
class”

Structure Definitions
• Structures
– Aggregate data types built using elements of other types
struct Time {
int hour;
int minute;
int second;
};
– Members of the same structure must have unique names
– Two different structures may contain members of the same
name
– Each structure definition must end with a semicolon
Structure tag
Structure members

Structure Definitions
• Self-referential structure
– Contains a member that is a pointer to the same structure
type
– Used for linked lists, queues, stacks and trees
• struct
– Creates a new data type that is used to declare variables
– Structure variables are declared like variables of other
types
– Example:
Time timeObject, timeArray[ 10 ],
*timePtr, &timeRef = timeObject;

Accessing Members of Structures
• Member access operators:
– Dot operator (.) for structures and objects
– Arrow operator (->) for pointers
– Print member hour of timeObject:
cout << timeObject.hour;
OR
timePtr = &timeObject;
cout << timePtr->hour;
– timePtr->hour is the same as ( *timePtr ).hour
– Parentheses required: * has lower precedence than .

1 // Fig. 6.1: fig06_01.cpp
2 // Create a structure, set its members, and print it.
3 #include <iostream>
4
5 using std::cout;
6 using std::endl;
7
8 struct Time { // structure definition
9 int hour; // 0-23
10 int minute; // 0-59
11 int second; // 0-59
12 };
13
14 void printMilitary( const Time & ); // prototype
15 void printStandard( const Time & ); // prototype
16
17 int main()
18 {
19 Time dinnerTime; // variable of new type Time
20
21 // set members to valid values
22 dinnerTime.hour = 18;
23 dinnerTime.minute = 30;
24 dinnerTime.second = 0;
25
26 cout << "Dinner will be held at ";
27 printMilitary( dinnerTime );
28 cout << " military time,nwhich is ";
29 printStandard( dinnerTime );
30 cout << " standard time.n";
31
Dinner will be held at 18:30 military time,
which is 6:30:00 PM standard time.

32 // set members to invalid values
33 dinnerTime.hour = 29;
34 dinnerTime.minute = 73;
35
36 cout << "nTime with invalid values: ";
37 printMilitary( dinnerTime );
38 cout << endl;
39 return 0;
40 }
41
42 // Print the time in military format
43 void printMilitary( const Time &t )
44 {
45 cout << ( t.hour < 10 ? "0" : "" ) << t.hour << ":"
46 << ( t.minute < 10 ? "0" : "" ) << t.minute;
47 }
48
49 // Print the time in standard format
50 void printStandard( const Time &t )
51 {
52 cout << ( ( t.hour == 0 || t.hour == 12 ) ?
53 12 : t.hour % 12 )
54 << ":" << ( t.minute < 10 ? "0" : "" ) << t.minute
55 << ":" << ( t.second < 10 ? "0" : "" ) << t.second
56 << ( t.hour < 12 ? " AM" : " PM" );
57 }
Time with invalid values: 29:73

Program Output
Dinner will be held at 18:30 military time,
which is 6:30:00 PM standard time.
Time with invalid values: 29:73

Implementing a Time Abstract Data
Type with a Class
• Classes
– Model objects that have attributes (data
members) and behaviors (member functions)
– Defined using keyword class
– Have a body delineated with braces ({ and })
– Class definitions terminate with a semicolon
– Example:

1 class Time {
2 public:
3 Time();
4 void setTime( int, int, int );
5 void printMilitary();
6 void printStandard();
7 private:
8 int hour; // 0 - 23
9 int minute; // 0 - 59
10 int second; // 0 - 59
11 };
Public: and Private: are
member-access specifiers.
setTime, printMilitary, and
printStandard are member
functions.
Time is the constructor.
hour, minute, and
second are data members.

Implementing a Time Abstract Data
Type with a Class
• Member access specifiers
– Classes can limit the access to their member functions and data
– The three types of access a class can grant are:
• Public — Accessible wherever the program has access to an object of
the class
• private — Accessible only to member functions of the class
• Protected — Similar to private and discussed later
• Constructor
– Special member function that initializes the data members of a
class object
– Cannot return values
– Have the same name as the class

Objects
• Class definition and declaration
– Once a class has been defined, it can be used as
a type in object, array and pointer declarations
– Example:
Time sunset, // object of type Time
arrayOfTimes[ 5 ], // array of Time objects
*pointerToTime, // pointer to a Time object
&dinnerTime = sunset; // reference to a Time object
Note: The class name
becomes the new type
specifier.

1 // Fig. 6.3: fig06_03.cpp
2 // Time class.
4
5 using std::cout;
6 using std::endl;
7
8 // Time abstract data type (ADT) definition
9 class Time {
10 public:
11 Time(); // constructor
12 void setTime( int, int, int ); // set hour, minute, second
13 void printMilitary(); // print military time format
14 void printStandard(); // print standard time format
15 private:
16 int hour; // 0 – 23
17 int minute; // 0 – 59
18 int second; // 0 – 59
19 };
20
21 // Time constructor initializes each data member to zero.
22 // Ensures all Time objects start in a consistent state.
23 Time::Time() { hour = minute = second = 0; }
24
25 // Set a new Time value using military time. Perform validity
26 // checks on the data values. Set invalid values to zero.
27 void Time::setTime( int h, int m, int s )
28 {
29 hour = ( h >= 0 && h < 24 ) ? h : 0;
30 minute = ( m >= 0 && m < 60 ) ? m : 0;
31 second = ( s >= 0 && s < 60 ) ? s : 0;
32 }
Note the :: preceding
the function names.

33
34 // Print Time in military format
35 void Time::printMilitary()
36 {
37 cout << ( hour < 10 ? "0" : "" ) << hour << ":"
38 << ( minute < 10 ? "0" : "" ) << minute;
39 }
40
41 // Print Time in standard format
42 void Time::printStandard()
43 {
44 cout << ( ( hour == 0 || hour == 12 ) ? 12 : hour % 12 )
45 << ":" << ( minute < 10 ? "0" : "" ) << minute
46 << ":" << ( second < 10 ? "0" : "" ) << second
47 << ( hour < 12 ? " AM" : " PM" );
48 }
49
50 // Driver to test simple class Time
51 int main()
52 {
53 Time t; // instantiate object t of class Time
54
55 cout << "The initial military time is ";
56 t.printMilitary();
57 cout << "nThe initial standard time is ";
58 t.printStandard();
59

60 t.setTime( 13, 27, 6 );
61 cout << "nnMilitary time after setTime is ";
63 cout << "nStandard time after setTime is ";
65
66 t.setTime( 99, 99, 99 ); // attempt invalid settings
67 cout << "nnAfter attempting invalid settings:"
68 << "nMilitary time: ";
70 cout << "nStandard time: ";
72 cout << endl;
73 return 0;
74 }
The initial military time is 00:00
The initial standard time is 12:00:00 AM
Military time after setTime is 13:27
Standard time after setTime is 1:27:06 PM
After attempting invalid settings:
Military time: 00:00
Standard time: 12:00:00 AM

Implementing a Time ADT with a Class
• Destructors
– Functions with the same name as the class but preceded
with a tilde character (~)
– Cannot take arguments and cannot be overloaded
– Performs “termination housekeeping”
• Binary scope resolution operator (::)
– Combines the class name with the member function
name
– Different classes can have member functions with the
same name
• Format for defining member functions
ReturnType ClassName::MemberFunctionName( ){
…
}

• If a member function is defined inside the class
– Scope resolution operator and class name are not
needed
– Defining a function outside a class does not change
it being public or private
• Classes encourage software reuse
– Inheritance allows new classes to be derived from
old ones
Implementing a Time ADT with a Class

Class Scope and Accessing Class
Members
• Class scope
– Data members and member functions
• File scope
– Nonmember functions
• Inside a scope
– Members accessible by all member functions
• Referenced by name
• Outside a scope
– Members are referenced through handles
• An object name, a reference to an object or a pointer to an object

Class Scope and Accessing Class
Members
• Function scope
– Variables only known to function they are defined in
– Variables are destroyed after function completion
• Accessing class members
– Same as structs
– Dot (.) for objects and arrow (->) for pointers
– Example:
• t.hour is the hour element of t
• TimePtr->hour is the hour element

1 // Fig. 6.4: fig06_04.cpp
2 // Demonstrating the class member access operators . and ->
3 //
4 // CAUTION: IN FUTURE EXAMPLES WE AVOID PUBLIC DATA!
6
7 using std::cout;
8 using std::endl;
9
10 // Simple class Count
11 class Count {
12 public:
13 int x;
14 void print() { cout << x << endl; }
15 };
16
17 int main()
18 {
19 Count counter, // create counter object
20 *counterPtr = &counter, // pointer to counter
21 &counterRef = counter; // reference to counter
22
23 cout << "Assign 7 to x and print using the object's name: ";
24 counter.x = 7; // assign 7 to data member x
25 counter.print(); // call member function print
26
27 cout << "Assign 8 to x and print using a reference: ";
28 counterRef.x = 8; // assign 8 to data member x
29 counterRef.print(); // call member function print
30

31 cout << "Assign 10 to x and print using a pointer: ";
32 counterPtr->x = 10; // assign 10 to data member x
33 counterPtr->print(); // call member function print
34 return 0;
35 }
Assign 7 to x and print using the object's name: 7
Assign 8 to x and print using a reference: 8
Assign 10 to x and print using a pointer: 10

Separating Interface from
Implementation
• Separating interface from implementation
– Makes it easier to modify programs
– Header files
• Contains class definitions and function prototypes
– Source-code files
• Contains member function definitions

1 // Fig. 6.5: time1.h
2 // Declaration of the Time class.
3 // Member functions are defined in time1.cpp
4
5 // prevent multiple inclusions of header file
6 #ifndef TIME1_H
7 #define TIME1_H
8
9 // Time abstract data type definition
10 class Time {
11 public:
12 Time(); // constructor
16 private:
17 int hour; // 0 - 23
18 int minute; // 0 - 59
19 int second; // 0 - 59
20 };
21
22 #endif

23 // Fig. 6.5: time1.cpp
24 // Member function definitions for Time class.
26
27 using std::cout;
28
29 #include "time1.h"
30
31 // Time constructor initializes each data member to zero.
32 // Ensures all Time objects start in a consistent state.
33 Time::Time() { hour = minute = second = 0; }
34
35 // Set a new Time value using military time. Perform validity
36 // checks on the data values. Set invalid values to zero.
38 {
39 hour = ( h >= 0 && h < 24 ) ? h : 0;
40 minute = ( m >= 0 && m < 60 ) ? m : 0;
41 second = ( s >= 0 && s < 60 ) ? s : 0;
42 }
43
44 // Print Time in military format
45 void Time::printMilitary()
46 {
47 cout << ( hour < 10 ? "0" : "" ) << hour << ":"
48 << ( minute < 10 ? "0" : "" ) << minute;
49 }
50
51 // Print time in standard format
52 void Time::printStandard()
53 {
54 cout << ( ( hour == 0 || hour == 12 ) ? 12 : hour % 12 )
55 << ":" << ( minute < 10 ? "0" : "" ) << minute
56 << ":" << ( second < 10 ? "0" : "" ) << second
57 << ( hour < 12 ? " AM" : " PM" );
58 }
Source file uses #include
to load the header file
Source file contains
function definitions

Controlling Access to Members
• public
– Presents clients with a view of the services the class
provides (interface)
– Data and member functions are accessible
• private
– Default access mode
– Data only accessible to member functions and friends
– private members only accessible through the public
class interface using public member functions

1 // Fig. 6.6: fig06_06.cpp
2 // Demonstrate errors resulting from attempts
3 // to access private class members.
5
6 using std::cout;
7
9
10 int main()
11 {
12 Time t;
13
14 // Error: 'Time::hour' is not accessible
15 t.hour = 7;
16
17 // Error: 'Time::minute' is not accessible
18 cout << "minute = " << t.minute;
19
20 return 0;
21 }
Compiling...
Fig06_06.cpp
D:Fig06_06.cpp(15) : error C2248: 'hour' : cannot access private
member declared in class 'Time'
D:Fig6_06time1.h(18) : see declaration of 'hour'
D:Fig06_06.cpp(18) : error C2248: 'minute' : cannot access private
member declared in class 'Time'
D:time1.h(19) : see declaration of 'minute'
Error executing cl.exe.
test.exe - 2 error(s), 0 warning(s)
Attempt to access private member
variable minute.
Attempt to modify private member
variable hour.

Access Functions and Utility Functions
• Utility functions
– private functions that support the operation of
public functions
– Not intended to be used directly by clients
• Access functions
– public functions that read/display data or check
conditions
– Allow public functions to check private data
• Following example
– Program to take in monthly sales and output the total
– Implementation not shown, only access functions

87 // Fig. 6.7: fig06_07.cpp
88 // Demonstrating a utility function
89 // Compile with salesp.cpp
90 #include "salesp.h"
91
92 int main()
93 {
94 SalesPerson s; // create SalesPerson object s
95
96 s.getSalesFromUser(); // note simple sequential code
97 s.printAnnualSales(); // no control structures in main
98 return 0;
99 }
OUTPUT
Enter sales amount for month 1: 5314.76
The total annual sales are: $60120.59
Create object s, an instance
of class SalesPerson

Class definition
class class_name {
public:
constructor and destructor
member functions
private:
data members
};

Initializing Class Objects: Constructors
• Constructors
– Initialize class members
– Same name as the class
– No return type
– Member variables can be initialized by the constructor or
set afterwards
• Passing arguments to a constructor
– When an object of a class is declared, initializers can be
provided
– Format of declaration with initializers:
Class-type ObjectName( value1,value2,…);
– Default arguments may also be specified in the
constructor prototype

1 // Fig. 6.8: time2.h
3 // Member functions are defined in time2.cpp
4
5 // preprocessor directives that
7 #ifndef TIME2_H
8 #define TIME2_H
9
10 // Time abstract data type definition
11 class Time {
12 public:
13 Time( int = 0, int = 0, int = 0 ); // default constructor
17 private:
18 int hour; // 0 - 23
19 int minute; // 0 - 59
20 int second; // 0 - 59
21 };
22
23 #endif

61 // Fig. 6.8: fig06_08.cpp
62 // Demonstrating a default constructor
63 // function for class Time.
65
66 using std::cout;
67 using std::endl;
68
70
71 int main()
72 {
73 Time t1, // all arguments defaulted
74 t2(2), // minute and second defaulted
75 t3(21, 34), // second defaulted
76 t4(12, 25, 42), // all values specified
77 t5(27, 74, 99); // all bad values specified
78
79 cout << "Constructed with:n"
80 << "all arguments defaulted:n ";
81 t1.printMilitary();
82 cout << "n ";
83 t1.printStandard();
84
85 cout << "nhour specified; minute and second defaulted:"
86 << "n ";
88 cout << "n ";
90
91 cout << "nhour and minute specified; second defaulted:"
92 << "n ";

OUTPUT
Constructed with:
all arguments defaulted:
00:00
12:00:00 AM
hour specified; minute and second defaulted:
02:00
2:00:00 AM
hour and minute specified; second defaulted:
21:34
9:34:00 PM
hour, minute, and second specified:
12:25
12:25:42 PM
all invalid values specified:
00:00
12:00:00 AM
When only hour
is specified,
minute and
second are set
to their default
values of 0.
94 cout << "n ";
96
97 cout << "nhour, minute, and second specified:"
98 << "n ";
100 cout << "n ";
102
103 cout << "nall invalid values specified:"
104 << "n ";
106 cout << "n ";
108 cout << endl;
109
110 return 0;
111}

Using Destructors
• Destructors
– Are member function of class
– Perform termination housekeeping before the system
reclaims the object’s memory
– Complement of the constructor
– Name is tilde (~) followed by the class name (i.e.,
~Time)
• Recall that the constructor’s name is the class name
– Receives no parameters, returns no value
– One destructor per class
• No overloading allowed

When Constructors and Destructors Are
Called
• Constructors and destructors called automatically
– Order depends on scope of objects
• Global scope objects
– Constructors called before any other function (including main)
– Destructors called when main terminates (or exit function
called)
– Destructors not called if program terminates with abort
• Automatic local objects
– Constructors called when objects are defined
– Destructors called when objects leave scope
• i.e., when the block in which they are defined is exited
– Destructors not called if the program ends with exit or abort

• Static local objects
– Constructors called when execution reaches the
point where the objects are defined
– Destructors called when main terminates or
the exit function is called
– Destructors not called if the program ends with
abort
When Constructors and Destructors Are
Called

1 // Fig. 6.9: create.h
2 // Definition of class CreateAndDestroy.
3 // Member functions defined in create.cpp.
4 #ifndef CREATE_H
5 #define CREATE_H
6
7 class CreateAndDestroy {
8 public:
9 CreateAndDestroy( int ); // constructor
10 ~CreateAndDestroy(); // destructor
11 private:
12 int data;
13 };
14
15 #endif

16 // Fig. 6.9: create.cpp
17 // Member function definitions for class CreateAndDestroy
19
20 using std::cout;
21 using std::endl;
22
23 #include "create.h"
24
25 CreateAndDestroy::CreateAndDestroy( int value )
26 {
27 data = value;
28 cout << "Object " << data << " constructor";
29 }
30
31 CreateAndDestroy::~CreateAndDestroy()
32 { cout << "Object " << data << " destructor " << endl; }
Constructor and Destructor changed to
print when they are called.

33 // Fig. 6.9: fig06_09.cpp
34 // Demonstrating the order in which constructors and
35 // destructors are called.
37
38 using std::cout;
39 using std::endl;
40
41 #include "create.h"
42
43 void create( void ); // prototype
44
45 CreateAndDestroy first( 1 ); // global object
46
47 int main()
48 {
49 cout << " (global created before main)" << endl;
50
51 CreateAndDestroy second( 2 ); // local object
52 cout << " (local automatic in main)" << endl;
53
54 static CreateAndDestroy third( 3 ); // local object
55 cout << " (local static in main)" << endl;
56
57 create(); // call function to create objects
58
59 CreateAndDestroy fourth( 4 ); // local object
60 cout << " (local automatic in main)" << endl;
61 return 0;
62 }

63
64 // Function to create objects
65 void create( void )
66 {
67 CreateAndDestroy fifth( 5 );
68 cout << " (local automatic in create)" << endl;
69
70 static CreateAndDestroy sixth( 6 );
71 cout << " (local static in create)" << endl;
72
73 CreateAndDestroy seventh( 7 );
74 cout << " (local automatic in create)" << endl;
75 }
OUTPUT
Object 1 constructor (global created before main)
Object 2 constructor (local automatic in main)
Object 3 constructor (local static in main)
Object 5 constructor (local automatic in create)
Object 6 constructor (local static in create)
Object 7 constructor (local automatic in create)
Object 7 destructor
Object 5 destructor
Object 4 constructor (local automatic in main)
Object 4 destructor
Object 2 destructor
Object 6 destructor
Object 3 destructor
Object 1 destructor
Notice how the order of the
constructor and destructor call
depends on the types of variables
(automatic, global and static)
they are associated with.

Using Data Members and Member
Functions
• Member functions
– Allow clients of the class to set (i.e., write) or get (i.e., read)
the values of private data members
– Example:
Adjusting a customer’s bank balance
• private data member balance of a class BankAccount
could be modified through the use of member function
computeInterest
• A member function that sets data member interestRate could
be called setInterestRate, and a member function that returns
the interestRate could be called getInterestRate
– Providing set and get functions does not make private
variables public
– A set function should ensure that the new value is valid

A Subtle Trap: Returning a Reference to
a Private Data Member
• Reference to an object
– Alias for the name of the object
– May be used on the left side of an assignment statement
– Reference can receive a value, which changes the
original object as well
• Returning references
– public member functions can return non-const
references to private data members
• Should be avoided, breaks encapsulation

1 // Fig. 6.11: time4.h
3 // Member functions defined in time4.cpp
4
5 // preprocessor directives that
7 #ifndef TIME4_H
8 #define TIME4_H
9
10 class Time {
11 public:
12 Time( int = 0, int = 0, int = 0 );
13 void setTime( int, int, int );
14 int getHour();
15 int &badSetHour( int ); // DANGEROUS reference return
16 private:
17 int hour;
18 int minute;
19 int second;
20 };
21
22 #endif
Notice how member function
badSetHour returns a reference
(int & is the return type).

1. Load header
1.1 Function definitions
23 // Fig. 6.11: time4.cpp
24 // Member function definitions for Time class.
26
27 // Constructor function to initialize private data.
28 // Calls member function setTime to set variables.
29 // Default values are 0 (see class definition).
30 Time::Time( int hr, int min, int sec )
31 { setTime( hr, min, sec ); }
32
33 // Set the values of hour, minute, and second.
35 {
36 hour = ( h >= 0 && h < 24 ) ? h : 0;
37 minute = ( m >= 0 && m < 60 ) ? m : 0;
38 second = ( s >= 0 && s < 60 ) ? s : 0;
39 }
40
41 // Get the hour value
42 int Time::getHour() { return hour; }
43
44 // POOR PROGRAMMING PRACTICE:
45 // Returning a reference to a private data member.
46 int &Time::badSetHour( int hh )
47 {
48 hour = ( hh >= 0 && hh < 24 ) ? hh : 0;
49
50 return hour; // DANGEROUS reference return
51 }
badSetHour returns a
reference to the private
member variable hour.
Changing this reference
will alter hour as well.

52 // Fig. 6.11: fig06_11.cpp
53 // Demonstrating a public member function that
54 // returns a reference to a private data member.
55 // Time class has been trimmed for this example.
57
58 using std::cout;
59 using std::endl;
60
62
63 int main()
64 {
65 Time t;
66 int &hourRef = t.badSetHour( 20 );
67
68 cout << "Hour before modification: " << hourRef;
69 hourRef = 30; // modification with invalid value
70 cout << "nHour after modification: " << t.getHour();
71
72 // Dangerous: Function call that returns
73 // a reference can be used as an lvalue!
74 t.badSetHour(12) = 74;
75 cout << "nn*********************************n"
76 << "POOR PROGRAMMING PRACTICE!!!!!!!!n"
77 << "badSetHour as an lvalue, Hour: "
78 << t.getHour()
79 << "n*********************************" << endl;
80
81 return 0;
82 }
Hour after modification: 30
*********************************
POOR PROGRAMMING PRACTICE!!!!!!!!
badSetHour as an lvalue, Hour: 74
*********************************

Program Output
Hour before modification: 20
Hour after modification: 30
*********************************
POOR PROGRAMMING PRACTICE!!!!!!!!
badSetHour as an lvalue, Hour: 74
*********************************

Assignment by Default Memberwise
Copy
• Assigning objects
– An object can be assigned to another object of
the same type using the assignment operator (=)
– Member by member copy
• Objects may be
– Passed as function arguments
– Returned from functions (call-by-value default)

1 // Fig. 6.12: fig06_12.cpp
2 // Demonstrating that class objects can be assigned
3 // to each other using default memberwise copy
5
6 using std::cout;
7 using std::endl;
8
9 // Simple Date class
10 class Date {
11 public:
12 Date( int = 1, int = 1, int = 1990 ); // default constructor
13 void print();
14 private:
15 int month;
16 int day;
17 int year;
18 };
19
20 // Simple Date constructor with no range checking
21 Date::Date( int m, int d, int y )
22 {
23 month = m;
24 day = d;
25 year = y;
26 }
27
28 // Print the Date in the form mm-dd-yyyy
29 void Date::print()
30 { cout << month << '-' << day << '-' << year; }

31
32 int main()
33 {
34 Date date1( 7, 4, 1993 ), date2; // d2 defaults to 1/1/90
35
36 cout << "date1 = ";
37 date1.print();
38 cout << "ndate2 = ";
39 date2.print();
40
41 date2 = date1; // assignment by default memberwise copy
42 cout << "nnAfter default memberwise copy, date2 = ";
43 date2.print();
44 cout << endl;
45
46 return 0;
47 }
date1 = 7-4-1993
date2 = 1-1-1990
After default memberwise copy, date2 = 7-4-1993

Software Reusability
• Software resusability
– Implementation of useful classes
– Class libraries exist to promote reusability
• Allows for construction of programs from existing, well-
defined, carefully tested, well-documented, portable, widely
available components
– Speeds development of powerful, high-quality
software

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