Classes and objects constructor and destructor

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”

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.
3 #include <iostream>
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! 5 #include <iostream>
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

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
8 #include "time1.h"
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.

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
2 // Declaration of the Time class.
3 // Member functions are defined in time2.cpp
4
5 // preprocessor directives that
6 // prevent multiple inclusions of header file
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
14 void setTime( int, int, int ); // set hour,
minute, second
15 void printMilitary(); // print military
time format
16 void printStandard(); // print standard
time format
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
69 #include "time2.h"
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

Classes and objects constructor and destructor

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