Classes and object

1
• A pointer variable is a variable whose value is the
address of a location in memory
int x;
x = 5;
int* ptr1;
ptr1 = &x;
int* ptr2;
ptr2 = ptr1;
*ptr1 = 6;
cout << ptr1 << endl;
cout << *ptr2 << endl;
Review: Pointers & Dynamic Data
int* ptr3;
ptr3 = new int;
*ptr3 = 5;
delete ptr3;
ptr3 = NULL;
int *ptr4;
ptr4 = new int[5];
ptr4[0] = 100;
ptr4[4] = 123;
delete [] ptr4;
ptr4 = NULL;

2
void increment(int b1, int &b2, int *b3)
{
b1 += 2;
b2 += 2
*b3 += 2;
}
Review: Reference Types
• Reference Types
– Alias for another variable
– Must be initialized when declared
– Are primarily used as function parameters
int main (void){
int a1 = 5, a2 = 10;
int *a3 = new int;
*a3 = 15;
int &a4 = a3;
cout << a1 << a2 << a3 << endl;
increment(a1, a2, a3);
cout << a1 << a2 << a3 << endl;
delete a3; a3 = NULL;
return 0;
}

3
Object-Oriented Programming
Introduction to Classes
• Class Definition
• Class Examples
• Objects
• Constructors
• Destructors

4
Class
• The class is the cornerstone of C++
– It makes possible encapsulation, data hiding and inheritance
• Type
– Concrete representation of a concept
• Eg. float with operations like -, *, + (math real numbers)
• Class
– A user defined type
– Consists of both data and methods
– Defines properties and behavior of that type
• Advantages
– Types matching program concepts
• Game Program (Explosion type)
– Concise program
– Code analysis easy
– Compiler can detect illegal uses of types
• Data Abstraction
– Separate the implementation details from its essential properties

5
class Rectangle
{
private:
int width;
int length;
public:
void set(int w, int
l);
int area();
};
Classes & Objects
Rectangle r1;
Rectangle r2;
Rectangle r3;
……
int a;
Objects: Instance of a class

6
Define a Class Type
class class_name
{
permission_label:
member;
permission_label:
member;
...
};
class Rectangle
{
private:
int width;
int length;
public:
void set(int w, int
l);
int area();
};
Body
Header

7
Class Definition
Data Members
• Can be of any type, built-in or user-defined
• non-static data member
– Each class object has its own copy
• static data member
– Acts as a global variable
– One copy per class type, e.g. counter

8
class Rectangle
{
private:
int width;
int length;
static int count;
public:
void set(int w, int
l);
int area();
}
Static Data Member
Rectangle r1;
Rectangle r2;
Rectangle r3;
width
length
width
length
width
length
r1
r3
r2
count

9
Class Definition
Member Functions
• Used to
– access the values of the data members (accessor)
– perform operations on the data members
(implementor)
• Are declared inside the class body
• Their definition can be placed inside the class
body, or outside the class body
• Can access both public and private members of
the class
• Can be referred to using dot or arrow member
access operator

10
Define a Member Function
class Rectangle
{
private:
int width, length;
public:
void set (int w, int l);
int area() {return width*length; }
};
void Rectangle :: set (int w, int l)
{
width = w;
length = l;
}
inline
class name
member function name
scope operator
r1.set(5,8);
rp->set(8,10);

11
• const member function
– declaration
• return_type func_name (para_list) const;
– definition
• return_type func_name (para_list) const { … }
• return_type class_name :: func_name (para_list) const { … }
– Makes no modification about the data members (safe
function)
– It is illegal for a const member function to modify a
class data member
Class Definition
Member Functions

12
Const Member Function
class Time
{
private :
int hrs, mins, secs ;
public :
void Write ( )
const ;
} ;
void Time :: Write( ) const
{
cout <<hrs << “:” << mins << “:” << secs << endl;
}
function declaration
function definition

13
• Information hiding
– To prevent the internal representation from direct
access from outside the class
• Access Specifiers
– public
• may be accessible from anywhere within a program
– private
• may be accessed only by the member functions, and friends
of this class
– protected
• acts as public for derived classes
• behaves as private for the rest of the program
Class Definition - Access Control

14
class Time Specification
class Time
{
public :
void Set ( int hours , int minutes , int seconds ) ;
void Increment ( ) ;
void Write ( ) const ;
Time ( int initHrs, int initMins, int initSecs ) ; // constructor
Time ( ) ; // default constructor
private :
int hrs ;
int mins ;
int secs ;
} ;
14

15
Class Interface Diagram
Private data:
hrs
mins
secs
Set
Increment
Write
Time
Time
Time class

16
• The default access specifier is private
• The data members are usually private or protected
• A private member function is a helper, may only be
accessed by another member function of the same
class (exception friend function)
• The public member functions are part of the class
interface
• Each access control section is optional, repeatable,
and sections may occur in any order
Class Definition
Access Control

17
What is an object?
OBJECT
Operations
Data
set of methods
(member functions)
internal state
(values of private data members)

18
class Rectangle
{
private:
int width;
int length;
public:
void set(int w, int
l);
int area();
};
Declaration of an Object
main()
{
Rectangle r1;
Rectangle r2;
r1.set(5, 8);
cout<<r1.area()<<endl;
r2.set(8,10);
cout<<r2.area()<<endl;
}

19
Another Example
#include <iostream.h>
class circle
{
private:
double radius;
public:
void store(double);
double area(void);
void display(void);
};
// member function definitions
void circle::store(double r)
{
radius = r;
}
double circle::area(void)
{
return 3.14*radius*radius;
}
void circle::display(void)
{
cout << “r = “ << radius << endl;
}
int main(void) {
circle c; // an object of circle class
c.store(5.0);
cout << "The area of circle c is " << c.area() << endl;
c.display();
}

20
class Rectangle
{
private:
int width;
int length;
public:
void set(int w, int
l);
int area();
};
main()
{
Rectangle r1;
r1.set(5, 8);
}
r1 is statically allocated
width
length
r1
width = 5
length = 8

21
class Rectangle
{
private:
int width;
int length;
public:
void set(int w, int
l);
int area();
};
main()
{
Rectangle r1;
r1.set(5, 8);
Rectangle *r2;
r2 = &r1;
r2->set(8,10);
}
r2 is a pointer to a Rectangle object
width
length
r1
width = 5
length = 8
5000
???
r2
6000
5000
width = 8
length =
10
//dot notation
//arrow notation

22
class Rectangle
{
private:
int width;
int length;
public:
void set(int w, int
l);
int area();
};
main()
{
Rectangle *r3;
r3 = new Rectangle();
r3->set(80,100);
delete r3;
r3 = NULL;
}
r3 is dynamically allocated
???
r3
6000
width
length
5000
5000
width = 80
length = 100
???NULL
//arrow notation

23
class circle
{
public:
double radius;
};
Object Initialization
int main()
{
circle c1; // Declare an instance of the class circle
c1.radius = 5; // Initialize by assignment
}
1. By Assignment
• Only work for public data
members
• No control over the operations
on data members

24
class circle
{
private:
double radius;
public:
void set (double r)
{radius = r;}
double get_r ()
{return radius;}
};
int main(void) {
circle c; // an object of circle class
c.set(5.0); // initialize an object with a public member function
cout << "The radius of circle c is " << c.get_r() << endl;
// access a private data member with an accessor
}
2. By Public Member Functions

25
class Rectangle
{
private:
int width;
int length;
public:
void set(int w, int
l);
int area();
}
main()
{
Rectangle r1;
r1.set(5, 8);
Rectangle *r2;
r2 = &r1;
r2->set(8,10);
}
r2 is a pointer to a Rectangle object
//dot notation
//arrow notation
r1 and r2 are both initialized by
public member function set

26
class Rectangle
{
private:
int width;
int length;
public:
Rectangle();
Rectangle(const Rectangle
&r);
Rectangle(int w, int l);
void set(int w, int l);
int area();
}
3. By Constructor
• Default constructor
• Copy constructor
• Constructor with parameters
There is no return type
Are used to initialize class data
members
Have the same name as the class
They are publicly accessible
They have different signatures

27
class Rectangle
{
private:
int width;
int length;
public:
int area();
};
• Default constructor
When a class is declared with no constructors,
the compiler automatically assumes default
constructor and copy constructor for it.
Rectangle :: Rectangle() { };
• Copy constructor
Rectangle :: Rectangle (const
Rectangle & r)
{
width = r.width; length = r.length;
};

28
class Rectangle
{
private:
int width;
int length;
public:
int area();
}
• Initialize with default constructor
Rectangle r1;
Rectangle *r3 = new Rectangle();
• Initialize with copy constructor
Rectangle r4;
r4.set(60,80);
Rectangle r5 = r4;
Rectangle r6(r4);
Rectangle *r7 = new Rectangle(r4);

29
class Rectangle
{
private:
int width;
int length;
public:
Rectangle(int w, int l)
{width =w; length=l;}
int area();
}
If any constructor with any number of
parameters is declared, no default
constructor will exist, unless you
define it.
Rectangle r4; // error
• Initialize with constructor
Rectangle r5(60,80);
Rectangle *r6 = new Rectangle(60,80);

30
class Rectangle
{
private:
int width;
int length;
public:
Rectangle();
Rectangle(int w, int l);
int area();
}
Write your own constructors
Rectangle :: Rectangle()
{
width = 20;
length = 50;
};
Rectangle *r7 = new Rectangle();
width
length
width = 20
length = 50
5000???
r7
6000
5000

31
class Account
{
private:
char *name;
double balance;
unsigned int id;
public:
Account();
Account(const Account
&a);
Account(const char
*person);
}
With constructors, we have more
control over the data members
Account :: Account()
{
name = NULL; balance = 0.0;
id = 0;
};
Account :: Account(const Account &a)
{
name = new char[strlen(a.name)+1];
strcpy (name, a.name);
balance = a.balance;
id = a.id;
};
Account :: Account(const char *person)
{
name = new char[strlen(person)+1];
strcpy (name, person);
balance = 0.0;
id = 0;
};

32
So far, …
• An object can be initialized by a class
constructor
– default constructor
– copy constructor
– constructor with parameters
• Resources are allocated when an object is
initialized
• Resources should be revoked when an
object is about to end its lifetime

33
Cleanup of An Object
class Account
{
private:
char *name;
double balance;
unsigned int id; //unique
public:
Account();
Account(const Account
&a);
Account(const char
*person);
~Account();
}
Destructor
Account :: ~Account()
{
delete[] name;
}
• Its name is the class name
preceded by a ~ (tilde)
• It has no argument
• It is used to release dynamically
allocated memory and to perform
other "cleanup" activities
• It is executed automatically when
the object goes out of scope

34
Putting Them Together
class Str
{
char *pData;
int nLength;
public:
//constructors
Str();
Str(char *s);
Str(const Str &str);
//accessors
char* get_Data();
int get_Len();
//destructor
~Str();
};
Str :: Str() {
pData = new char[1];
*pData = ‘0’;
nLength = 0;
};
Str :: Str(const Str &str) {
int n = str.nLength;
pData = new char[n+1];
nLength = n;
strcpy(pData,str.pData);
};
Str :: Str(char *s) {
pData = new
char[strlen(s)+1];
strcpy(pData, s);
nLength = strlen(s);
};

35
class Str
{
char *pData;
int nLength;
public:
//constructors
Str();
Str(char *s);
//accessors
char* get_Data();
int get_Len();
//destructor
~Str();
};
char* Str :: get_Data()
{
return pData;
};
Str :: ~Str()
{
delete[] pData;
};
int Str :: get_Len()
{
return nLength;
};

36
class Str
{
char *pData;
int nLength;
public:
//constructors
Str();
Str(char *s);
//accessors
char* get_Data();
int get_Len();
//destructor
~Str();
};
int main()
{
int x=3;
Str *pStr1 = new Str(“Joe”);
Str *pStr2 = new Str();
}

38
Working with Multiple Files
• To improve the readability, maintainability and
reusability, codes are organized into modules.
• When working with complicated codes,
– A set of .cpp and .h files for each class groups
• .h file contains the prototype of the class
• .cpp contains the definition/implementation of the class
– A .cpp file containing main() function, should include
all the corresponding .h files where the functions used
in .cpp file are defined

39
Example : time.h
// SPECIFICATION FILE ( time .h )
// Specifies the data members and
// member functions prototypes.
#ifndef _TIME_H
#define _TIME_H
class Time
{
public:
. . .
private:
. . .
} ;
#endif

40
// IMPLEMENTATION FILE ( time.cpp )
// Implements the member functions of class Time
#include “ time.h” // also must appear in client code
… …
bool Time :: Equal ( Time otherTime ) const
// Function value == true, if this time equals otherTime
// == false , otherwise
{
return ( (hrs == otherTime.hrs) && (mins == otherTime.mins)
&& (secs == otherTime.secs) ) ;
}
. . .
Example : time.cpp

41
Example : main.cpp
// Client Code ( main.cpp )
#include “ time.h”
// other functions, if any
int main()
{
… …
}
Compile and Run
g++ -o mainExec main.cpp time.cpp

42
Separate Compilation and Linking of Files
time.h
main.cpp time.cpp
main.o
mainExec
time.o
Compiler Compiler
Linker
#include “time.h”
implementation file
specification file
main program

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