classess

1. 4. Classes and Objects In C+
+

2. Introduction
 A structure in C is a collection of similar &
dissimilar data types. C++ extends the reach of
structures by allowing the inclusion of even
functions within structures
 Placing data & functions together into a single
entity is the central idea in object oriented
programming.

3. Classes & Objects In C++
 There is almost no difference in the syntax of a
structure and a class, hence at least in principle they
can be used interchangeably.
 But most C++ programmers use structures to
exclusively hold data and classes to hold both data &
functions.

4. Object
 An object is an instance of a class, and the process
of creating an object is called instantiation.
 rectangle r1,r2,r3 //known as instance variables.
 In simplest terms a class is like a data type, where as
an object is like a variable of that data type.

5. Specifying a Class
 Generally, a class specification has two parts,
 Class Declaration
 Class function definitions.
class class_name
{
private:
variable declarations;
function declarations;
public:
variable declarations;
function declarations;
}

6. Defining member functions
 Member functions can be defined in two places.
 Outside the class definition
 Inside the class definition
Outside the class definition
Return-type class-name:: function-name (argument declaration)
{
function body
}

7. #include<iostream>
using namespace std;
Class rectangle
{
private:
int len,br;
public:
void getdata()
{
cout<<“Enter length & breadth”;
cin>>len>>br;
}
void setdata(int l,int b)
{
len=l;
br=b;
}
void displaydata()
{
cout<<“length= “<<len;
cout<<“breadth=“<<br;
}

8. void area_peri()
{
int a,p;
a=len*br;
p=2*(len+br);
cout<<“area=“<<a;
cout<<“perimeter=“<<p;
}
};
int main()
{
rectangle r1,r2,r3; //define 3 objects of class rectangle
r1.setdata(10,20); //set data in elements of the objects
r1.displaydata(); //display the data set by setdata()
r1.area_peri(); //calculate and print area & perimeter
return 0;
}

9. From example
 Class declaration. // class rectangle
 Class is delimited by braces and terminated by a
semicolon.
 The body of the class contains two unfamiliar
keywords:
 private & public, they are used in C++ to implement a
concept called data hiding.
 In the class the data items len & br follow the
keyword private, so they can be accessed from
within the class, but not from outside it.

10. From example
 A function declared within the definition of a class is
called a “member function” or “method”. The two
terms are interchangeable; member function is a C+
+ term & method is a general OOP’s term.
 Usually the data within a class is private and the
functions are public.

11. From example
 The first statement in main()
 Rectangle r1,r2,r3;
 Defining an object is similar to defining a variable of
any data type: space is set aside for it in memory.
 To use a member function, the dot operator connects
the object name and the member function.
 r1.setdata(10,20);
 The dot operator is also called “class member access
operator”.

12. Static Data Members
 A static member variable has certain special characteristics. These are
 It is initialized to zero when the first object of its class is created.

Only one copy of the member is created for the entire class and is shared
by all the objects of that class, no matter how many objects are created.
 It is visible only within the class, but its lifetime is the entire program.
int item :: count; //definition of static data member

13. Static Member Functions
 A member function that is declared static has the following properties
 A static function can have access to only other static members (functions or
variables) declared in the same class.
 A static member function can be called using the class name (instead of its
objects) as follows
class name:: function-name;

14. Arrays of Objects
 We can have arrays of variables that are of the type class. Such
variables are called arrays of objects
class employee
{
char name[30];
float age;
public:
void getdata(void);
void putdata(void);
};
employee manager[3]; //array of manager
employee worker[75]; //array of workers

15. Objects as Function Arguments
 Like any other data type, an object may be used as a function
argument.
Example

16. Friendly functions
 Private members cannot be accessed from outside the class.
 That is, a non-member function cannot have an access to the private
data of a class.
In such situations, C++ allows the function to be friend with the both
classes, thereby allowing the function to have access to the private
data of these classes.

17. Friendly functions
 General form to write friend function
Class ABC
{
…….
…….
public:
…….
…….
friend void xyz(void); //declaration
};

18. Friendly function
 A friend function possesses certain special characteristics.
 It is not in the scope of the class to which it has been declared as friend.
 Since it is not in the scope of the class, it cannot be called using the object
of that class
 It can be invoked like a normal function without the help of any object.
 Unlike member functions, it cannot access the member names directly and
has to use an object name and dot membership operator with each member
name.
 It can be declared either in the public or the private part of a class without
affecting its meaning.
 Usually, it has the objects as arguments.