WE are doing programming in C++

1. Static Data Member
• A type of data member that is shared among all objects of class is
known as static data member.
• The static data member is defined in the class with static keyword.
• When a data member is defined as static, only one variable is created
in the memory even if there are many objects of that class.
• A static data item is useful when all objects of the same class must
share a common item of information.
• The characteristics of a static data member are same as normal static
variable.

2. Static Data Member
• It is visible only in the class, in which it is defined but its lifetime
• Starts when the program starts its execution.
• Ends when the entire program is terminated.
• It is normally used to share some data among all objects of a particular
class.
• The main difference between normal data member and static data
member is that
• each object has its own variable of normal data member.
• On the other hand, static data member is shared among all objects of the class.
• Only one memory location is created for static data member that is shared
among all objects.

3. Difference Between Normal & Static Data
Members
Object with three normal data
members
Object with two normal data members
and one static data member
200
n
1
a
10
b
Object b1
2
a
20
b
Object b2
1
a
10
b
100
n
Object b1
2
a
20
b
200
n
Object b2

4. Uses of Static Class Data
• Why would you want to use static member data?
• As an example, suppose an object needed to know how many other
objects of its class were in the program.
• for example :
• In a road-racing game, a race car might want to know how many other cars
are still in the race.
• In this case a static variable count could be included as a member of
the class. All the objects would have access to this variable.
• It would be the same variable for all of them; they would all see the
same count.

5. Separate Declaration and Definition
• Static member data requires an unusual format.
• Ordinary variables are usually declared and defined in the same
statement.
• Static member data, on the other hand, requires two separate
statements.
• The variable’s declaration appears in the class definition, but the
• Variable is defined outside the class, in much the same way as a global
variable.
• Why is this two-part approach used?
• If static member data were defined inside the class, it would violate the idea
that a class definition is only a blueprint and does not set aside any memory.

6. Separate Declaration and Definition
• Putting the definition of static member data outside the class also
serves to emphasize that
• the memory space for such data is allocated only once, before the program
starts to execute, and that
• one static member variable is accessed by an entire class; each object does
not have its own version of the variable, as it would with ordinary member
data.
• In this way a static member variable is more like a global variable.

7. Write a program that counts the number of
objects created of a particular class (1/2)
class yahoo
{
private:
static int n;
public:
yahoo()
{ n++; }
void show()
{
cout<<“you have created ”<<n<<“ objects so far ”<<endl;
}
};

8. Write a program that counts the number of
objects created of a particular class (2/2)
int yahoo::n=0;
void main()
{
yahoo x,y;
x.show();
yahoo z;
x.show();
}
OUTPUT:
• You have created 2 objects so far.
• You have created 3 objects so far.

9. How it Works
• The program declares a static data member n to count the number of objects
that have been created.
• The statement int yahoo::n=0;defines the variable and initializes it to 0.
• The variable is defined outside the class because it will be not part of any object.
• It is created only once in the memory and is shared among all objects of the class.
• The variable definition outside the class must be preceded with class name and
scope resolution operator ::.
• The compiler does not display any error if the static data member is not defined.
• The linker will generate an error when the program is executed.
• The above program creates three objects x, y and z. each time an object is
created, the constructor is executed that increases the value of n by 1.

10. Write a program that creates three objects of class
student. Each of them must assigned a unique roll
number. (Hint: use static data member for unique roll number) (1/2)
class Student
{
private:
static int r;
int rno,marks;
char name[30];
public:
Student()
{ r++;
Rno =r; }
void in()
{
cout<<“enter name:”;
gets(name);
cout<<“enter marks:”;
cin>>marks;
}
void show()
{
cout<<“Roll No:”<<rno<<endl;
cout<<“Name:”<<name<<endl;
cout<<“Marks:”<<marks<<endl;
}
};

11. Write a program that creates three objects of class
student. Each of them must assigned a unique roll
number. (Hint: use static data member for unique roll number) (2/2)
int Student::r=0;
void main()
{
Student s1,s2,s3;
s1.in();
s2.in();
s3.in();
s1.show();
s2.show();
s3.show();
}

12. How it Works
• The above program uses a static data member r to assign unique roll
numbers to each object of the class Student.
• The static data member is initialized to 0.
• The constructor increments its value by 1 when an object is created
and then assigns the updated value of r to the data member rno.
• It ensures that each object gets a unique roll number.

13. Static member function:
• A member function that is declared static has the following properties:
 A static function can have access to only other static members(function or variable)
declared in the same class.
 A static member function can be called using the class name.
like, class_name :: Function_name();
test :: getdata();

14. • Static Member Function in C++
• Static Member Function in a class is the function that is declared as static because
of which function attains certain properties as defined below:
• A static member function is independent of any object of the class.
• A static member function can be called even if no objects of the class exist.
• A static member function can also be accessed using the class name through the
scope resolution operator.
• A static member function can access static data members and static member
functions inside or outside of the class.
• Static member functions have a scope inside the class and cannot access the
current object pointer.
• You can also use a static member function to determine how many objects of the
class have been created.

15. • Static members are frequently used to store information that is
shared by all objects in a class.
• For instance, you may keep track of the quantity of newly generated
objects of a specific class type using a static data member as a
counter. This static data member can be increased each time an
object is generated to keep track of the overall number of objects.

16. • // C++ Program to show the working of
• // static member functions
• #include <iostream>
• using namespace std;
• class Box
• {
• private:
• static int length;
• static int breadth;
• static int height;
•
• public:
•
• static void print()
• {
• cout << "The value of the length is: " << length << endl;
• cout << "The value of the breadth is: " << breadth << endl;
• cout << "The value of the height is: " << height << endl;
• }
• };

17. • // initialize the static data members
• int Box :: length = 10;
• int Box :: breadth = 20;
• int Box :: height = 30;
• // Driver Code
• int main()
• {
•
• Box b;
•
• cout << "Static member function is called through Object name: n" << endl;
• b.print();
•
• cout << "nStatic member function is called through Class name: n" << endl;
• Box::print();
•
• return 0;
• }