lecture-18staticdatamember-160705095116.pdf

Static Data Members
Muhammad Hammad Waseem
m.hammad.wasim@gmail.com

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.

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.

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

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.

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.

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.

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;
}
};

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.

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.

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;
}
};

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();
}
OUTPUT
• Enter name: Farhan Khan
• Enter marks: 600
• Enter name: Zeeshan
• Enter name: Ali
• Roll no: 1
• Name: Farhan Khan
• Marks: 600
• Roll no: 2
• Name: Farhan Khan
• Marks: 786
• Roll no: 3
• Name: Ali
• Marks: 567

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.

lecture-18staticdatamember-160705095116.pdf

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