Static Data Members in C++

Static data members are class members that are declared using the static keyword. There is only one copy of the static data member in the class, even if there are many class objects. This is because all the objects share the static data member. The static data member is always initialized to zero when the first class object is created.

The syntax of the static data members is given as follows −

static data_type data_member_name;

In the above syntax, static keyword is used. The data_type is the C++ data type such as int, float etc. The data_member_name is the name provided to the data member.

A program that demonstrates the static data members in C++ is given as follows −

Example

#include <iostream>
#include<string.h>

using namespace std;
class Student {
   private:
   int rollNo;
   char name[10];
   int marks;
   public:
   static int objectCount;
   Student() {
      objectCount++;
   }

   void getdata() {
      cout << "Enter roll number: "<<endl;
      cin >> rollNo;
      cout << "Enter name: "<<endl;
      cin >> name;
      cout << "Enter marks: "<<endl;
      cin >> marks;
   }

   void putdata() {
      cout<<"Roll Number = "<< rollNo <<endl;
      cout<<"Name = "<< name <<endl;
      cout<<"Marks = "<< marks <<endl;
      cout<<endl;
   }
};
int Student::objectCount = 0;
int main(void) {
   Student s1;
   s1.getdata();
   s1.putdata();
   Student s2;

   s2.getdata();
   s2.putdata();
   Student s3;

   s3.getdata();
   s3.putdata();
   cout << "Total objects created = " << Student::objectCount << endl;
   return 0;
}

Output

The output of the above program is as follows −

Enter roll number: 1
Enter name: Mark
Enter marks: 78
Roll Number = 1
Name = Mark
Marks = 78

Enter roll number: 2
Enter name: Nancy
Enter marks: 55
Roll Number = 2
Name = Nancy
Marks = 55

Enter roll number: 3
Enter name: Susan
Enter marks: 90
Roll Number = 3
Name = Susan
Marks = 90
Total objects created = 3

In the above program, the class student has three data members denoting the student roll number, name and marks. The objectCount data member is a static data member that contains the number of objects created of class Student. Student() is a constructor that increments objectCount each time a new class object is created.

There are 2 member functions in class. The function getdata() obtains the data from the user and putdata() displays the data. The code snippet for this is as follows −

class Student {
   private:
   int rollNo;
   char name[10];
   int marks;
   public:
   static int objectCount;
   Student() {
      objectCount++;
   }

   void getdata() {
      cout << "Enter roll number: "<<endl;
      cin >> rollNo;
      cout << "Enter name: "<<endl;
      cin >> name;
      cout << "Enter marks: "<<endl;
      cin >> marks;
   }

   void putdata() {
      cout<<"Roll Number = "<< rollNo <<endl;
      cout<<"Name = "<< name <<endl;
      cout<<"Marks = "<< marks <<endl;
      cout<<endl;
   }
};

In the function main(), there are three objects of class Student i.e. s1, s2 and s3. For each of these objects getdata() and putdata() are called. At the end, the value of objectCount is displayed. This is given below −

int main(void) {
   Student s1;
   s1.getdata();
   s1.putdata();

   Student s2;
   s2.getdata();
   s2.putdata();

   Student s3;
   s3.getdata();
   s3.putdata();

   cout << "Total objects created = " << Student::objectCount << endl;

   return 0;
}