Static data member and member function

Static data member

A static data member is declared with the keyword static inside a class. Only one copy exists and it is shared by all objects of the class, instead of each object having its own copy. It is created once, before any object, and initialised to zero by default. It must be defined outside the class using the scope resolution operator:

int Counter::count = 0;

Static member function

A static member function is declared with static. It can access only static data members (and other static functions), because it has no this pointer. It can be called without any object, using the class name: ClassName::function().

When to use

• Static data member: when data belongs to the whole class, such as a count of objects created, a total, or a shared constant (interest rate).
• Static function: when we need to work on static data without creating an object, e.g. to display the object count.

Example

#include <iostream>
using namespace std;

class Student
{
    int roll;
    static int count;          // shared by all objects
public:
    Student() { roll = ++count; }
    static void showCount()    // static function
    {
        cout << "Objects created = " << count << endl;
    }
    void show() { cout << "Roll = " << roll << endl; }
};

int Student::count = 0;        // definition of static member

int main()
{
    Student::showCount();      // 0, no object needed
    Student s1, s2, s3;
    s1.show();
    s3.show();
    Student::showCount();      // 3
    return 0;
}

Output

Objects created = 0
Roll = 1
Roll = 3
Objects created = 3