Static Data Members and Functions in C++

In daily life, some things are shared by everyone rather than belonging to just one person. For example, in a classroom the notice board is the same for all students. If one student puts a notice, every student can see it. Similarly, in a society, the water tank is shared by all houses. In C++, such shared values or functions are handled using the static keyword.

A static member in C++ belongs to the class itself rather than to individual objects, and its value is shared among all objects of that class.

Why Do We Need Static?

• Shared Data: Some values are common for all objects. For example, the total number of students in a class is the same information for all students.
• Memory Efficiency: Instead of each object having its own copy, only one shared copy is created.
• Persistence: A static variable keeps its value throughout the program execution, instead of being created and destroyed repeatedly.

Static Variables Inside Functions

Normally, when a variable is declared inside a function, it is created every time the function is called and destroyed when the function ends. But if it is declared as static, it is created only once and remembers its value between function calls.

#include <iostream>
using namespace std;

void demo() {
    static int x = 0;  // created only once
    x++;
    cout << "x = " << x << endl;
}

int main() {
    demo();
    demo();
    demo();
    return 0;
}

Output:

x = 1
x = 2
x = 3

Static Data Members in Classes

When a variable is declared as static inside a class, it is shared by all objects of that class. Only one copy exists in memory.

Program Example: Counter

#include <iostream>
using namespace std;

class counter {
public:
    static int count;   // Declaration of static variable

    counter() {
        count++;  // Every new object increases the same shared count
    }

    static void show() {  // static function
        cout << "The value of counter is: " << count << endl;
    }
};

// Definition of static variable outside the class
int counter::count = 0;

int main() {
    counter C1;
    C1.show();

    counter C2;
    C2.show();

    counter C3;
    counter::show();   // Can also be called using class name

    cout << "Try programiz.pro" << endl;

    return 0;
}

Explanation

• static int count; → declared inside the class.
• int counter::count = 0; → defined outside the class.
• Every time a new object (C1, C2, C3) is created, the constructor increments count.
• The function show() is declared static, so it can be called without creating an object (e.g., counter::show();).
• All objects share the same count.

Daily Life Example

Think of class attendance:

Each student can be thought of as an object, while the attendance register in the classroom is a single shared record. No matter how many students there are, they all refer to and update the same register when marked present. In the same way, the static variable count is like that register—every object (student) updates the same shared value.

Static Member Functions

A static function inside a class can be called without creating an object. It only works with static data members, since those are also shared.

class MyClass {
public:
    static int value;
    static void display() {
        cout << value << endl;
    }
};

Comparison: Normal vs Static Members

Aspect	Normal Member	Static Member
Storage	Each object has a copy	Shared by all objects
Lifetime	Created/destroyed with obj	Exists till program ends
Access	Through objects only	Through objects or class
Memory Efficiency	Less efficient (duplicates)	More efficient (single copy)

Practice Exercises

1. Write a program to count how many objects of a class are created using a static variable.
2. Create a static function in a class that displays the total number of books in a library.
3. Demonstrate a static variable inside a function that keeps its value across multiple calls.