C++ : static keyword

Static data members of a class

– When the static keyword is used inside a class definition, it indicates that the members are not tied to the class objects. This is particularly useful when we want to provide only a single copy of a class variable between all the objects of a class, rather than having each object maintain its own copy.
– As static data members are not bound to class objects, they exist even if no class objects are created.

Example
– A static data member of a class could be used to keep track of the number of class objects that are created.
– A static data member could be a pointer pointing to the error-handling or logging function for that class.

Accessing a static data members

If a static data member is declared inside the public section of the class, then it can be accessed outside the class using ClassName :: static_member;
If a static data member is declared inside the private section of the class, then it can be accessed outside the class using
- non-static member functions.
- static member functions.

Defining a static data members

Static data member must be definied in a .cpp file. If this is not done a linked error is reported.

Static member functions of a class

– When a function is declared as static inside a class definition, it is not bound to any class object.
– As a static member function is not bound to any class object, it does not receive the this pointer.

Key points

Static Data Members	Static Member Functions
- Data to be shared by all objects of a class, is stored in static data members. Thus static variable is a class variable. - Only a single copy of the static data member exists. - Static data member could be defined inside the public or the private section of a class. - For declaration a static data member is preceded with the keyword static.	- Static member functions can only access static data members. A static member function is called using the class name i.e Class_Name :: static_function - this pointer never gets passed to a static member function. - As static member functions never receive the this pointer, they cannot access non-static data members.

Below C++ program demonstrates the use of static data members and static member functions of a class.
Class definition : Employee.h

#include<iostream>
#include<string>

class Employee {

    private:
    int m_emp_id;
    std :: string m_emp_name; 

    public:
    // Declaration of a static member of a class. It needs to be defined in the cpp file.
    static std :: string m_company;

    Employee (int id, std :: string emp_name) : m_emp_id (id), m_emp_name(emp_name)
    {}

    void Employee_Details();
    static void Org_Details(); // Static member function
};

Employee.cpp

#include "Employee.h"

// Static data member m_company is defined here (i.e in this cpp file)
std :: string Employee :: m_company = "Algotree.org";

void Employee :: Employee_Details() {
    std :: cout << "Employee Id : " << m_emp_id << std :: endl;
    std :: cout << "Employee Name : " << m_emp_name << std :: endl;
}

void Employee :: Org_Details() {
    std :: cout << "Organization Name (displayed using static function) : " << m_company << std :: endl;
}

int main() {

    Employee e(25, "Deltoid");

    e.Employee_Details();
    Employee :: Org_Details();

    std :: cout << "Organization Name (displayed using class name) : " << Employee :: m_company << std :: endl;
    return 0;
}

Output

Employee Id : 25
Employee Name : Deltoid
Organization Name (displayed using static function) : Algotree.org
Organization Name (displayed using class name) : Algotree.org