Static members in C++

Static data member

A static data member belongs to the class, not to any object. It is shared by all the objects of the class.

To make a data member static, it should be defined with keyword static inside the class body.  

A static data MUST also be defined outside the body of the class because inside class, the declaration of static data is not definition.

class SimpleInterest
{
 static double rate;
 int amount;
 float period;
public:
 /*funnctions*/
};
double SimpleInterest::rate=0.07;/*definition*/

Static data members can be accessed using dot or arrow operator but they can also be accessed without an object, using class name followed by scope resolution operator.

int main()
{
 SimpleInterest::rate=0.08;
 SimpleInterest obj;
 float intrst = obj.amount*obj.period*obj.rate;
 cout<<intrst;
}

Static member functions

A static member function can be called directly without using any object,  with class name and scope resolution operator.

A function is made static by using static keyword within the class body in function declaration.

Rules for static functions.

1. A static function can not use "this" pointer. Hence it can not access non-static data members.
2. A static function can not be const
3. Static function can not be virtual

class A
{
 int n;
 static int m;
public:
 A(int m);
 static void print() ;/*static function*/
};
int A::m=4;

void A::print()
{
 cout<<"Hello world";
 cout<<n;/*error. Can not access n*/
 cout<<m;/*no error. m is static*/}
int main()
{
 A::print();
}

print() function of A class, which is static can not access n, as n is non-static data.