Initialization of Static Class members

If i try to initialize obj_s it asks me to make it const - and i cant do that for i have to keep count of my Created Objects.

#include<iostream>

class A
{
        static int obj_s=0;
public: 
        A(){ ++obj_s;co开发者_高级运维ut << A::obj_s << "\nObject(s) Created\n"; }
}; 

int main()
{
A a,b,c,d;
}    

The code below keeps giving me the following error:

  [Linker error] undefined reference to `A::obj_s' 

[Solved]

The code is giving the error because the object is not getting created in the second case, and in the first its not initializing, the way its supposed to - Here's the fixed Code:

#include<iostream>
class A
{
        static int obj_s;
public: 
        A()
{  obj_s++;  std::cout << A::obj_s << "\nObject(s) Created\n" ;  }
}; 

int A::obj_s=0;  // This is how you intialize it

int main()
{
A a ,b,c,d;
}