What is the problem with this code?

Question

#include<stdio.h>

class A { public: int a;};
class B: public A { 
public:
    static int b;
    B(){
        b++;
        printf("B:%d\n",b);
    }   
};

int main() {

    A* a1 = new B[100];
    A* a2 = new B();
    return 0;
}

Error:

In function `main':
undefined reference to `B::b'
undefined reference to `B::b'
undefined reference to `B::b'
undefined reference to `B::b'

Answer 1

Static variables need to be allocated outside the class. Add this line outside the class B:

int B::b;

Think of static variables as being declared with the extern keyword. They still need to be allocated somewhere. This means the allocation should never be in the header file!

Answer 2

Because it is static, you also need to define storage for B::b (in a class definition, all you have done is declared the variable).

You need to add:

int B::b;

Answer 3

You have to initialize your static member in the according .cpp file like int B::b = 0