Base class problem

I have a base class, “A”, that derive the class “B”:

example.h


class A
{
private:
protected:
public:
	A();
	virtual ~A();
	virtual void foo() = 0;
};

class B : public A
{
private:
       int number;

protected:
public:
	B(int number);
	~B();
	void foo();
};

example.cpp


B::B(int number) : number(number) {}

B::~B() {}

void A::foo()
{
	// How can I get the variable "number" from here???
}

This has absolutely nothing to do with OpenGL.