How can I refer to a static private object of a class?

Question

This is the context:

I have a class A which should say 'Hi", but since A does not knows to speech, it uses an object of class B to speech for him. Since the only purpose of A holding an B is to B speech for it, there's no need of each A hold it's own B object; because of this I choose to use an unique static private B for this.

Like this:

class A {
    static B b;
public:
    void sayHi();
};

void A::sayHi()
{
    b.sayHi();
}

And B goes like this:

class B {
public:
    void sayHi();
};

void B::sayHi() 
{
    std::cout << "Hi!" << std::endl;
}

The problem is when I try to compile this code with g++ compiler...

int main() {
    A a;
    a.sayHi();
    return 0;
}

I get an "undefined reference" error. I'm not sure why this is not working, I was wondering that the compiler thinks I'm referring to an non-static B in A, but I don't know how it should be.

P.S.: In my code, the declaration of B comes before declaration of A.

Answer 1

You need to actually create the static object somewhere in your code. All you've done is say the class has one. Add this to a .cpp file:

B A::b;

This assumes the object should be default constructed.