Virtual overridden function must be implemented in header?

[Halsafar]

Lets say:

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

class B : public A {
void foo();
}


Now MUST foo()'s implimentation for class 'B' be written in the header?
It seems I have having unresolved external symbols if I try to declare them in a source file.