passing this from constructor to other constructor yields undefined reference on linking

I am trying to compile code with the following structure under codeblocks:

class a : public cppcms::application (has virtual members) {
 public:
   a() : b_inst(this) {}
   ...
   b b_inst;

}

class b {
  public:
    b(a* ptr) : a_ptr(ptr) {}
  private:
    a* a_ptr;
  ...
}

I am trying to put this together with codeblocks/g++ and get the following error message (at the linking stage):

undefined reference to `vtable for b (In function b: ...)

I tried pulling of the same thing with a reference, same result. I tried to change a::b_inst to a pointer and creating an instance of b with new in the constructor (codepart), same result. What is the right way to do this?

By the way, if I do not add the pointer passing on construction, the code works, so i think it is not开发者_开发技巧 resolved by the answer here

undefined reference to `vtable for b (In function b: ...)

That means probably you haven't implemented all of b's virtual methods. gcc emits the vtable when the first one is defined AFAIK.

That means it has nothing to do with the implementation of your constructor.