Question

I have a superclass with a class defined inside it. like:

class A {
 public:
  class B {public: bool value;};

  A() {
      DoStuff(b_);
  }
  B b_;
 private:
  virtual void DoStuffImpl(B& b) = 0;
  void DoStuff(B& b) { return DoStuffImpl(b); }
};

class X : public A {
  // ...
 private:
  virtual void DoStuffImpl(B& b);
  void UseBForSomethingElse(B& b);
};

void X::DoStuffImpl(B& b) {
    UseBForSomethingElse(b);
}

void X::UseBForSomethingElse(B& b) {
    b.value = true;
}

int main(){
    X x;
    return x.b_.value;
}

My compiler seems to understand that DoStuffImpl() just fine. But, when I added UseBForSomethingElse(), the compiler could not find the definition for the B class. I tried to further specify by doing bool UseBForSomethingElse(A::B& b). This compiled, but then failed during linking.

How do I correctly specify the parent B, and why does it work for the virtual function but not the other one?

Answer 1

Your updated post doesn’t properly qualify UseBForSomethingElse().

void UseBForSomethingElse(const B& b)

should be

void X::UseBForSomethingElse(const B& b)

Once this is fixed, you still have a problem (and heaven help me if I get this wrong).

You’re firing a virtual method from a base class constructor without the derived class finishing construction. I.e, UseBForSomethingElse (non-virtual) is fired from DoStuffImpl() (virtual, pure @ A, defined in X) before X is finished base-construction (you’re, in fact in X’s base-construction when you make the call). This will trigger a ‘pure virtual function called’ since X’s vtable isn’t fixed up until its constructor is actually entered.

This does happen on my machine as well, btw.