Thread: forward declaration error?

Code:

class Bar;

class Foo
{
  public:
     Foo() { p_Bar = NULL;}
    ~Foo() {}

    void setBar(Bar *b) { p_Bar = b;}
    void doSomething()
    {
      std::cout<< "Hey I'm Foo and I'm doing something\n";
      notifyBar();
    }
  private:
    Bar *p_Bar;
    void notifyBar() {if (p_Bar != NULL) p_Bar->update();}

  protected:

};


class ControlFoo
{
  public:
    ControlFoo() {}
    ~ControlFoo() {}

    void setFoo(Foo *f) {m_f = f;}

    void controlSomething() {m_f->doSomething();}
  private:
    Foo *m_f;
  protected:

};


class Bar
{
  public:
    Bar() {}
    ~Bar() {}
    virtual void update() = 0;
    void setControlFoo(ControlFoo *cf) {m_cf = cf;}
  private:

  protected:
    ControlFoo *m_cf;
};

class BarBar: Bar
{
  public:
    BarBar(): Bar() {}
    ~BarBar() {}

    virtual void update();
    void doBBSomething();
  private:
  protected:

};

void
BarBar::update()
{
  std::cout << "Hey! we got some update!\n";
}

void
BarBar::doBBSomething()
{
  m_cf->controlSomething();
}

The following example gives me the error
t1.hpp: In member function ‘void Foo::notifyBar()’:
t1.hpp:19: error: invalid use of incomplete type ‘struct Bar’
t1.hpp:3: error: forward declaration of ‘struct Bar

Can anyone help me understand this? Is this because Bar is not yet an complete type and the call in notifyBar requires it to be?

Thanks!

I think that is the cause. The Bar::update function isn't defined

In the implementation of Foo::notifyBar() you are attempting to call p_Bar->update()

Doing that requires the compiler to be able to see the definition of class Bar before that line. A forward declaration is not enough. Defining class Bar later in code is also not enough.

The solution will be to move the implementation of Foo::notifyBar() out of the class, and to a point after both classes Foo and Bar are defined.