simple class question

I have a simple class question/problem. Here is an example of my code:

class a
{
  int someVariable;
  void aFunction(void);
}

class b
{
  int someOtherVariable;
  a classA;
}

I want to create a pointer to class b inside class a. This way the functions from class a can use variables from class b. This piece of code does not work because at the time class a is compiled the compiler does not know class b yet.

class a
{
  int someVariable;
  b classB;
  void aFunction(void);
}

Does anyone know if it is possible and how it''s done?? Thanx!