How to define a nested class outside its parent in C++

Question

I have two classes, A and B. Class B has no meaning except to class A and requires private access to A's members, so I feel it should be a private nested class.

Class A is already complicated, so I would like to keep the definition of Class B outside of Class A, maybe in a separate header.

I tried this...

class A;

class A::B
{
  int i;
};

class A
{
  class B;

  B my_b;
  int i;
};

int main (void)
{
  A my_a;
  return 0;
}

And get error: qualified name does not name a class before ‘{’ token.

I try this...

class A
{
  class B;

  B my_b;
  int i;
};

class A::B
{
  int i;
};

int main (void)
{
  A my_a;
  return 0;
}

And get error: field ‘my_b’ has incomplete type ‘A::B’.

This is similar to How to write the actual code from a nested class outside the main class, but complicated by the fact that class A has a A::B as a member.

Answer 1

You can have a pointer to B as a member, or a smart pointer. The reason you cannot have a member variable of type B and have it defined outside of A is that if the compiler have not seen the definition of a class it does not know its size therefore cannot figure out layout for A.

Another approach to the whole thing is to use a pimpl idiom, I think it would be ideal here.