Ошибка c2079 использует не имеющую определения структуру class

Think about the computer’s memory for a second here.

class B;

class A {
    byte aa;
    B ab;
};

class B {
    byte bb;
    A ba;
};

A x;

Now the question the compiler needs to answer is How much space should I reserve for x?

Let’s see. The first byte of x is byte aa;. Easy enough. That’s 1 byte.
Next comes B ab;. Let’s see what’s in there.
The first byte of x.ab is a byte bb;. That’s 2 bytes for x so far.
Next, is a A ba;. Let’s see what’s in there.
The first byte of x.ab.ba is a byte aa;. That’s 3 bytes for x so far.
And so on and so forth ad infinitum.
How big is x? The correct answer is of course *** OUT OF CHEESE ERROR ***.

The compiler doesn’t actually do this because it knows it can’t handle this case — so the syntax doesn’t allow circular containment in the first place.

Here’s a diagram of the contents of x in this example:

UPDATE

Apparently, I forgot to include a solution here. Now that you understand what the problem is, the solution should be pretty simple. Use pointers. Either use a pointer from A to B and let B include A as it already does, or vice versa, or use two pointers. Then, you won’t have circular inclusion — if B doesn’t include a copy of A but just a pointer to it, that fixes the entire issue here.

Another method is use an intermediate class, plus pointers, its more long, but, it works:

This is header file, ( yes I know, «*.hpp» extension is not standard ):

ForwardClassExample.hpp

class ForwardClass {
public:
  virtual void DoSomething();
};

class ContainerClass {
   ForwardClass* Item;

   /* constructor */ ContainerClass();
   /* destructor */ ~ContainerClass();
};

class RealClass: ForwardClass {
  /* override */ virtual void DoSomething();
};

This is body file:

ForwardClassExample.cpp

/* constructor */ ContainerClass::ContainerClass()
{
  // create reference to forwaded class item
  this.Item = new RealClass();
}

/* destructor */ ContainerClass::~ContainerClass()
{
  // deletereference to forwaded class item
  free this.Item();
}

void ForwardClass::DoSomething()
{
  // ...
}

void RealClass::DoSomething()
{
  // ...
}

Note:

I suggest to get used to apply pointers to variables, instead of direct fields, its may looks more difficult, at start, but, eventually allows to do more stuff.

It also prepare you to use «references» in case, one day you have to work with other programming languages.

Cheers.