Parent Child Relationship Causes Class Does Not Name Type Error

Question

If I have a simple two-class relationship like this, I will get this error: ‘Parent’ does not name a type. I do understand why this error occurs, but how can I prevent this?

parent.h

#ifndef PARENT_H
#define PARENT_H

#include "child.h"

class Parent {
        Child b;
};

#endif

child.h

#ifndef CHILD_H
#define CHILD_H

#include "parent.h"

class Child {
        Parent a;
};

#endif

impl.cpp

#include "parent.h"

Parent b;

int main() {

}