Проблема с зависимостями [C++]

// a.hpp
#ifndef __A_HPP
#define __A_HPP
#include "b.hpp"

class A
{
        private:
                B *b;
        public:
                A(B *_b) { b = _b; };
                ~A() { };
}
#endif

// b.hpp
#ifndef __B_HPP
#define __B_HPP
#include "a.hpp"

class B
{
        private:
                A *a;
        public:
                B(A *_a) { a = _a; };
                ~B() { };
}
#endif

// prog.cpp
#include "a.hpp"
#include "b.hpp"

int main(int argc, char **argv)
{
        A *a = new A();
        delete a;
        return 0;
}

// a.hpp
#ifndef __A_HPP
#define __A_HPP
class B;

class A
{
        private:
                B *b;
        public:
                A(B *_b) { b = _b; };
                ~A() { };
};
#endif

// b.hpp
#ifndef __B_HPP
#define __B_HPP
class A;

class B
{
        private:
                A *a;
        public:
                B(A *_a) { a = _a; };
                ~B() { };
};
#endif

// prog.cpp
#include "a.hpp"
#include "b.hpp"

int main(int argc, char **argv)
{
        A *a = new A();
        delete a;
        return 0;
}

Огромное спасибо! Всё заработало!