#include <QSlider>

class QSlider;

// a.h
#include "b.h"
struct A { B * a;  }

// b.h
#include "a."  // circlular include reference
struct B { A * a;  }

// Solution: break circular reference by forward delcaration of B or A

// foo.h
#include <qslider>
class Foo
{
   QSlider * someSlider;
}

// bar.h
#include "foo.h"
class Bar
{
   Foo * kungFoo;
   // ...
}

// bar.h
class Foo;
class Bar
{
   Foo * kungFoo;
   // ...
}

// bar.cpp
#include "bar.h"
#include "foo.h"
// ...

/* --- --- --- Y.hpp */
class X;
class Y {
    X *x;
};

/* --- --- --- Y.cpp */
#include <x.hpp>
#include <y.hpp>

...