I'm trying to write a very simple program that makes use of QSharedDataPointer and QSharedData.

Here is my class:
Qt Code: 
  1. #ifndef BUILDING_H
  2. #define BUILDING_H
  3.  
  4. #include <QSharedData>
  5. #include <QSharedDataPointer>
  6.  
  7. class BuildingPrivate;
  8.  
  9. class Building
  10. {
  11. public:
  12. 	Building();
  13. 	~Building();
  14.  
  15. 	int value() const;
  16. 	void setValue(int value);
  17.  
  18. private:
  19. 	QSharedDataPointer<BuildingPrivate> d;
  20. };
  21.  
  22. #endif // BUILDING_H
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Qt Code: 
  1. #include "building.h"
  2.  
  3. class BuildingPrivate : public QSharedData {
  4. public:
  5. 	BuildingPrivate() {
  6. 		value = 0;
  7. 	}
  8. 	BuildingPrivate(const BuildingPrivate &other) {
  9. 		value = other.value;
  10. 	}
  11. 	~BuildingPrivate() {}
  12.  
  13. 	int value;
  14. };
  15.  
  16. Building::Building()
  17. 	: d(new BuildingPrivate)
  18. {
  19. }
  20.  
  21. Building::~Building() { }
  22.  
  23. int Building::value() const
  24. {
  25. 	return d->value;
  26. }
  27.  
  28. void Building::setValue(int value)
  29. {
  30. 	d->value = value;
  31. }
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And here is my main.cpp:
Qt Code: 
  1. #include <QtCore/QCoreApplication>
  2. #include <QDebug>
  3.  
  4. #include "myobject.h"
  5.  
  6. Building build() {
  7. 	Building test;
  8. 	test.setValue(888);
  9. 	return test;
  10. }
  11.  
  12. int main(int argc, char *argv[])
  13. {
  14. 	QCoreApplication a(argc, argv);
  15.  
  16. 	qDebug() << build().value();
  17.  
  18. 	return 0;
  19. }
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When I try to compile that I get:
Qt Code: 
  1. In file included from /usr/include/qt4/QtCore/QSharedData:2,
  2.                  from building.h:5,
  3.                  from myobject.h:6,
  4.                  from main.cpp:5:
  5. /usr/include/qt4/QtCore/qshareddata.h: In copy constructor ‘QSharedDataPointer<T>::QSharedDataPointer(const QSharedDataPointer<T>&) [with T = BuildingPrivate]’:
  6. building.h:10:   instantiated from here
  7. /usr/include/qt4/QtCore/qshareddata.h:90: error: invalid use of incomplete type ‘struct BuildingPrivate’
  8. building.h:7: error: forward declaration of ‘struct BuildingPrivate’
  9. /usr/include/qt4/QtCore/qshareddata.h: In destructor ‘QSharedDataPointer<T>::~QSharedDataPointer() [with T = BuildingPrivate]’:
  10. building.h:10:   instantiated from here
  11. /usr/include/qt4/QtCore/qshareddata.h:87: error: invalid use of incomplete type ‘struct BuildingPrivate’
  12. building.h:7: error: forward declaration of ‘struct BuildingPrivate’
  13. /usr/include/qt4/QtCore/qshareddata.h:87: warning: possible problem detected in invocation of delete operator:
  14. /usr/include/qt4/QtCore/qshareddata.h:87: warning: invalid use of incomplete type ‘struct BuildingPrivate’
  15. building.h:7: warning: forward declaration of ‘struct BuildingPrivate’
  16. /usr/include/qt4/QtCore/qshareddata.h:87: note: neither the destructor nor the class-specific operator delete will be called, even if they are declared when the class is defined.
  17. make: *** [main.o] Error 1
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What am I missing?