for completeness, here is the InteractiveQGraphicsFrameItem class, which works without problems:

Qt Code: 
  1. #ifndef INTERACTIVEQGRAPHICSFRAMEITEM_H
  2. #define INTERACTIVEQGRAPHICSFRAMEITEM_H
  3.  
  4. #include <QSharedData>
  5. #include "GraphicItemsSharedLibrary.h"
  6. #include "AbstractInteractiveQGraphicsItem.h"
  7.  
  9. class QRectF;
  10. class QColor;
  11. class QPainter;
  13. class QWidget;
  16. class QKeyEvent;
  18.  
  19. class InteractiveQGraphicsFrameItemData;
  20.  
  21. class GRAPHICITEMS_SHAREDLIB_LIB_EXPORT InteractiveQGraphicsFrameItem : public AbstractInteractiveQGraphicsItem
  22. {
  23.   public:
  24.     InteractiveQGraphicsFrameItem(AbstractInteractiveQGraphicsCoordinationItem* parent = 0, QGraphicsScene* scene = 0);
  25.     virtual ~InteractiveQGraphicsFrameItem();
  26.     virtual QRectF boundingRect() const;
  27.     virtual QRectF getRect();
  28.     virtual void paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget = 0);
  29.     virtual void setDrawNewRectOnClickOutOfTheFrame(bool b);
  30.     virtual void setFillColor(const QColor &color);
  31.     virtual void setFrame(const QRectF& rect);
  32.     virtual void setFrameColor(const QColor &color);
  33.     virtual void setHandleFillColor(const QColor &color);
  34.     virtual void setHandleFrameColor(const QColor &color);
  35.     virtual void setMaximumHandleSize(double size);
  36.     virtual void setOuterFillColor(const QColor &color);
  37.  
  38.   protected:
  39.     virtual void adjustHandleSize();
  40.     virtual void keyPressEvent(QKeyEvent* event);
  41.     virtual void keyReleaseEvent(QKeyEvent* event);
  42.     virtual void mousePressEvent(QGraphicsSceneMouseEvent* event);
  43.     virtual void mouseMoveEvent(QGraphicsSceneMouseEvent* event);
  44.     virtual void mouseReleaseEvent(QGraphicsSceneMouseEvent* event);
  45. //    virtual void mouseDoubleClickEvent(QGraphicsSceneMouseEvent* event);
  46. //    virtual void wheelEvent(QGraphicsSceneWheelEvent* event);
  47.  
  48.     virtual void updateHandles();
  49.  
  50.   protected:   
  51.     QSharedDataPointer<InteractiveQGraphicsFrameItemData> d;
  52. };
  53.  
  54. #endif // INTERACTIVEQGRAPHICSFRAMEITEM_H
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Qt Code: 
  1. #ifndef INTERACTIVEQGRAPHICSFRAMEITEMDATA_H
  2. #define INTERACTIVEQGRAPHICSFRAMEITEMDATA_H
  3.  
  4. #include <QSharedData>
  5. #include <QRectF>
  6. #include <QPointF>
  7. #include <QVector>
  8. #include <QColor>
  9.  
  10. class InteractiveQGraphicsFrameItemData : public QSharedData
  11. {
  12.   public:
  13.     InteractiveQGraphicsFrameItemData();
  14.  
  15.     QRectF frame;
  16.     bool mouseDown;
  17.     bool mouseInObject;
  18.     bool newSelection;
  19.     bool lockAspectRatio;
  20.     bool drawNewRectOnClickOutOfTheFrame;
  21.     double handleSize;
  22.     double maximumHandleSize;
  23.     QRectF* mouseOverHandle;
  24.     QPointF dragStartPoint;
  25.     QRectF  frameBeforeDrag;
  26.     QColor fillColor;
  27.     QColor frameColor;
  28.     QColor handleFillColor;
  29.     QColor handleFrameColor;
  30.     QColor outerFillColor;
  31.  
  32.     // naming convention for handles
  33.     // T top, B bottom, R Right, L left
  34.     // 2 letters: a corner
  35.     // 1 letter: the handle on the middle of the corresponding side
  36.     QRectF TLHandle, TRHandle, BLHandle, BRHandle;
  37.     QRectF LHandle, THandle, RHandle, BHandle;
  38.  
  39.     QVector<QRectF*> handles;
  40. };
  41.  
  42. #endif // INTERACTIVEQGRAPHICSFRAMEITEMDATA_H
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Now where is the difference to the InteractiveQGraphicsLineItem class, which makes the one compile and the other not?