invalid static_cast from type 'QCell*' to type 'QWidget*'

Question

this error appears in line 4:

void QPiece::setPosition( QPoint value )
{
    _position = value;
    QWidget* parentWidget = static_cast<QWidget *>( _board->Cells[value.x() ][ value.y() ]);
if (parentWidget->layout()) {
    parentWidget->layout()->addWidget( this ); }
else { 
     QHBoxLayout *layout = new QHBoxLayout( parentWidget );
     layout->setMargin(0); 
     layout->addWidget(this); 
     parentWidget->setLayout(layout);
}
    this->setParent( _board->Cells[ value.x() ][ value.y() ] );
}

Here is definition of function Cells():

class QBoard : public QWidget
{
     Q_OBJECT
     public:
          QCell *Cells[8][8];
          QBoard(QWidget *parent = 0);
          void drawCells();

     private:
          void positionCells();
};

I think I do something wrong, but what? Thanks in advance. Here is type of QCell, and i think QWidget is parent to QLabel

class QCell:public QLabel

{
     Q_OBJECT

public:
     QCell( QPoint position, QWidget *parent = 0 );

private:
     QGame *_game;
     QPoint _position;
protected:
      void mousePressEvent( QMouseEvent *ev );
 };

Answer 1

This should work without a cast at all, the conversion from derived to base is implicit.

A likely cause of this error would be that you only have a forward declaration of QCell visible in that compilation unit, which would trigger this error. You need to have the complete class declaration visible for the compiler to know whether that conversion is legal or not.

Example:

#include <QWidget>

class QCell;

int main(int argc, char **argv)
{
    QCell *w = 0;
    QWidget *q = static_cast<QWidget*>(w);
}

main.cpp: In function ‘int main(int, char**)’:
main.cpp:8:41: error: invalid static_cast from type ‘QCell*’ to type ‘QWidget*’