Puzzle Example on Qt's website

[tbscope]

There isn't a QGridLayout used, but I assume you already know that.

The widget is devided into several target squares, each 80x80.
A position is calculated by using the piece location. This is done with the following function:

const QRect PuzzleWidget::targetSquare(const QPoint &position) const
 {
     return QRect(position.x()/80 * 80, position.y()/80 * 80, 80, 80);
 }

const QRect PuzzleWidget::targetSquare(const QPoint &position) const
 {
     return QRect(position.x()/80 * 80, position.y()/80 * 80, 80, 80);
 }

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If you look at the code, you'll see several lists, one for the pieces themselves, one for the position, etc...
When you drop a piece, the location and the piece are added to these lists. Then when you select a piece and move it around, this location is used to calculate if you're on the target rect or not.