data.cpp

void PuzzleWidget::mouseMoveEvent(QMouseEvent *event)
{
  // ...
  
  QByteArray src;      
  QDataStream io(&src, QIODevice::WriteOnly);
  io << data;
  mime->setData("myapp/data", src);
  // ...
}

drag.cpp

QDrag *drag = new QDrag(this);
QMimeData *mime = new QMimeData;

mime->setText("Hello World");

drag->setMimeData(mime);

drop.cpp

void PuzzleWidget::dropEvent(QDropEvent *event)
{
  // ...

  if ( event->mimeData()->formats().contains("mygame/point"))
  {
    QByteArray data = event->mimeData()->data("mygame/point");
    QDataStream io(data);
    QPoint pc;
    io >> pc;
  }
}

mime.cpp

void DndDemoWidget::dropEvent(QDropEvent *ev)
{
    qDebug() << "DROP";

    const QMimeData *mime = ev->mimeData();
    if (mime->hasText())
    {
        qDebug() << "Got text: " << mime->text();
    }
}