Suggestion:
It's better to have a construct function to new a QLineF use "P1", "Angle", "Length".
For example :
QLineF ( const QPointF & p1, qreal angle, qreal length)
QLineF ( qreal x1, qreal y1, qreal angle, qreal length)

And it's better to support interfaces to set QLine within these 3 parameters.
void setLine ( qreal x1, qreal y1, qreal angle, qreal length)
void setPoints ( const QPointF & p1, qreal angle, qreal length)

Reason:
During developing games these interfaces are needed.
It waste time to separately call 3 functions.
If support this API, the performance would be better.