Reimplement resize event - c++

Question

My widget is frameless because of the sleek looks and so I need to reimplement the resize behaviour.
If I drag the bottom right corner it already works with this code:
void mouseMoveEvent(QMouseEvent *event) {
resize(event->pos().x(), event->pos().y());
}
But what about all the other corners? For example the bottom left corner.
As one expects, it should behave as if the top right corner of the widget would be fixed.
But in the resize function, the upper left corner is fixed, because there is the event.pos()==0.
My idea was to resize the window and then move it, so that it looks as if it would not move, but only change size around the upper left corner. As this leads to flickering and even not the perfect result, is there a better way?
EDIT: Solution:
a) You can define in mousePressEvent:
offset = event->pos();
initialFixedCornerPosX = this->width()+this->pos().x();
initialFixedCornerPosY = this->pos().y();
and in mouseMoveEvent
int x = event->globalX();
int y = event->globalY();
int x_w = offset.x();
int y_w = offset.y();
setGeometry(x-x_w,initialFixedCornerPosY,initialFixedCornerPosX-x+x_w,y-initialFixedCornerPosY);
or
b) in mouseMoveEvent
QRect rect = geometry();
rect.setBottomLeft(event->globalPos());
setGeometry(rect);

Answer 1

To both resize and move the window in one step without flickering, you should use QWidget::setGeometry(QRect) by providing a modified rectangle previously fetched using the corresponding getter function QWidget::geometry().
QRect rect = geometry();
// (then modify...)
setGeometry(rect);
To modify a corner of a QRect, you could either modify each edge or the corners directly. Depending on the rest of your logic, one makes more sense than the other. I'd prefer the second of the following two options:
Example using corners:
If you detect that the user drags the bottom left corner, use
rect.setBottomLeft(event->pos());
However, you of course need to consider edges too, and if you consider corners as separate cases this results in eight cases to be considered in the mouse event.
Example using only edges:
If you detect that the mouse is on the left edge (it might be as well on the top or bottom corner, which are only special cases, so for now we ignore that):
rect.setLeft(event->pos().x());
and if you detect it is on the bottom edge, then
rect.setBottom(event->pos().y());
so if both cases are true, this effectively moves the corner of the rect. So you only need to consider four cases to drag all edges and corners of your window! This assumes that you have a single widget which handles the resize (i.e. your top level widget, which has a margin on the layout to have the children not touch the window edge), and do not add a widget for each corner / edge.