Shift key click in qt? - c++

Question

I am working on a paint program in QT5 C++ and I am trying to modify a function that draws a line to incorporate the 45 degree, horizontal or vertical special line that should be drawn if the shift key is clicked.
Below is what I have but for some reason the key handler is not working for me.
I received an error but I don't understand what I need to do to fix it attached below is the error and the code for the paint function that I've modified after that. I have wrapped the modification I did in comments for readability
void LineInstrument::paint(ImageArea &imageArea, bool isSecondaryColor, bool)
{
QPainter painter(imageArea.getImage());
if(isSecondaryColor)
{
painter.setPen(QPen(DataSingleton::Instance()->getSecondaryColor(),
DataSingleton::Instance()->getPenSize() * imageArea.getZoomFactor(),
Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin));
}
else
{
painter.setPen(QPen(DataSingleton::Instance()->getPrimaryColor(),
DataSingleton::Instance()->getPenSize() * imageArea.getZoomFactor(),
Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin));
}
if(mStartPoint != mEndPoint) // here is where the line is drawn
{
painter.drawLine(mStartPoint, mEndPoint); // let the line be drawn
// my modifications start here
if (QApplication::keyboardModifiers().testFlag(Qt::ShiftModifier) == true) { // check if shift key is active
QMouseEvent *mouse;
if (mouse->pos().x() > mouse->pos().y()){
// transform to a horizontal line
painter.save(); // save current painter state
painter.rotate(180);
painter.restore(); // restores painter state
}
else if (mouse->pos().x() < mouse->pos().y()){
// transfomr to a vertical line
painter.save();
painter.rotate(90);
painter.restore();
}
else{
// transform to a 45 degree line
painter.save();
painter.rotate(45);
painter.restore();
}
}// and end here
}
if(mStartPoint == mEndPoint)
{
painter.drawPoint(mStartPoint);
}
imageArea.setEdited(true);
// int rad(DataSingleton::Instance()->getPenSize() + round(sqrt((mStartPoint.x() - mEndPoint.x()) *
// (mStartPoint.x() - mEndPoint.x()) +
// (mStartPoint.y() - mEndPoint.y()) *
// (mStartPoint.y() - mEndPoint.y()))));
// mPImageArea->update(QRect(mStartPoint, mEndPoint).normalized().adjusted(-rad, -rad, +rad, +rad));
painter.end();
imageArea.update();
}

Answer 1

The error is pretty clear.
QMouseEvent *mouse; - you declare a pointer to the a QMouseEvent, but where is it instantiated? This is only a pointer which points to something.
If you want to handle mouse events you probably have to overload some kind of widget's mouse event (mouseMoveEvent, mousePressEvent, etc.). Those will provide you a valid QMouseEvent input.