Shuffling rendered primitives - c++

Question

This is just an experiment I'm doing, but suppose that we have three rectangles that are drawn:
void thisDialog::paintEvent(QPaintEvent *ev)
{
rectangle = new QRect(40, 80, 40, 90);
painter = new QPainter(this);
brush = new QBrush(QColor(QColor::green);
painter->fillRect(...);
painter->drawRect(...);
rectangle2 = new QRect(140, 80, 40, 90);
// and draw
rectangle3 = new QRect(240, 80, 40, 90);
// and draw...
}
And now I want to somehow shuffle the rectangles drawn. My solution is to use std::random_shuffle. The plan is we push all the rectangles into a vector thereof:
vector<QRect*> vecRect;
vecRect.push_back(rectangle);
// push back other rectangles
std::random_shuffle(vecRect.begin(), vecRect.end());
At this point, the order of the rectangles have been shuffled. The only problem is that although they're shuffled I have to somehow reflect that in their positions and also update what's shown on-screen for the changes to be apparent. So, I propose creating a function called shuffle. I want only the x-values to change, so the y-values and the spacings of the rectangles should be left alone.
void shuffle()
{
std::random_shuffle(vecRect.begin(), vecRect.end());
for (auto it : vecRect)
{
// do something to the x-values in rectangle; e.g.
it->setX(/*set something...*/);
}
}
To shuffle the rectangles along the x-axis and maintain the y-axis, and maintain the spacing in between I'd have to get a random number and add the spacing and ensure that it is within the window boundaries say 800x600.
Since I am new to Qt I have to know how Qt handles primitive positions, and so I guess that setX() from the docs would modify the x-position of the rectangle. My other enquiry is: how do we update the positions so they're reflected on what's drawn on-screen? Whenever I update positions it doesn't seem to take effect. I've tried update() but that doesn't seem to work.
Btw, I call the foresaid function through a key press, so if the key 'S' is pressed then the function is called.
TLDR; How can I shuffle x-positions in these rectangles and update their positions rendered?