Cannot use QRect::setWidth

[WilliamSpiderWeb]

When I try to set the width of a modified QDockWidget

int iWidth = 500;
this->geometry().setWidth(iWidth);

int iWidth = 500;
this->geometry().setWidth(iWidth);

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I get the following error.

passing 'cost QRect' as 'this' argument of 'void QRect::setWidth(int)' discards qualifier

I have no idea what that error means and why I can't use such a simple function.