The size of a QWidget is limited. How can I change it?

Question

I want to build a Qt5.2 Application in C++ with Visual Studio 2013. I apply the QWidget::resize function to set the size of the QWidget object in the image above to 1200*800. But it seems that the QWidget object can't be that large(there are vertical and horizontal scroll bars).

How can I get the size of the QWidget object expanded to 1200*800 and remove the scroll bars? And how can I get the object at the centre of the application both vertically and horizontally?

Answer 1

Add your widget to QLayout:

QVBoxLayout *layout = new QVBoxLayout;
layout->addWidget(widget);
mainWindow->setLayout(layout);

Answer 2

You need to provide a minimal example of what's going on. We shouldn't have to guess.

All I can see is up to three widgets: the top level one, the QScrollArea, and whatever widget is inside of the scroll area. If that's the case, then the scroll area is not managed by a layout, and when you resize the top level widget, the scroll area's size remains unchanged.

I see two solutions, assuming that MyContentsWidget is the widget that draws your genetics thingamajingy (if that's what it is).

1. Get rid of the toplevel widget and use the QScrollArea as a toplevel widget:

   int main(int argc, char ** argv) {
     QApplication app(argc, argv);
     QScrollArea area;
     MyContentsWidget contents;
     area.setWidget(&contents);
     area.show();
     return app.exec();
   }
   

2. Add a layout to the toplevel widget, so that it'll resize the scroll area appropriately:

   class MyWindow : public QWidget {
     QGridLayout m_layout;
     QScrollArea m_area;
     MyContentsWidget m_contents;
   public:
     MyWindow(QWidget * parent = 0) : QWidget(parent), m_layout(this) {
       m_layout.addWidget(&m_area, 0, 0);
       m_area.setWidget(&m_contents);
     }
   };
   

In both cases, the order of declaration is the opposite of the order of destruction, and it is important since you must ensure that MyContentsWidget is destructed before the scroll area.