Basic usage:
main window examples

(...) uses multiple Ui's, and a main window?
It's rather easy if you know how to create a widget that uses Ui form created with Designer.
Lets say that you have n forms, form_1.ui, form_2.ui, ..., form_n.ui.
foreach form f:
1) Add f into .pro file.
2) create and add to .pro a class that setups ui for f (include header "ui_form_n.h" and call 'setupUi()' ), example of n-th class (depends on names you use in .ui file, 'WidgetForm_n' is the name of main ui widget):
Qt Code: 
  1. #ifndef _WIDGET_N_H__
  2. #define _WIDGET_N_H__
  3.  
  4. #include <QWidget>
  5. #include "ui_form_n.ui"
  6.  
  7. class Widget_n : public QWidget{
  8. Q_OBJECT
  9. public:
  10.     Widget_n( QWidget * parent = NULL ) : QWidget(parent){
  11.         _ui.setupUi(this);
  12.     }
  13. protected:
  14.     Ui::WidgetForm_n _ui;
  15. };
  16.  
  17. #endif
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3) include headers for classes created in 2) in your mainWindow sources
4) call QMainWindow::setCentralWidget( new Widget_n() ) to set the widget representing n-th form as central for your main window at appropriate time.