Quote Originally Posted by Pieter from Belgium
At a certain point in the program I would like to find out whether or not the Control key is pressed.
(How) can I do this in Qt3?

So I am looking for a Qt 3 approach for Qt 4's QApplication::keyboardModifiers().
I needed the same thing. Here's my implementation (I subclassed QApplication to do this).
Qt Code: 
  1. #ifdef Q_WS_WIN
  2. #    include <windows.h>
  3. #    include <shellapi.h>
  4. #else /* Q_WS_X11 */
  5. #    include <X11/Xlib.h>
  6. #endif
  7. #include <qpaintdevice.h>
  8.  
  9. // Implement a way to easily deduce which mousebuttons and modifier
  10. // keys are held down
  11. static Qt::ButtonState get_modifiers()
  12. {
  13.     int result = Qt::NoButton;
  14.  
  15. #ifdef Q_WS_WIN
  16.  
  17.     result |= GetAsyncKeyState(VK_SHIFT) < 0 ? Qt::ShiftButton : 0;
  18.     result |= GetAsyncKeyState(VK_MENU) < 0 ? Qt::AltButton : 0;
  19.     result |= GetAsyncKeyState(VK_CONTROL) < 0 ? Qt::ControlButton : 0;
  20.  
  21.     // Check if left and right mousebuttons have been swapped
  22.     bool swap_buttons = GetSystemMetrics(SM_SWAPBUTTON) != 0;
  23.  
  24.     if (swap_buttons)
  25.     {
  26.         result |= GetAsyncKeyState(VK_LBUTTON) < 0 ? Qt::RightButton : 0;
  27.         result |= GetAsyncKeyState(VK_RBUTTON) < 0 ? Qt::LeftButton : 0;
  28.     }
  29.     else
  30.     { 
  31.         result |= GetAsyncKeyState(VK_LBUTTON) < 0 ? Qt::LeftButton : 0;
  32.         result |= GetAsyncKeyState(VK_RBUTTON) < 0 ? Qt::RightButton : 0;   
  33.     }
  34.     result |= GetAsyncKeyState(VK_MBUTTON) < 0 ? Qt::MidButton : 0;
  35.  
  36. #else /* Q_WS_X11 */
  37.  
  38.     Window root, child;
  39.     int root_x, root_y;
  40.     int win_x, win_y;
  41.     uint keys_buttons;
  42.     bool status = XQueryPointer(QPaintDevice::x11AppDisplay(),
  43.             QPaintDevice::x11AppRootWindow(),
  44.             &root, &child,
  45.             &root_x, &root_y,
  46.             &win_x, &win_y,
  47.             &keys_buttons);
  48.  
  49.     if (status)
  50.     {        
  51.         result |=  keys_buttons & ShiftMask ? Qt::ShiftButton : 0;
  52.         result |=  keys_buttons & Mod1Mask ? Qt::AltButton : 0;
  53.         result |=  keys_buttons & Mod4Mask ? Qt::MetaButton : 0;
  54.         result |=  keys_buttons & ControlMask ? Qt::ControlButton : 0; 
  55.  
  56.         result |=  keys_buttons & Button1Mask ? Qt::LeftButton : 0;
  57.         result |=  keys_buttons & Button2Mask ? Qt::MidButton : 0; 
  58.         result |=  keys_buttons & Button3Mask ? Qt::RightButton : 0; 
  59.     }
  60. #endif
  61.     return Qt::ButtonState(result);
  62. }
  63.  
  64. Qt::ButtonState MyApplication::keyboardModifiers()
  65. {
  66.     return Qt::ButtonState(get_modifiers() & Qt::KeyButtonMask);
  67. }
  68.  
  69. Qt::ButtonState MyApplication::mouseButtons()
  70. {
  71.     return Qt::ButtonState(get_modifiers() & Qt::MouseButtonMask);
  72. }
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One solution I can think of is to filter all keyboard events and to do explicit bookkeeping of whether the CTRL key is pressed, but I wonder if there is a simpler approach.
That's exactly how they implement it in Qt4!