QtSingleApplication

[mule]

(Qt user for 6 months, but this makes me realize I am still a newbie ...
Thank you in advance for your assistance.)

Environment:
Qt 4.1.0
Windows 2000 SP4
Microsoft VC 6.0

What is desired:
I am using the QtSingleApplication class to guarantee only one instance of an application
is running.

When a second instance is started, I want the original instance to become the application
of focus and the second instance to terminate.

What is observed:
The second instance terminates; however, the original instance does not gain
the input focus. The original instance's taskbar button usually flashes, but its
main window remains hidden behind other windows.

vRaiseWindowForThisInst() is being called because the window title is changed.

I have attempted to strip down the source files to the essentials ...

File main.h:

extern  QtSingleApplication *  pApp;

extern  QtSingleApplication *  pApp;

To copy to clipboard, switch view to plain text mode 

File main.cpp:

QtSingleApplication *  pApp;
 
int  main( int  argc,  char *  argv  [] )
{
   pApp = new QtSingleApplication( "UniqueName", argc, argv );
   if ( pApp->sendMessage( "RUNNING?" ) ) {
      return  0;
   }
   pApp->initialize();
 
   pMain = new clMain();
 
   return  pApp->exec();
}

QtSingleApplication *  pApp;

int  main( int  argc,  char *  argv  [] )
{
   pApp = new QtSingleApplication( "UniqueName", argc, argv );
   if ( pApp->sendMessage( "RUNNING?" ) ) {
      return  0;
   }
   pApp->initialize();

   pMain = new clMain();

   return  pApp->exec();
}

To copy to clipboard, switch view to plain text mode 

File clMain.h:

class  clMain  : public QMainWindow
{
   Q_OBJECT
 
   public:
      clMain( QWidget *  parent  = 0 );
      virtual  ~clMain( void );
 
   private:
      void  vRaiseWindowForThisInst( void );
      void  customEvent( QEvent *  event );
 
   private slots:
      void  vMessageReceivedFromOtherInst( const QString &  msg );
};

class  clMain  : public QMainWindow
{
   Q_OBJECT

   public:
      clMain( QWidget *  parent  = 0 );
      virtual  ~clMain( void );

   private:
      void  vRaiseWindowForThisInst( void );
      void  customEvent( QEvent *  event );

   private slots:
      void  vMessageReceivedFromOtherInst( const QString &  msg );
};

To copy to clipboard, switch view to plain text mode 

File clMain.cpp:

clMain::clMain( QWidget *  parent )
      : QMainWindow( parent )
{
   QPushButton *  pExitAllButton  = new QPushButton( "Exit All" );
   setCentralWidget( pExitAllButton );
 
   connect( pExitAllButton, SIGNAL( clicked() ),
            this,           SLOT( vExitAllPressed() ) );
 
   connect( pApp, SIGNAL( messageReceived( const QString & ) ),
            this, SLOT( vMessageReceivedFromOtherInst( const QString & ) ) );
 
   setWindowTitle( "Original Title" );
   show();
}
 
clMain::~clMain( void )
{
   ;
}
 
void  clMain::vRaiseWindowForThisInst( void )
{
   setWindowTitle( "CHANGED TITLE" );
 
   //setFocus( Qt::OtherFocusReason );
   //show();
   raise();
}
 
enum  QEvent::Type  {
   Event_Main_Raise  = QEvent::User +  1
};
 
void  clMain::customEvent( QEvent *  event )
{
   switch ( event->type() ) {
      default :
         QMainWindow::customEvent( event );
         break;
 
      case Event_Main_Raise :
         vRaiseWindowForThisInst();
         break;
   }
}
 
void  clMain::vMessageReceivedFromOtherInst( const QString &  msg )
{
   QApplication::postEvent( this, new QEvent( Event_Main_Raise ) );
}
 
void  clMain::vExitAllPressed( void )
{
   close();
}

clMain::clMain( QWidget *  parent )
      : QMainWindow( parent )
{
   QPushButton *  pExitAllButton  = new QPushButton( "Exit All" );
   setCentralWidget( pExitAllButton );

   connect( pExitAllButton, SIGNAL( clicked() ),
            this,           SLOT( vExitAllPressed() ) );

   connect( pApp, SIGNAL( messageReceived( const QString & ) ),
            this, SLOT( vMessageReceivedFromOtherInst( const QString & ) ) );

   setWindowTitle( "Original Title" );
   show();
}

clMain::~clMain( void )
{
   ;
}

void  clMain::vRaiseWindowForThisInst( void )
{
   setWindowTitle( "CHANGED TITLE" );

   //setFocus( Qt::OtherFocusReason );
   //show();
   raise();
}

enum  QEvent::Type  {
   Event_Main_Raise  = QEvent::User +  1
};

void  clMain::customEvent( QEvent *  event )
{
   switch ( event->type() ) {
      default :
         QMainWindow::customEvent( event );
         break;

      case Event_Main_Raise :
         vRaiseWindowForThisInst();
         break;
   }
}

void  clMain::vMessageReceivedFromOtherInst( const QString &  msg )
{
   QApplication::postEvent( this, new QEvent( Event_Main_Raise ) );
}

void  clMain::vExitAllPressed( void )
{
   close();
}

To copy to clipboard, switch view to plain text mode 

[jacek]

Did you try to invoke the QWidget::activateWindow() method (together with raise())?

[mule]

Yes, I see I failed to copy&paste all the things I tried. Here they are:
activateWindow();
setFocus( Qt::OtherFocusReason );
show();
setWindowState( Qt::WindowActive );
update();
repaint();
setVisible( true );
raise();

As you can see, I was getting frustrated/desperate/confused ...

[jacek]

According to the docs, you should call QWidget::setActiveWindow() (or rather QApplication::setActiveWindow() as it should be written):

void QtSingleApplication::activateMainWidget () [virtual slot]
Calls QWidget::setActiveWindow() on this application's mainWidget(). This function does nothing if no main widget has been set.
This slot is typically connected to the messageReceived() signal.
See also messageReceived().

So such combo:

show();
raise();
activateWindow();

show();
raise();
activateWindow();

To copy to clipboard, switch view to plain text mode 

should be enough. Maybe somebody else knows the answer, but you can always try the OS specific functions as the last resort.

[mule]

Thank you for your help, jacek, and thank you for redirecting me back to the doc. I had placed the doc aside a while back after many failed attempts and was just trying things by brute force and trial/error.

I need to go back and re-think this from the beginning. The doc mentions setMainWidget() and its relation to activateMainWidget(). Since setMainWidget() was abandoned from Qt3 to Qt4 (I am using Qt4) I was trying to do the equivalent with QMainWindow, setCentralWidget(), and such.

If/When I figure this out, I will post the solution.

Other input/thoughts/suggestions still welcome ...

[mule]

(Admin: Originally posted in Newbie forum, but now believe beyond that. If this violates forum policy, please pull this thread. However, this post does contain more complete information and code.)

Qt 4.1.0 / Ms VC 6.0 / Windows 2000 SP4.

What: Using QtSingleApplication class to guarantee single instance of application.
Desired: When second instance starts, it should terminate and the original instance should become the application of focus.
Observed: When the second instance starts, it terminates; however, the original instance is not brought forward as the application of focus.

The QtSingleApplication doc states that activateMainWidget() ...

Calls QWidget::setActiveWindow() on this application's mainWidget(). This function does nothing if no main widget has been set.

The example in the doc also shows setMainWidget() being invoked by main().

Questions:
1) Since setMainWidget() was removed from Qt3 to Qt4, is QMainWindow::setCentralWidget() not functionally equivalent? If it is not, what is?
2) Since this Solution was only tested using Qt 4.1.1 and I am using 4.1.0, is this a version issue? (I did not see anything in the changelog for 4.1.1 that would indicate this.) (I cannot go to 4.1.1 at this time.)

File main.h

#ifndef  _main_h_
#define  _main_h_
 
#include  <QtSingleApplication>
 
extern  QtSingleApplication *  pApp;
 
#endif  //_main_h_

#ifndef  _main_h_
#define  _main_h_

#include  <QtSingleApplication>

extern  QtSingleApplication *  pApp;

#endif  //_main_h_

To copy to clipboard, switch view to plain text mode 

File main.cpp

#include  "main.h"
 
#include  "clMain.h"
 
QtSingleApplication *  pApp;
 
int  main( int  argc,  char *  argv  [] )
{
   pApp = new QtSingleApplication( "UniqueName", argc, argv );
   if ( pApp->sendMessage( "RUNNING?" ) ) {
      return  0;
   }
   pApp->initialize();
 
   pMain = new clMain();
 
   return  pApp->exec();
}

#include  "main.h"

#include  "clMain.h"

QtSingleApplication *  pApp;

int  main( int  argc,  char *  argv  [] )
{
   pApp = new QtSingleApplication( "UniqueName", argc, argv );
   if ( pApp->sendMessage( "RUNNING?" ) ) {
      return  0;
   }
   pApp->initialize();

   pMain = new clMain();

   return  pApp->exec();
}

To copy to clipboard, switch view to plain text mode 

File CustomEvent.h

#ifndef  _customevent_h_
#define  _customevent_h_
 
#include  <QEvent>
 
enum  QEvent::Type  {
   Event_Main_Raise  = QEvent::User +  1
};
 
#endif  //_customevent_h_

#ifndef  _customevent_h_
#define  _customevent_h_

#include  <QEvent>

enum  QEvent::Type  {
   Event_Main_Raise  = QEvent::User +  1
};

#endif  //_customevent_h_

To copy to clipboard, switch view to plain text mode 

File clMain.h

#ifndef  _clMain_h_
#define  _clMain_h_
 
#include  <QMainWindow>
#include  <QWidget>
#include  <QPushButton>
 
class  clMain  : public QMainWindow
{
   Q_OBJECT
 
   public:
 
      clMain( QWidget *  parent  = 0 );
      virtual  ~clMain( void );
 
   private:
 
      QPushButton *  pExitAllButton;
      QWidget *  pMainWidget;
 
      void  vRaiseWindowForThisInst( void );
 
      void  customEvent( QEvent *  event );
 
   private slots:
 
      void  vMessageReceivedFromOtherInst( void );
 
      void  vExitAllClicked( void );
};
 
extern  clMain *  pMain;
 
#endif  //_clMain_h_

#ifndef  _clMain_h_
#define  _clMain_h_

#include  <QMainWindow>
#include  <QWidget>
#include  <QPushButton>

class  clMain  : public QMainWindow
{
   Q_OBJECT

   public:

      clMain( QWidget *  parent  = 0 );
      virtual  ~clMain( void );

   private:

      QPushButton *  pExitAllButton;
      QWidget *  pMainWidget;

      void  vRaiseWindowForThisInst( void );

      void  customEvent( QEvent *  event );

   private slots:

      void  vMessageReceivedFromOtherInst( void );

      void  vExitAllClicked( void );
};

extern  clMain *  pMain;

#endif  //_clMain_h_

To copy to clipboard, switch view to plain text mode 

File clMain.cpp
( NOTE to all the #define options:
Used to select one of many attempts.
Set ONLY ONE non-zero at a time.
See connect() attempts in clMain().
See various method attempts in vRaiseWindowForThisInst(). )

// != 0 -> use activateMainWidget() directly
         // == 0 -> use indirect method (following ...)
#define  _dir_activateMainWidget                                              0
 
         // indirectly ...
 
         // != 0 -> use activateMainWidget()
#define  _indir_activateMainWidget                                            0
 
         // indirectly on this ...
 
         // != 0 -> use show(); raise(); activateWindow()
#define  _indir_this_show_raise_activateWindow                                0
         // != 0 -> use show(); raise(); activateWindow(); setFocus()
#define  _indir_this_show_raise_activateWindow_setFocus                       0
         // != 0 -> use setWindowState(); show(); raise(); activateWindow()
#define  _indir_this_setWindowState_show_raise_activateWindow                 0
         // != 0 -> use setWindowState(); show(); raise(); activateWindow(); setFocus()
#define  _indir_this_setWindowState_show_raise_activateWindow_setFocus        1
 
         // indirectly on pMainWidget widget ...
 
         // != 0 -> use show(); raise(); activateWindow()
#define  _indir_MainWidget_show_raise_activateWindow                          0
         // != 0 -> use show(); raise(); activateWindow(); setFocus()
#define  _indir_MainWidget_show_raise_activateWindow_setFocus                 0
         // != 0 -> use setWindowState(); show(); raise(); activateWindow()
#define  _indir_MainWidget_setWindowState_show_raise_activateWindow           0
         // != 0 -> use setWindowState(); show(); raise(); activateWindow(); setFocus()
#define  _indir_MainWidget_setWindowState_show_raise_activateWindow_setFocus  0
 
         // indirectly on pExitAllButton button ...
 
         // != 0 -> use show(); raise(); activateWindow()
#define  _indir_ExitAllButton_show_raise_activateWindow                       0
         // != 0 -> use show(); raise(); activateWindow(); setFocus()
#define  _indir_ExitAllButton_show_raise_activateWindow_setFocus              0
 
#include  "clMain.h"
 
#include  "Main.h"
#include  "CustomEvent.h"
#include  <QGridLayout>
 
clMain *  pMain  = 0;
 
clMain::clMain( QWidget *  parent )
      : QMainWindow( parent )
{
   pExitAllButton = new QPushButton( "Exit All" );
 
   QGridLayout *  pLayout  = new QGridLayout;
   pLayout->addWidget( pExitAllButton, 0, 0, 1, 1 );
 
   pMainWidget = new QWidget();
   pMainWidget->setLayout( pLayout );
 
   setCentralWidget( pMainWidget );
 
   connect( pExitAllButton, SIGNAL( clicked() ),
            this,           SLOT( vExitAllClicked() ) );
 
   #if  _dir_activateMainWidget
      connect( pApp, SIGNAL( messageReceived( const QString & ) ),
               pApp, SLOT( activateMainWidget() ) );
   #else  //_USE_activateMainWidget
      connect( pApp, SIGNAL( messageReceived( const QString & ) ),
               this, SLOT( vMessageReceivedFromOtherInst() ) );
   #endif  //_USE_activateMainWidget
 
   setWindowTitle( "Original Title" );
   show();
}
 
clMain::~clMain( void )
{
   ;
}
 
void  clMain::vRaiseWindowForThisInst( void )
{
   // indirectly ...
 
   #if  _indir_activateMainWidget
      setWindowTitle( "** CHANGED #1 **" );
 
      pApp->activateMainWidget();
   #endif
 
   // indirectly on this ...
 
   #if  _indir_this_show_raise_activateWindow
      setWindowTitle( "** CHANGED #2a **" );
 
      show();
      raise();
      activateWindow();
   #endif
 
   #if  _indir_this_show_raise_activateWindow_setFocus
      setWindowTitle( "** CHANGED #3a **" );
 
      show();
      raise();
      activateWindow();
 
      setFocus( Qt::OtherFocusReason );
   #endif
 
   #if  _indir_this_setWindowState_show_raise_activateWindow
      setWindowTitle( "** CHANGED #4a **" );
 
      setWindowState( ( windowState() & ~ Qt::WindowMinimized ) | Qt::WindowActive );
 
      show();
      raise();
      activateWindow();
   #endif
 
   #if  _indir_this_setWindowState_show_raise_activateWindow_setFocus
      setWindowTitle( "** CHANGED #5a **" );
 
      setWindowState( ( windowState() & ~ Qt::WindowMinimized ) | Qt::WindowActive );
 
      show();
      raise();
      activateWindow();
 
      setFocus( Qt::OtherFocusReason );
   #endif
 
   // indirectly on pMainWidget widget ...
 
   #if  _indir_MainWidget_show_raise_activateWindow
      setWindowTitle( "** CHANGED #2b **" );
 
      pMainWidget->show();
      pMainWidget->raise();
      pMainWidget->activateWindow();
   #endif
 
   #if  _indir_MainWidget_show_raise_activateWindow_setFocus
      setWindowTitle( "** CHANGED #3b **" );
 
      pMainWidget->show();
      pMainWidget->raise();
      pMainWidget->activateWindow();
 
      pMainWidget->setFocus( Qt::OtherFocusReason );
   #endif
 
   #if  _indir_MainWidget_setWindowState_show_raise_activateWindow
      setWindowTitle( "** CHANGED #4b **" );
 
      pMainWidget->setWindowState(
         ( pMainWidget->windowState() & ~ Qt::WindowMinimized ) | Qt::WindowActive );
 
      pMainWidget->show();
      pMainWidget->raise();
      pMainWidget->activateWindow();
   #endif
 
   #if  _indir_MainWidget_setWindowState_show_raise_activateWindow_setFocus
      setWindowTitle( "** CHANGED #5b **" );
 
      pMainWidget->setWindowState(
         ( pMainWidget->windowState() & ~ Qt::WindowMinimized ) | Qt::WindowActive );
 
      pMainWidget->show();
      pMainWidget->raise();
      pMainWidget->activateWindow();
 
      pMainWidget->setFocus( Qt::OtherFocusReason );
   #endif
 
   // indirectly on pExitAllButton button ...
 
   #if  _indir_ExitAllButton_show_raise_activateWindow
      setWindowTitle( "** CHANGED #2c **" );
 
      pExitAllButton->show();
      pExitAllButton->raise();
      pExitAllButton->activateWindow();
   #endif
 
   #if  _indir_ExitAllButton_show_raise_activateWindow_setFocus
      setWindowTitle( "** CHANGED #3c **" );
 
      pExitAllButton->show();
      pExitAllButton->raise();
      pExitAllButton->activateWindow();
 
      pExitAllButton->setFocus( Qt::OtherFocusReason );
   #endif
}
 
void  clMain::customEvent( QEvent *  event )
{
   switch ( event->type() ) {
      default :
         QMainWindow::customEvent( event );
         break;
 
      case Event_Main_Raise :
         vRaiseWindowForThisInst();
         break;
   }
}
 
void  clMain::vMessageReceivedFromOtherInst( void )
{
   QApplication::postEvent( this, new QEvent( Event_Main_Raise ) );
}
 
void  clMain::vExitAllClicked( void )
{
   close();
}

// != 0 -> use activateMainWidget() directly
         // == 0 -> use indirect method (following ...)
#define  _dir_activateMainWidget                                              0

         // indirectly ...

         // != 0 -> use activateMainWidget()
#define  _indir_activateMainWidget                                            0

         // indirectly on this ...

         // != 0 -> use show(); raise(); activateWindow()
#define  _indir_this_show_raise_activateWindow                                0
         // != 0 -> use show(); raise(); activateWindow(); setFocus()
#define  _indir_this_show_raise_activateWindow_setFocus                       0
         // != 0 -> use setWindowState(); show(); raise(); activateWindow()
#define  _indir_this_setWindowState_show_raise_activateWindow                 0
         // != 0 -> use setWindowState(); show(); raise(); activateWindow(); setFocus()
#define  _indir_this_setWindowState_show_raise_activateWindow_setFocus        1

         // indirectly on pMainWidget widget ...

         // != 0 -> use show(); raise(); activateWindow()
#define  _indir_MainWidget_show_raise_activateWindow                          0
         // != 0 -> use show(); raise(); activateWindow(); setFocus()
#define  _indir_MainWidget_show_raise_activateWindow_setFocus                 0
         // != 0 -> use setWindowState(); show(); raise(); activateWindow()
#define  _indir_MainWidget_setWindowState_show_raise_activateWindow           0
         // != 0 -> use setWindowState(); show(); raise(); activateWindow(); setFocus()
#define  _indir_MainWidget_setWindowState_show_raise_activateWindow_setFocus  0

         // indirectly on pExitAllButton button ...

         // != 0 -> use show(); raise(); activateWindow()
#define  _indir_ExitAllButton_show_raise_activateWindow                       0
         // != 0 -> use show(); raise(); activateWindow(); setFocus()
#define  _indir_ExitAllButton_show_raise_activateWindow_setFocus              0

#include  "clMain.h"

#include  "Main.h"
#include  "CustomEvent.h"
#include  <QGridLayout>

clMain *  pMain  = 0;

clMain::clMain( QWidget *  parent )
      : QMainWindow( parent )
{
   pExitAllButton = new QPushButton( "Exit All" );

   QGridLayout *  pLayout  = new QGridLayout;
   pLayout->addWidget( pExitAllButton, 0, 0, 1, 1 );

   pMainWidget = new QWidget();
   pMainWidget->setLayout( pLayout );

   setCentralWidget( pMainWidget );

   connect( pExitAllButton, SIGNAL( clicked() ),
            this,           SLOT( vExitAllClicked() ) );

   #if  _dir_activateMainWidget
      connect( pApp, SIGNAL( messageReceived( const QString & ) ),
               pApp, SLOT( activateMainWidget() ) );
   #else  //_USE_activateMainWidget
      connect( pApp, SIGNAL( messageReceived( const QString & ) ),
               this, SLOT( vMessageReceivedFromOtherInst() ) );
   #endif  //_USE_activateMainWidget

   setWindowTitle( "Original Title" );
   show();
}

clMain::~clMain( void )
{
   ;
}

void  clMain::vRaiseWindowForThisInst( void )
{
   // indirectly ...

   #if  _indir_activateMainWidget
      setWindowTitle( "** CHANGED #1 **" );

      pApp->activateMainWidget();
   #endif

   // indirectly on this ...

   #if  _indir_this_show_raise_activateWindow
      setWindowTitle( "** CHANGED #2a **" );

      show();
      raise();
      activateWindow();
   #endif

   #if  _indir_this_show_raise_activateWindow_setFocus
      setWindowTitle( "** CHANGED #3a **" );

      show();
      raise();
      activateWindow();

      setFocus( Qt::OtherFocusReason );
   #endif

   #if  _indir_this_setWindowState_show_raise_activateWindow
      setWindowTitle( "** CHANGED #4a **" );

      setWindowState( ( windowState() & ~ Qt::WindowMinimized ) | Qt::WindowActive );

      show();
      raise();
      activateWindow();
   #endif

   #if  _indir_this_setWindowState_show_raise_activateWindow_setFocus
      setWindowTitle( "** CHANGED #5a **" );

      setWindowState( ( windowState() & ~ Qt::WindowMinimized ) | Qt::WindowActive );

      show();
      raise();
      activateWindow();

      setFocus( Qt::OtherFocusReason );
   #endif

   // indirectly on pMainWidget widget ...

   #if  _indir_MainWidget_show_raise_activateWindow
      setWindowTitle( "** CHANGED #2b **" );

      pMainWidget->show();
      pMainWidget->raise();
      pMainWidget->activateWindow();
   #endif

   #if  _indir_MainWidget_show_raise_activateWindow_setFocus
      setWindowTitle( "** CHANGED #3b **" );

      pMainWidget->show();
      pMainWidget->raise();
      pMainWidget->activateWindow();

      pMainWidget->setFocus( Qt::OtherFocusReason );
   #endif

   #if  _indir_MainWidget_setWindowState_show_raise_activateWindow
      setWindowTitle( "** CHANGED #4b **" );

      pMainWidget->setWindowState(
         ( pMainWidget->windowState() & ~ Qt::WindowMinimized ) | Qt::WindowActive );

      pMainWidget->show();
      pMainWidget->raise();
      pMainWidget->activateWindow();
   #endif

   #if  _indir_MainWidget_setWindowState_show_raise_activateWindow_setFocus
      setWindowTitle( "** CHANGED #5b **" );

      pMainWidget->setWindowState(
         ( pMainWidget->windowState() & ~ Qt::WindowMinimized ) | Qt::WindowActive );

      pMainWidget->show();
      pMainWidget->raise();
      pMainWidget->activateWindow();

      pMainWidget->setFocus( Qt::OtherFocusReason );
   #endif

   // indirectly on pExitAllButton button ...

   #if  _indir_ExitAllButton_show_raise_activateWindow
      setWindowTitle( "** CHANGED #2c **" );

      pExitAllButton->show();
      pExitAllButton->raise();
      pExitAllButton->activateWindow();
   #endif

   #if  _indir_ExitAllButton_show_raise_activateWindow_setFocus
      setWindowTitle( "** CHANGED #3c **" );

      pExitAllButton->show();
      pExitAllButton->raise();
      pExitAllButton->activateWindow();

      pExitAllButton->setFocus( Qt::OtherFocusReason );
   #endif
}

void  clMain::customEvent( QEvent *  event )
{
   switch ( event->type() ) {
      default :
         QMainWindow::customEvent( event );
         break;

      case Event_Main_Raise :
         vRaiseWindowForThisInst();
         break;
   }
}

void  clMain::vMessageReceivedFromOtherInst( void )
{
   QApplication::postEvent( this, new QEvent( Event_Main_Raise ) );
}

void  clMain::vExitAllClicked( void )
{
   close();
}

To copy to clipboard, switch view to plain text mode 

[jacek]

Originally posted in Newbie forum, but now believe beyond that. If this violates forum policy, please pull this thread. However, this post does contain more complete information and code.

Next time, please, just ask to move the thread instead of starting a new one.

1) Since setMainWidget() was removed from Qt3 to Qt4, is QMainWindow::setCentralWidget() not functionally equivalent?

No, QMainWindow::setCentralWidget() just sets the central widget for the main window (i.e. the widget that fills the area between toolbars and statusbar).

Since this Solution was only tested using Qt 4.1.1 and I am using 4.1.0, is this a version issue?

It looks like the solution itself works. This might be a problem with Qt itself (a bug or limitation).

[Chicken Blood Machine]

What is observed:
The second instance terminates; however, the original instance does not gain
the input focus. The original instance's taskbar button usually flashes, but its
main window remains hidden behind other windows.

There's no bug in your code. It's a restriction of Windows 2000 onwards, see here:

http://msdn.microsoft.com/library/de...oundwindow.asp

[mule]

That's it! Thank you, Chicken Blood Machine. I will design a different approach.
(Anybody know of any hair-restoring products?)