Hi Chris,

you are right. The problem was that my code is really large and I don't know where the problem is so I would have to post all code with all files. I think I'd never get an answer this way...

Well, I found the problem. What I did not tell you is, that I subclassed QWidget's show-function and called a QDialog in it, this way:
Qt Code: 
  1. void clsWindowMain::show()
  2. {
  3.   QWidget::show();
  4.   if(!this->windowInstall->isInstalled())
  5.   {
  6.     this->windowInstall->exec();
  7.     if(!this->windowInstall->isInstalled())
  8.       this->close();
  9.   }
  10. }
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I'd like to let QWidget check whether my program is already installed on the computer or not. If not, then QWidget should automatically call a QDialog that holds all installation settings.
The problem with it is, that the main event loop has never started (application.exec()), because QDialog has its own event loop and even when I close the QWidget after QDialog terminated, it will return to main function and start the main event loop.

The problem is, that I can't see a good way to prevent this. How can I create the main QWidget + starting its event loop (application.exec()) and after that automatically checking for installation status? As soon as I start the event loop, I can't do anything till event loop returns...
Qt Code: 
  1. int main(int argc, char *argv[])
  2. {
  3.   QApplication application(argc, argv);
  4.  
  5.   clsWindowMain windowMain(settings);
  6.   windowMain.show();
  7.   int ret = application.exec();
  8.  
  9.   if(!windowMain.windowInstall->isInstalled())  // <-- will not work because main function stops and waits at application.exec() till windowMain is closed...
  10.     windowMain.windowInstall->exec();
  11.  
  12.   return ret;
  13. }
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My only solution is something like that:
Qt Code: 
  1. int main(int argc, char *argv[])
  2. {
  3.   QApplication application(argc, argv);
  4.  
  5.   clsWindowMain windowMain(settings);
  6.   windowMain.show();
  7.   windowMain.windowInstall->exec();
  8.   int ret;
  9.   if(windowMain.windowInstall->isInstalled())
  10.     ret = application.exec();
  11.  
  12.   return ret;
  13. }
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But I think this method is dirty. I'd like to manage that stuff with methods of windowMain and not manually from the main function...

How can I show windowMain, start its event loop (application.exec()) and automatically let it check for installation?