How to not show widget

[amoswood]

I have already tried to subclass QObject. The problem is then that QObject has no exec(), so the program can not have return Client.exec() in main.cpp.

I don't know what you are doing in your Client class, but you can work around the exec() function with the code or the Qt class QEventLoop:

while(true)
{
  QApplication::instance()->processEvents();
  QApplication::instance()->sendPostedEvents();
}

while(true)
{
  QApplication::instance()->processEvents();
  QApplication::instance()->sendPostedEvents();
}

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[gQt]

The class QEventLoop solved the problem, thanks Client can subclass QEventLoop, and QEventLoop has also an exec() function, so it is still possible to have the Client.exec() in main.