Does Q*Application always needed?

[DTF]

Hello.
Please excuse me for my poor english.

I am trying to write a program, and one of its functions will be manipulations with windowses of other programs.
In particular, moving them and making its screenshots.

I am trying to make screenshot with code like this one:

for (const auto& screen : QGuiApplication::screens()) {
   if (screen) {
      QPixmap screenshot = screen->grabWindow(window_id);
      if (!screenshot.isNull() && screenshot.save("screenshot.png")) {
         cout << "saved" << endl;
      }
   }
}

for (const auto& screen : QGuiApplication::screens()) {
   if (screen) {
      QPixmap screenshot = screen->grabWindow(window_id);
      if (!screenshot.isNull() && screenshot.save("screenshot.png")) {
         cout << "saved" << endl;
      }
   }
}

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However, to make it work, I have to create a Q*Application object:

QApplication a(argc, argv);

QApplication a(argc, argv);

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Without it, nothing works (QGuiApplication::screens() is empty).

In case where a program activelu uses Qt it is not a problem.
However, I am trying to make a small library, which can manipulate with windovs via qt, and use the library in my program.

So I want to make my program qt-independent (with exception of the manipulating library ofc).
In this case making global object of a qt type is looking bad.

So what is correct way to solve the task?
Make a singleton with QApplication inside the library?

[d_stranz]

An application class instance (something derived from QCoreApplication) is needed because it supplies an event loop to process events (Windows messages on Windows, X-Windows messages on linux, etc.). Almost everything in Qt depends on having an event loop running. There is only one QCoreApplication instance allowed, so if you implement one in your library, it means your library probably can't be used with another Qt app.

If you want to do anything GUI-related, you need a class based on QGuiApplication, because this class handles messages from the windowing system.

[anda_skoa]

QGuiApplication also loads the QPA (Qt Platform Adapter) plugin, which provides the intergration with the windowing system.

The QPA is the place that actually interfaces with the native API to get the screens, etc.

A Q*Application instance is in effect a singleton, i.e. only one instance can be created.

Cheers,
_