Qt without QApplication

[jakebman]

How much of Qt can be run without calling QApplication::exec in the main thread? We can call it in another thread, just not the initial one.

Can we still create windows? Can we draw in them? Does sound still work?

How much would I break if I never call QApplication::exec?

[wysota]

If you never call exec() or processEvents(), events won't be delivered, so using any GUI or network functionality will not be possible. May I ask why you don't want to call QApplication::exec()?

[jakebman]

I'm trying to rewrite an existing graphics library for an introductory C++ class. The library is just a set of function - not event-driven, like Qt. I'd like to keep the API the same, which means I have to drop QApplication::exec, or do something clever (read "stupid") to it.

The class covers standard C++ on the command line. They write main(){...} and bring up a drawing window by calling a library function, InitGraphics(). Their code is mainly driven by cin/getline and cout/printf, so it has to have a connection to the console.

The way I see it, unless I can get around QApplication::exec (hence the question), I have to set up another executable (ugh) or figure out another way to run QApplication::exec in another thread from main().

Mostly I'm curious if anyone has experience with Qt outside of the event loop.

[fullmetalcoder]

running QApplication does not forbid you to use command line input/output as far as I know.

if I understand well the what you want to do is rather a kind of interpreter or am I missing something? If you need to keep the CLI, the simplest thing you could do to refactor the I/O would be to use a QTextStream, initialized as follows :

QTextStream cli_input(stdin);

QTextStream cli_input(stdin);

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However, you might as well drop the CLI and write a small input window where the user would type some commands and then pass them to your library.

Hope this helps, even though I'm not sure I fully understood what you want to do.

[JimDaniel]

Not sure if this helps, but we are writing an app for real-time 3D graphics, and we don't call app.exec(), as we need control over the timing of the loop, so we create our own loop and call app.processEvents() and app.sendPostedEvents() when we are ready, then app.exit() when the program shuts down.

[wysota]

Actually the correct approach would be to merge both event loops by integrating Qt's abstract event dispatching mechanism. With your current approach you can't have things using local event loops like modal dialogs.

[Brandybuck]

If all you want is to draw in a window somehow (like logo's turtle graphics), then maybe you can make a simple Qt server application that has a window, and the student's write client applications that issue drawing commands over IPC (qtdbus).