How to make Qt GUI apps in C++ without memory leaks
I haven't been able to create a Qt GUI app that didn't have over 1K 'definitely lost' bytes in valgrind. I have experimented with this, making minimal apps that just show one QWidget, that extend QMainWindow; that just create a QApplication object without showing it or without executing it or both, but they always leak.
Trying to figure this out I have read that it's because X11 or glibc has bugs, or because valgrind gives false positives. And in one forum thread it seemed to be implied that creating a QApplication-object in the main function and returning the object's exec()-function, as is done in tutorials, is a "simplified" way to make GUIs (and not necessarily good, perhaps?).
The valgrind output does indeed mention libX11 and libglibc, and also libfontconfig. The rest of the memory losses, 5 loss records, occurs at ??? in libQtCore.so during QLibrary::setFileNameAndVersion.
If there is a more appropriate way to create GUI apps that prevents even just some of this from happening, what is it? And if any of the valgrind output is just noise, how do I create a suppression file that suppresses the right things?
EDIT: Thank you for comments and answers!
I'm not worrying about the few lost kB themselves, but it'll be easier to find my own memory leaks if I don't have to filter several screens of errors but can normally get an "OK" from valgrind. And if I'm going to suppress warnings, I'd better know what they are, right?
Interesting to see how accepted leaks can be!
Answer
It is not uncommon for large-scale multi-thread-capable libraries such as QT, wxWidgets, X11, etc. to set up singleton-type objects that initialize once when a process is started and then make no attempt to effort to clean up the allocation when the process shuts down.
I can assure you that anything "leaked" from a function such as QLibrary::setFileNameAndVersion() has been left so intentionally. The bits of memory left behind by X11/glibc/fontConfig are probably not bugs either.
It could be seen as bad coding practice or etiquette, but it can also greatly simplify certain types of tasks. Operating systems these days offer a very strong guarantee for cleaning up any memory or resources left open by a process when its killed (either gracefully or by force), and if the allocation in question is very likely to be needed for the duration of the application, including shutdown procedures -- and various core components of QT would qualify -- then it can be a boon to performance to have the library set up some memory allocations as soon as it is loaded/initialized, and allow those to persist indefinitely. Among other things, this allows the memory to be present for use by any other C++ destructors that might reference that memory.
Since those allocations are only set up once, and from one point in the code, there is no risk of a meaningful memory leak. Its just memory that belongs to the process and is thus cleaned up when the process is closed by the operating system.
Conclusion: if the memory leak isn't in your code, and it doesn't appear to get significantly larger over time (and by significant these days, think megabytes), and/or is clearly orginating from first-time initialization setup code that is only ever invoked once within your app, then don't worry about it. It is probably intentional.