Just a note about a non-Valgrind, windows-specific leak-checking method:

https://docs.microsoft.com/en-us/visualstudio/debugger/finding-memory-leaks-using-the-crt-library?view=vs-2019

It has been a while since I have done any extensive work on a Microsoft platform, but in my recollection crtdbg.h can do many helpful diagnostic things, although it is sometimes tricky to get all the debug build settings configured in the way that will trigger the features you seek.

Thanks to both.
@SGaist
I don't give a parent to QListWidgetItem constructor if I use insertItem method.

@mrjj
I overridden operator<.

It works now.

Then you should also implement the sizeHint method of your delegate