Hi everyone, sorry if most of you know about this already, but since it seems that QStringLiterals are being introduced everywhere right now, I think that it is important to raise awareness about the fact that this might be more dangerous that it seems at first sight.
QStringLiteral has the nice property that it stores the string data in read-only memory and avoids heap allocations when it is used to construct a QString. The QString, and any copies that are made, just contain a pointer to read-only data. There is a very nice overview by Olivier at https://woboq.com/blog/qstringliteral.html However, QString is still a non-POD type, even if it has been constructed with QStringLiteral (or copied from such a QString), so its destructor is being run, which accesses the read-only data, then finds out that it's been made with QStringLiteral, such that no refcount updates and deallocations are needed. This becomes a problem if the read-only data that the QString refers to are not there any more, which can happen if the QString was stored in a global static object from one library, and the QStringLiteral is from another library, which might have been unloaded before the global static object was destroyed. I believe that this is just what happens right now with a global static KIconLoader from kicontnkhemes and a QStringLiteral from frameworkintegration: https://bugs.kde.org/show_bug.cgi?id=359758 If my analysis is wrong, please let me know! If this crash is really caused by the QStringLiteral, then we should think about how we want to treat QStringLiteral in the future. The current approach seems to be "use it everywhere", but this might cause more crashes in the future. Possible solutions could be a) Everyone who makes QString -> QStringLiteral replacements should be extremely careful (which is very difficult, since it is not always obvious if passing a QString to a function will result in the string being stored in a global static object). Automated tools like clazy should then not recommend to use QStringLiteral any more. b) Classes like KIconLoader, which are used as global static objects, should copy all strings that they get to the heap in order to prevent such crashes (which might also be quite difficult to do consistently). Accidental mistakes are very easy to make either way, so I'm wondering if the use of QStringLiteral should be restricted to situations where it's 100% clear that there will be no risk (this will often not be the case, I think), and it also seems likely that using it will have an impact on the performance and/or memory usage (because it's a string that will be copied a lot, or because it's in a hot code path). Opinions? Regards, Frank _______________________________________________ Kde-frameworks-devel mailing list Kde-frameworks-devel@kde.org https://mail.kde.org/mailman/listinfo/kde-frameworks-devel
