[SOLVED]Qt 5.7 string encoding issues

[MasterBLB]

Hey mates

I read a value from database which contains special national characters,say "zażółć gęślą jaźń",and try to display it as a QString.
In old days Qt4 there were setCodecForCString() and setCodecForTr() whose worked well,sadly these are obsolete in Qt 5.7.

I've googled for a solution,but either found advices to use the above,obsolete functions,or some like QTextCodec::setCodecForLocale(QTextCodec::codecFor Name("utf-8")),or ensure Qt Creator uses UTF-8 encoding.Unfortunately,nothing worked for me :/

Therefore I'm asking you for help,how nowadays in Qt 5.7 national characters should be handled properly in an application?