I have application implement my custom FileEngine,and as I kown where are some Zip-based AbstractFileEngine implements and so on.
So I don't understand why we set QAbstractFileEngine "private".

Ref:http://doc-snapshot.qt-project.org/5.0/qtdoc/sourcebreaks.html
QFSFileEngine, QAbstractFileEngine, QAbstractFileEngineIterator, and QAbstractFileEngineHandler are no longer public. We recommend you to avoid using these classes as there is no compatibility promise.