While trying to implement custom protocol handlers for QNetworkAccessManager, I spotted this:

QNetworkReply::isFinished() is non-virtual, and is implemented to return d_func()->isFinished()

Since d_func isn't accessible from outside Qt sources, the custom protocol handlers can't overload QNetworkReplyPrivate::isFinished(), therefore isFinished() always returns false (as per its definition in the base class in qnetworkreply_p.h).

Because of this, some functions (e.g. a synchronous download along the lines of

QNetworkReply *r=nam->get(whatever);
while(!r->isFinished())
QCoreApplication::instance()->processEvents();
// do something)

work fine for the builtin handlers, but there's no chance for a handler implemented outside of Qt to work correctly in this context.