What's the difference between "-no-stl" and "-stl"
If I build the QT Lib with "-no-stl" option what will happen?
eg: The "std::string QString::toStdString() const" function will be unusable?
This option is pretty much obsolete (it has been removed in Qt5).
If you disable STL, Qt containers will use their own, custom implementation. I think the QString method you mention is not affected by this, but please test.