About the namespace Qt
I noticed in this book sometimes he uses Qt before an object so the compiler would know where to look , but what if I used the namespace qt in every class instead of writing qt:: every time ? is that ok ? or it's considered a bad habit and I should quit it ?
I'm not 100% sure what you mean. Can you give an example?
The "Qt" namespace contains some official enums: http://qt-project.org/doc/qt-5/qt.html