OK, I guess because this is a non-virtual method, it is resolved statically at compile-time and thus has no effect on either the class layout or the vtable. If the library is compiled with the code in place, it will generate a reference to a method that is never called externally by a caller using code compiled with the constructor commented out.

It is still hard for me to believe you can mix and match code from Qt, an app, and third-party libraries using different values for the flag and not end up getting into trouble somewhere.