I'm fairly sure that as soon as it's no longer POD (plain old data), it can no longer be passed in a register, though, as it MUST have a permanent memory addres

I'm fairly sure that as soon as it's no longer POD (plain old data), it can no longer be passed in a register, though, as it MUST have a permanent memory address. A struct or class loses its POD-ness by having constructors, a destructor, being a derived class, having virtual functions, public/protected/private sections or a bunch of other very strict conditions. Simple iterators may well be POD in many STL implementations, though.