All the above discussion is really a discussion about what the assignment of an object that has a const member means.  Well, in C++, it really doesn't make sense and I agree.

That's why I believe there's some tiny piece missing from the serialization library.  We should also be able to unserialize instances through a factory that returns an actual object rather than having to use operator>>.

Here's some code showing this idea that works along with the previous example.