[Boost-users] [serialization] link error and exception on get_key

I have a derived class that exports its type and overrides get_key for serialization through a base class pointer. If I move the definition of the overridden get_key function out of the header into a cpp file, an assertion fails on line 49 of extended_type_info.cpp. If I define any other functions in the cpp file, I get a linker error saying that the get_key symbol is already defined. It looks to me that any serializable derived class must be completely defined in the header file. Is this correct? If so, why does such a limitation exist? Thank you