Users of the persistence layer may want to examine the structure of persistent classes at run-time. This is not of a big issue in reflective languages like Java or C#, but in the C++, which is not reflective language, this requirement may pose a problem. Yet not necessarily, because the persistence layer must know about the structure of classes it is mapping to database. So, it only depends on the particular implementation of a C++ O/R mapping library whether it provides access to this information and how.
If the library puts these introspection features behind a unified interface and allows to inspect wider set of classes than only the persistent classes, it may provide at least simpler alternative to reflection features offered by reflective languages. This may be a big advantage, because developers tend to include O/R mapping features into their application frameworks and O/R mapping is often one of the pillars of infrastructural part of business applications. Therefore it reduces the need for other reflection library.