Does anyone know of any convenient interface/API (Database) that
transparently encodes/decodes (serialization) graph-like C++
structures and relations between them to and from disk? I believe this
ought to have something to do with OODBMS, right?

Of course the programmer needs explicitly specify the serialization of
the data-members of the struct/class typically using the member
functions

- Obj::encode(std:stream & os)
- Obj::decode(std:stream & os).

I have also implemented "automatic management" of two-way-relations to
realized unordered graph structures.
These are realized as enum-typed double-linked/way pointers.
These could be (un)serialized automatically

Could we reuse boost::graph and boost::serialization somehow?

If not, what structures (balanced tree, hashmap, sorted arrays, ...)
should I use to construct file-format?

Thanks in advance,
Nordlöw