As I'm sure you know, the OIDs are the only thing that's stored in the SNMP agent's database, and the only thing that's transmitted in the SNMP datagrams. What the OID's actually "mean" (in human readable text) is stored elsewhere (e.g. a text file that you import nito your SNMP manager, so you can supplement the standard MIB with your site-specific OIDs).
I've worked with SNMP in the past (distant past ;-)), but I'm unfamiliar with SNMP++ per se.
I'm sure you've already Read The Fine Manual for SNMP++, and it didn't tell you.
If your SNMP++ library is a purchased product, definitely ask your vendor's tech support (that's what they're paid for ;-).
Otherwise, here's a package, an "SMI MIB compiler" you might be able to integrate in to your SNMP++ based application: