I haven't used program_options yet. But if I understand correctly both libraries seem to support storing and accessing data with strings that might describe some kind of hierarchy. This seems to be the core idea of both libraries - is this correct?
Then it wouldn't matter much what container is used. However a generic tree which can store data hierarchically probably makes most sense. If I understand correctly both libraries could make use of such a class?
I think generic tree container is material for another library. Whether property_tree should be based on it or not is a matter of internal implementation, and generally of little interest to users. The biggest value of property_tree is in its easy to use interface, that should not be compromised, if at all possible. I have been already reassured in this view by quite many people who took their time to review the library. Best regards, Marcin