From these two web pages:
http://docs.mongodb.org/manual/core/capped-collections/
and:
http://docs.mongodb.org/manual/core/capped-collections/#capped-collections-options

it seems that the default order for a query in a capped collection is the natural order.

"If you perform a find() on a capped collection with no ordering specified, MongoDB guarantees that the ordering of results is the same as the insertion order."

Can you confirm that this applies also to the c++ driver?