From doc:

"This section describes the standard format of the MongoDB connection URI used to connect to a MongoDB database server. The format is the same for **all** official MongoDB drivers"

C Driver (and c++ too) seem not use connectionString. They use a old format connection string that isn't documented anywhere.

A lot of options listed here can't be setted in any way:
http://docs.mongodb.org/manual/reference/connection-string/