I think is wrong version in the installation page Mongo CXX Driver when you suggest download boost 1.59 to build properly the mongo cxx driver version r3.1.1 . I was unable to compile mongo-cxx driver because the file elements.cpp has a "wrong" initialization related to a certain optional.
I solve it downloading boost version 1.63, try to compile again, and this time, voilà, compile perfectly.
So, I think, it would be a great idea, change the boost version to 1.63.
Thanks in advance.