Per this SO post:

http://stackoverflow.com/questions/30330349/how-do-you-declare-a-variable-for-a-class-whose-constructor-takes-an-rvalue

There may be situations where it is useful to default construct a database, collection, or client. Such an object is not in a valid state, and should be equivalent to a moved-from object of the same type.

We should probably also provide an explicit operator bool so that one can test whether these vocabulary types are in valid states.