how to use $in in c++

I want to query a match on an element in an embedded object which is
in an array, the doc is like this:

"user": "admin",
"members": [ "user1", "user2" ]

I want to construct a query like this:

QUERY("user" << "admin" << "members" << opIN <<
"sam")

but there is not a label for opIN, only for GT,LT etc.
Is this supported? I really would prefer not to switch back to c from c
++ if possible.

thanks.

Sam