Given a single cursor, iterator construction and iteration result in the construction of bsoncxx::document::view objects wrapping the bson_t pointer returned by a call to mongoc_cursor_next. However, the validity of this bson_t pointer is only until the next call to mongoc_cursor_next (see docs).

Consider this example:

auto cursor = db["fo"].find({});
auto iter1 = cursor.begin();  // Calls mongoc_cursor_next().
auto iter2 = cursor.begin();  // Calls mongoc_cursor_next().

// At this point, iter1._doc violates the lifecycle constraints.

iter1++;

// At this point, iter2._doc violates the lifecycle constraints.

In short, only the most recently iterated iterator ever has a valid bsoncxx::document::view.