Take a look at the standard library reference for deque:
http://www.cplusplus.com/reference/deque/deque/
Look for the complexity field.
** EDIT **
I can't verify if those statements are right without looking at how your deque is implemented. You have different member functions than those for the std::deque data structure.
Then you wouldn't have to look through the items to find the first element, so it would be O(1).
But you would have to look through the items to find the last, so it would be O(n).