To be honest, your better off leaving it at home. A bass as carry on is still a large item, and it may or may not fit in the overhead. I am assuming you plan on bringing it in a gig bag? if you are lucky, and do get it to fit, there are still other people who will need to put the luggage into the overheads, which means other peoples crap will be pressing against your bass, and even in a gig bag this could cause minor damage.
There is also the fact that if you get there and it wont fit, they will make you check it, in a gig bag.
Even if it is able to get on as carryon, if you have to take a connector flight in a smaller plane, it will have to go into the luggage compartment.
If you absolutely have to bring the bass, I would just get a real flight case for it and check it, and insure it...