Shift damn you.

So, im writing a c++ program and i have it all done encept for some reason i cannot get the idea behind shifting a string down.
so i was wondering how you code to get
abc...yz   to become
bcd...za?

the thing that is messing with me is not so much the action of the cipher as it is that the assignment i have is making me take in a string to deal with it instead of cin.get or getline or an array. which i think would make this much easier.