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?

well i guess what i mean is then how would the programming work for it then. if you take the string from the user and get its size with something like input.size then you would have a cutoff for the letters but after you have this and you can operate on them, the way by which you shift the words is tricky. if i shift them by making a=0, b=1 and so forth then a shift over will make something become a letter that is not a number.

so that is what i mean by the idea of it. how to shift without the loss of that number and how to place it at the beginning. so that a user can input a shift value of lets say 6, and type xyz to get cde.