NSStrings are opaque--you always call accessor functions and never have access to the low level backing store. The reason they are good is that you can't get data into or out of them without specifying an encoding, which leaves the actual encoding of the backing store as an implementation detail.

The fact is, I don't even know (or see documented) that the backing is UTF-16--Apple is free to change that at their whim and no user programs would break.