[akersten]

I wouldn't prefer one more special case to test against (empty string / null string / 'invalid' string). Why can't those operations just return error codes instead? How about memcpy if you try to memcpy into a buffer that's too small - it writes an 'invalid buffer' type instead?

[codehero]

Propagating NAN is an elegant method in floating point and makes sense for well defined string encodings like UTF-8.

memcpy and company are strictly for raw unencoded buffers.