Comment by codehero

Efficiency looks past current deficiency.

We have the empty string: "\0"

We have the null string: NULL

There is no concept of an INVALID string, as float has NAN.

This would be the result of trying to copy a string to a buffer that is too small.

Or sprintf() into a small buffer.

Or a raw string parsed as UTF-8 and is invalid.

Correctness over efficiency.