I came up with a kind of clever data type for storing short strings in a fixed size struct so they can be stored on the stack or inline without any allocations.

C++ already does that for short strings while seamlessly switching to allocation for long strings.

It’s always null-terminated so it can be passed directly as a C-style string, but it also stores the string length without using any additional data (Getting the length would normally have to iterate to find the end).

Also the case in the standard library

The trick is to store the number of unused bytes in the last character of the buffer. When the string is full, there are 0 unused bytes and the size byte overlaps the null terminator.

Iirc, that trick was used in one implementation but discontinued because it was against the standard.

(Only works for strings < 256 chars excluding null byte)

If you need a niche for allocated string you can get to 254 but the typical choice seem to be around 16.