Using a char value >= 128

[Jonathan M Davis]

On Sunday, October 27, 2019 6:44:05 AM MDT Per Nordlöw via Digitalmars-d-
learn wrote:
> In which circumstances can a `char` be initialized a non-7-bit
> value (>= 128)? Is it possible only in non- at safe code?
>
> And, if so, what will be the result of casting such a value to
> `dchar`? Will that result in an exception or will it interpret
> the `char` using a 8-bit character encoding?
>
> I'm asking because I'm pondering about how to specialize the
> non-7-bit `needle`-case of the following array-overload of
> `startsWith` when `T` is `char`:
>
> bool startsWith(T)(scope const(T)[] haystack,
>                     scope const T needle)
> {
>      static if (is(T : char)) { assert(needle < 128); } // TODO
> convert needle to `char[]` and call itself
>      if (haystack.length >= 1)
>      {
>          return haystack[0] == needle;
>      }
>      return false;
> }

char is a value above 127 all the time, because specific values above 127
are used as the first byte in a multibyte code point in UTF-8. Also, as Adam
points out, the default value for char is 255 (in order to specifically give
it an invalid value).

That being said, it doesn't make sense to use startsWith with a single char
which isn't ASCII, because no such char would be valid UTF-8 on its own.

- Jonathan M Davis