In which circumstances can a `char` be initialized a non-7-bit value (>= 128)? Is it possible only in non-@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;
}

On Sunday, 27 October 2019 at 12:44:05 UTC, Per Nordlöw wrote:
> In which circumstances can a `char` be initialized a non-7-bit value (>= 128)? Is it possible only in non-@safe code?

All circumstances, `char`'s default initializer is 255.

char a; // is 255

> 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?

It will treat the numeric value as a Unicode code point then.

> 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`:

I'd say that is just plain invalid and it should throw; I'm of the opinion  the assert there is correct.

But you could also do cast into dchar, then call std.utf.encode

http://dpldocs.info/experimental-docs/std.utf.encode.1.html

to get it back to utf-8 and compare the values then. It'd spit out a two byte pair that is probably the closest thing to what the user intended.

But I'm just not convinced the library should be guessing what the user intended to begin with.

On Sunday, 27 October 2019 at 12:44:05 UTC, Per Nordlöw wrote:
> In which circumstances can a `char` be initialized a non-7-bit value (>= 128)? Is it possible only in non-@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 in D is always unsigned, it is not implementation-specific.
Therefore it can take values ​​up to (2^8)−1, If you want a signed 8 byte type you can use ubyte, which obviously can take up from -(2^7) to (2^7)-1

On Sunday, 27 October 2019 at 14:36:54 UTC, Ernesto Castellotti wrote:
> On Sunday, 27 October 2019 at 12:44:05 UTC, Per Nordlöw wrote:
>> [...]
>
> char in D is always unsigned, it is not implementation-specific.
> Therefore it can take values ​​up to (2^8)−1, If you want a signed 8 byte type you can use ubyte, which obviously can take up from -(2^7) to (2^7)-1

> signed 8 byte
correction: they are obviously 8 bits, not 8 byte

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-@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