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From: Ben Bacarisse <ben@bsb.me.uk>
Newsgroups: comp.lang.c
Subject: Re: multi bytes character - how to make it defined behavior?
Date: Wed, 14 Aug 2024 01:32:14 +0100
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Thiago Adams <thiago.adams@gmail.com> writes:

> static_assert('×' == 50071);

static_assert(U'×' == 215);

works, but then I don't know what you were trying to do.

> GCC -  warning multi byte
> CLANG - error character too large
>
> I think instead of "multi bytes" we need "multi characters" - not
> bytes.
>
> We decode utf8 then we have the character to decide if it is multi char or
> not.

These terms can be confusing and I don't know exactly how you are using
them.  Basically I simply don't know what that second sentence is
saying.

> decoding '×' would consume bytes 195 and 151 the result is the decoded
> Unicode value of 215.

Yes, Unicode 215 is UTF-8 encoded as two bytes with values 195 and 151.

> It is not multi byte : 256*195 + 151 = 50071

If that × is UTF-8 encoded then it might look, to the compiler, just
like an old-fashioned multi-character character constant just like 'ab'
does.  Then again, it might not.  gcc and clan take different views on
the matter.

You can get clang to that the same view a gcc by writing

  static_assert('\xC3\x97' == 50071);

instead.  Now both gcc and clang see it for what it is: an old-fashioned
multi-character character constant.

> O the other hand 'ab' is "multi character" resulting

The term for these things used to be "multi-byte character constant" and
they were highly non-portable.  The trouble is that the term "multi-byte
character" now refers to highly portable encodings like UTF-8.  Maybe
that's why gcc seems to have changed it's warning from what you gave to:

  warning: multi-character character constant [-Wmultichar]

> 256 * 'a' + 'b' = 256*97+98= 24930
>
> One consequence is that
>
> 'ab' == '𤤰'
>
> But I don't think this is a problem. At least everything is defined.
>

-- 
Ben.