Subject: Re: Is the concept of basic execution character sets useful?
From: Hubert Tong (hubert.reinterpretcast_at_[hidden])
Date: 2021-01-29 22:39:15
On Wed, Jan 27, 2021 at 3:57 AM Corentin via SG16 <sg16_at_[hidden]>
> Very quick reminder, using C++20 terminology
> We have:
> - basic source character set, which, while of limited use in the core
> language is used quite a bit in the library as a proxy for "displayable
> characters available in all encodings", which removal would then be
> slightly more involved.
> - The execution character set(s) which describe actual character sets used
> during evaluation and are therefore necessary.
> - The basic execution character set, which is a super set of the basic
> source character set
> and a subset of all execution character sets.
> It's strictly basic source character set + alert + backspace + carriage
> return + NULL
> Nowhere is it used in the library.
> It is not used in the core language either, except of course that we need
> to prescribe that NULL is encoded as 0 and that digits are encoded
> While alert + backspace + carriage return are mentioned in escape
> sequences, if a theoretical encoding would miss these characters, there
> would be no further ill-effect on the behavior of the standard.
> The main change on top of the C++20 wording would be as follow
> The basic execution character set and the basic execution wide-character
> set shall each contain all the members of the basic source character set, plus
> control characters representing alert, backspace, and carriage return, plus
> a null character (respectively, null wide character), whose value is 0. For
> each basic execution character set, the values of the members shall be
> non-negative and distinct from one another. In both the source and
> execution basic character sets,
You missed a "basic" as applied to "execution character set" here.
> the value of each character after 0 in the above list of decimal digits
> shall be one greater than the value of the previous. The execution
> character set and the execution wide-character
> set are implementation-defined supersets of the basic execution character
> set and the basic execution wide-character set, respectively. The values
> of the members of the execution character sets and the sets of additional
> members are locale-specific.
> Any reason why we should not do this?
Because the above does not update [intro.memory] and leaves a dangling
reference to the meaning of "basic execution character set". Also, the
above wording is currently meant to say (in part) that the characters
required as members of the basic execution character sets, when encoded
within a "narrow" possibly-multibyte string in any execution coded
character set supported by the implementation, are single bytes whose value
as read via a glvalue of type `char` is positive. The proposal seems to
leave the relevant sentence in a sad state.
That said, the idea of basic execution character sets (a "narrow" one and a
wide one) for which the characters have (encoding) values somewhat implies
but fails to really say certain things that are not true. We are,
therefore, indeed better off with shifting the talk of encoding values to
the locale-specific execution narrow/wide coded character sets. In other
words, I think the homework here is to be better at saying "coded character
set" when we want to.
> (As always, I'm interested in having a simple model with no
> unnecessary terminology as, as observed these past few months, it has a
> tendency to hinder our collective understanding)
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