C++ Logo

SG16

Advanced search

Subject: Re: Wording strategy for Unicode std::format
From: Corentin Jabot (corentinjabot_at_[hidden])
Date: 2021-04-26 15:01:42


On Mon, Apr 26, 2021 at 9:34 PM Tom Honermann via SG16 <
sg16_at_[hidden]> wrote:

> On 4/26/21 1:56 PM, Peter Brett via SG16 wrote:
>
> *There has been very little interest in char*_t overloads though.*
>
>
>
> I’ve been thinking about this quite a bit over the weekend (esp. during
> our impromptu Twitter-based SG16 meeting).
>
>
>
> I’ve come to the conclusion that char8_t is actually a solution in search
> of a problem. All you know when you see a u8string is that its associated
> encoding is UTF-8, but this isn’t actually very useful for anyone; you
> still have to treat it as a bag of bytes of indeterminate encoding because,
> by (lack of) contract, a u8string can contain any old nonsense.
>
> Some of this is true, but not all of it.
>
> It is correct that a char8_t sequence or a std::u8string may contain
> garbage. This is not unique to char8_t; it is true for all of the other
> character types as well. It is also true of Rust's std::string (see
> from_utf8_unchecked()
> <https://doc.rust-lang.org/std/string/struct.String.html#method.from_utf8_unchecked>);
> it is possible to put garbage in Rust's string types too. The Rust
> standard library has a precondition that std::string contains well-formed
> UTF-8. Rust's string type is much better designed to avoid such garbage
> than std::basic_string is; no one is going to claim otherwise.
>
Rust is putting preconditions on from_utf8_unchecked() - doing the same
thing in C++ would make the value proposition of char8_t A LOT MORE
interesting

> But that lack of a strict well-formed guarantee does not prevent solving
> real problems. The introduction of char8_t solved a real problem in the
> standard library itself; it enabled std::path constructor overloads to
> support UTF-8 and eliminated the std::u8path() factory function
> workaround.
>
> char8_t was never intended to solve all UTF-8 related problems. It is
> intended to enable type safe use of UTF-8 without accidentally mixing UTF-8
> text with text in other encodings (stored in char).
>
> Anyone using char8_t for anything other than UTF-8 (except for the
> special case of creating ill-formed UTF-8 text for the purposes of testing
> that such text is correctly rejected/handled) is abusing the type. Full
> stop.
>
We need the wording to explicitly say that. And enforce it

> We can certainly add a type that enforces well-formed UTF-8 using either
> char or char8_t; I suspect it would look a lot like Rust's string type,
> including unchecked functions that enable construction with garbage because
> we don't want to pay the overhead of constant re-validation. Functions
> that except text will always have to have a wide contract or a precondition
> on well-formed text. That cannot be avoided.
>
Yes. We need to put these preconditions in place.

>
>
> Therefore, if I have the option to build my code with UTF-8 as the literal
> encoding (and nearly everyone has that option), all char8_t does is to
> provide me with two mutually-incompatible ways to express “some unknown
> bytes that might be text.”
>
> The standard library does not have that option. Nor does a lot of
> commercial software, including nearly all software that runs in some
> particular ecosystems. There are 3-5 million C++ developers (or whatever
> the current estimation is) and experiences vary considerably. Please don't
> assume that the options available to you or that your experience translates
> widely throughout the community. I have personally worked on code bases
> that, for both migration cost and technical reasons, would find switching
> to UTF-8 literal encoding infeasible.
>
>
>
> I’ve therefore decided not to spend any more of my limited available
> committee time on any language or library changes related to charN_t.
> Instead I think it would be more productive for me to focus on adding
> features that provide text codecs and validation (thanks, JeanHeyd!)
>
> That is certainly your choice and your contributions are of course welcome.
>

The point I think Peter is making is that the number one issue with both
char, and uint8_t is that they carry no semantics.
uint8_t is just an integer, char is either a narrow code unit, a byte, an
integer or some kind, or a code unit in some other encoding.

The problem with that is they are not useful to establish expectation.
You get something, which library will interpret differently which will
result in mojibake.

The _only_ way we have to avoid this issue is to to the best we can to
ensure that there is an ecosystem-wide reasonable expectation that char8_t,
but more importantly u8string_view and u8string denotes
utf-8 (meaning, tautologically, valid utf-8).

This *doesn't* mean that there cannot be functions accepting these types
with a wide contract, such as validating/decoding/transcoding functions.
But from within the center of the Unicode sandwich, that expectation should
be materialized by preconditions that these types when passed to text
functions such as fmt denote UTF-8.
We should be giving a lot more consideration to P1880 even if it means
doing a survey of all text functions in the standard library (which we
should be doing anyhow).
Note that most functions operating on xxstring(_view) have no text
semantic, it's just code units manipulation.

The reasoning extends to ALL text functions, that there is a precondition
that whatever is passed to a function that accepts text is in the encoding
expected by that function
- and a text/character function is defined tautologically in a
roundabout way as something that interprets its input using an encoding.

This doesn't change the status quo, it just gives us a tool to explain
exactly why mojibake happens, let us recognize that mojibake is bad and the
consequence of a non-well-formed program,
and gives us an opportunity to trace a well defined path.

By the same logic

   - Narrow string literals have the same abstract values when interpreted
   as in the narrow literal or narrow execution encoding
   - Ditto for wide
   - Changing the locale changes the execution encoding and so affect
   preconditions, which in practice means that utf8 literal encoding
   interpreted as Big5 will it UB pretty fast, which will result in mojibake,
   which is status quo

And from there we realize that utf8 string literals do not do much more
than adding another footgun to our users' arsenal.
And it's problematic because string literals are created everywhere from
within the unicode sandwich so they make precondition violations more
likely.
And sure we could say to our users "don't do that" but users will, and
everything will keep being terrible.

Tom.
>
>
>
> Thanks again for all your valuable contributions to this discussion,
> Victor!
>
>
>
> Peter
>
>
>
> *From:* Victor Zverovich <victor.zverovich_at_[hidden]>
> <victor.zverovich_at_[hidden]>
> *Sent:* 26 April 2021 18:36
> *To:* Peter Brett <pbrett_at_[hidden]> <pbrett_at_[hidden]>
> *Cc:* sg16_at_[hidden]
> *Subject:* Re: [SG16] Wording strategy for Unicode std::format
>
>
>
> EXTERNAL MAIL
>
> > Corentin:
> > If our requirements are ... - all of which I agree with individually - I
> don't see a path forward.
>
>
>
> I don't see any problem. Since we are in the beginning of the C++23 cycle
> it makes sense to tackle more difficult problems such as locale first.
> Adding format overloads is not hard and makes more sense to do later when
> we have a better understanding and hopefully solution for locale issues.
>
>
>
> > A non-broken Unicode locale support calls for research and
> implementation experience outside of the standard.
>
>
>
> That's exactly what I intend to do, contributors are welcome.
>
>
>
> > Providing something that is consistent and *not more* broken than the
> narrow overload seems useful in the short term.
>
> First I don't think it's necessary per my comment above and second it will
> likely severely constrain our design space for fixing these issues in the
> future.
>
>
>
> > Peter:
>
> > Just for clarity, what does {fmt} currently do?
>
>
>
> Unfortunately {fmt} cannot fix the standard library so it does something
> completely crazy in char*_t overloads, namely uses a narrow locale and
> casts characters. There has been very little interest in char*_t overloads
> though.
>
>
>
> Cheers,
>
> Victor
>
>
>
> On Thu, Apr 22, 2021 at 8:57 AM Peter Brett <pbrett_at_[hidden]> wrote:
>
> Hi Victor,
>
>
>
> This is helpful, thank you. I will put full ‘L’ handling for UTF-8/16/32
> as the preferred option in the paper.
>
>
>
> Just for clarity, what does {fmt} currently do? Obviously if it currently
> does something different then I will have to do some work to demonstrate
> implementability.
>
>
>
> Best wishes,
>
>
>
> Peter
>
>
>
> *From:* SG16 <sg16-bounces_at_[hidden]> *On Behalf Of *Victor
> Zverovich via SG16
> *Sent:* 22 April 2021 15:27
> *To:* SG16 <sg16_at_[hidden]>
> *Cc:* Victor Zverovich <victor.zverovich_at_[hidden]>
> *Subject:* Re: [SG16] Wording strategy for Unicode std::format
>
>
>
> EXTERNAL MAIL
>
> > Peter:
>
> > What should the following code do?
>
>
>
> I think (1) is the only acceptable option because all the rest are
> inconsistent with existing std::format overloads.
>
>
>
> > “std::locale in its current form is pretty much useless,” may be a true
> statement but it doesn’t help me make progress.
>
>
>
> Maybe we are trying to make "progress" in the wrong direction? We don't
> have to quickly hack something together for new std::format overloads. We
> didn't have a chance to look at locale in C++20 but now is a great time.
>
>
>
> > Corentin:
>
> > Converting between UTF-X and UTF-Y is a lossless operation.
>
>
>
> Only valid ones. There is still a question of handling transcoding errors.
>
>
>
> > what is it that we gain by not allowing format(u8"{}", u"");
> and format(u8"{}", U"");?
>
>
>
> We gain consistency between all std::format overloads, simple
> specification, not having to deal with transcoding errors. I am not
> suggesting that it shouldn't be possible but that it should be explicit,
> e.g.
>
>
>
> format(u8"{}", xcode(u""))
>
>
>
> With explicit approach you can easily configure error handling.
>
>
>
> > we could provide only the u8 overload
>
>
>
> Sure provided that we have transcoding facilities. I don't think u16 and
> u32 overloads are particularly useful since you can't do much with the
> result.
>
>
>
> > mandates that the existing locale be specialize for char8_t
>
>
>
> If this specialization inherits all existing locale problems then I think
> it's not a good idea.
>
>
>
> - Victor
>
>
>
> On Mon, Apr 19, 2021 at 3:18 AM Corentin Jabot via SG16 <
> sg16_at_[hidden]> wrote:
>
> Talking with Peter, we realized we could provide only the u8 overload and
> mandates that the existing locale be specialize for char8_t
>
> We believe this would
>
> - Satisfy Victor's excellent remark about the need not to be
> gratuitously inconsistent
> - Put minimum strain on implementers
> - Let us move forward with having a Unicode overload in 23.
>
>
>
>
>
>
>
>
>
> --
> SG16 mailing list
> SG16_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/sg16
> <https://urldefense.com/v3/__https:/lists.isocpp.org/mailman/listinfo.cgi/sg16__;!!EHscmS1ygiU1lA!WueeYVkg4epLn98-McfxKUi3lJONY6lPzMPbUArFN5V6WCOZxR45PasGv15tlA$>
>
>
>
> --
> SG16 mailing list
> SG16_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/sg16
>



SG16 list run by sg16-owner@lists.isocpp.org