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Subject: Re: [SG16-Unicode] Source file encoding (was: What is the proper term for the locale dependent run-time character set/encoding used for the character classification and conversion functions?)
From: Tom Honermann (tom_at_[hidden])
Date: 2019-08-13 21:17:11


Niall, this is again off topic for this thread.  But now that you put
this out there, I feel obligated to respond.  But please start a new
thread with a different set of mailing lists if you wish to continue
this any further; this is not a CWG issue.

On 8/13/19 12:03 PM, Niall Douglas via Liaison wrote:
> On 13/08/2019 15:27, Herring, Davis via Core wrote:
>>> Is it politically feasible for C++ 23 and C 2x to require
>>> implementations to default to interpreting source files as either (i) 7
>>> bit ASCII or (ii) UTF-8? To be specific, char literals would thus be
>>> either 7 bit ASCII or UTF-8.
>> We could specify the source file directly as a sequence of ISO 10646 abstract characters, or even as a sequence of UTF-8 code units, but the implementation could choose to interpret the disk file to contain KOI-7 N1 with some sort of escape sequences for other characters. You might say "That's not UTF-8 on disk!", to which the implementation replies "That's how my operating system natively stores UTF-8." and the standard replies "What's a disk?".
> I think that's an unproductive way of looking at the situation.
>
> I'd prefer to look at it this way:
>
>
> 1. How much existing code gets broken if when recompiled as C++ 23, the
> default is now to assume UTF-8 input unless input is obviously not that?
*All* code built on non-ASCII platforms, some amount of code (primarily
in regions outside the US) that is currently built with the Microsoft
compiler and encoded according to the Windows Active Code Page for that
region, and source code encoded in Shift-JIS or GB18030.
>
> (My guess: a fair bit of older code will break, but almost all of it
> will never be compiled as C++ 23)

I think you'll need to find a way to measure the breakage if you want to
pursue such a change.

Personally, I don't think this is the right approach as adding more
assumptions about encodings seems likely to lead to even more problems. 
My preference is to focus on explicit solutions like adding an encoding
pragma similarly to what is done in Python and HTML and is existing
practice for IBM's xlC compiler
(https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.3.0/com.ibm.zos.v2r3.cbclx01/zos_pragma_filetag.htm).

>
>
> 2. How much do we care if code containing non-UTF8 high bit characters
> in its string literals breaks when the compiler language version is set
> to C++ 23 or higher?
>
> (My opinion: people using non-ASCII in string literals without an
> accompanying unit test to verify the compiler is doing what you assumed
> deserve to experience breakage)

Instead of non-ASCII, I think you mean characters outside the basic
source character set.

Testing practices have varied widely over time and across projects.  I
don't think it is acceptable to think it ok for other people's code to
break because it wasn't developed to your standards.

>
>
> 3. What is the benefit to the ecosystem if the committee standardises
> Unicode source files moving forwards?
>
> (My opinion: people consistently underestimate the benefit if they live
> in North America and work only with North American source code. I've had
> contracts in the past where a full six weeks of my life went on
> attempting mostly lossless up-conversions from multiple legacy encoded
> source files into UTF-8 source files. Consider that most, but not all,
> use of high bit characters in string literals is typically for testing
> that i18n code works right in various borked character encodings, so
> yes, fun few weeks. And by the way, there is an *amazing* Python module
> full of machine learning heuristics for lossless upconverting legacy
> encodings to UTF-8, it saved me a ton of work)
I agree we need to provide better means for handling source file
encodings.  But this all-or-nothing approach strikes me as very costly. 
Many applications are composed from multiple projects. Improving support
for UTF-8 encoded source files will require means to adopt them
gradually.  That means that there will be scenarios where a single TU is
built from differently encoded source files. We need a more fine grained
solution.
>
>
> But all the above said:
>
> 4. Is this a productive use of committee time, when it would displace
> other items?
>
> (My opinion: No, probably not, we have much more important stuff before
> WG21 for C++ 23. However I wouldn't say the same for WG14, personally, I
> think there is a much bigger bang for the buck over there. Hence I ask
> here for objections, if none, I'll ask WG14 what they think of the idea)

I think this is a productive use of SG16's time.  I don't think it is a
productive use of the rest of the committee's time until we have a
proposal to offer.

Tom.

>
>
> Niall
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