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Subject: Re: [SG16-Unicode] P1208R3 / source_location
From: Robert Douglas (rwdougla_at_[hidden])
Date: 2019-02-20 07:14:30


Will this prevent usage of printf ("func: %s", sl.function_name ()); ?

On Tue, Feb 19, 2019, 6:37 PM Corentin <corentin.jabot_at_[hidden]> wrote:

> It kinda is but the compiler can get a useful encoding from the source
> code but not from the source file, in the general case.
> It's mostly an issue with filesystems with no or poor encoding support.
>
> I don't believe the observable behavior will be widely different from
> __FILE__ and __func__ in practical terms
>
>
>
> On Tue, 19 Feb 2019 at 14:31 Robert Douglas <rwdougla_at_[hidden]> wrote:
>
>> So filename and functionname would neccessarily have different encodings?
>> Does that not seem awful?
>>
>> On Tue, Feb 19, 2019, 6:25 PM Axel Naumann <Axel.Naumann_at_[hidden]> wrote:
>>
>>> Thanks everyone, this is what I'll take to Core.
>>> Axel.
>>>
>>> On 19.02.19 13:58, Corentin wrote:
>>>
>>> After talking with Tom, I'd like to modify function_name to be a
>>> NTMBS as it is something we can actually guarantee and I don't think
>>> __func__ should constrain the design of source location. It would
>>> consistent with thTstatisfy the NB comment (whose resolution was adopted in
>>> that direction this morning)
>>>
>>> Tom convinced me that filename cannot and should not be a NTMBS
>>>
>>>
>>> On Tue, 19 Feb 2019 at 13:22 Robert Douglas <rwdougla_at_[hidden]> wrote:
>>>
>>>> Agree.
>>>>
>>>> On Tue, Feb 19, 2019 at 5:17 PM Tom Honermann <tom_at_[hidden]>
>>>> wrote:
>>>>
>>>>> On 2/18/19 1:17 PM, Robert Douglas wrote:
>>>>>
>>>>> Historical footnote, these are intended to be as drop-in as possible
>>>>> for existing facilities. __FILE__ is a "character string literal," which
>>>>> gets it's null termination in phase 7. Since we are accessing these at
>>>>> run-time, we should thus expect these to be NTBS. Changes to this
>>>>> expectation would be a deviation from these being a drop-in replacement to
>>>>> __FILE__ and __func__. Note that [dcl.fct.def.general]
>>>>> p 8 defines __func__ as an implementation-defined string as if static
>>>>> const char __func__[] = "function-name "; which implies, also, an
>>>>> NTBS. This is the reasoning for NTBS. To do otherwise, would deviate this
>>>>> feature from __FILE__ and __func__, which it is designed to replace.
>>>>>
>>>>> Agreed. Certainly guaranteeing that these have a null terminator is
>>>>> required given that file_name() returns const char*. I don't agree with
>>>>> associating these with NTMBSs though since multi-byte has encoding
>>>>> implications.
>>>>>
>>>>> Tom.
>>>>>
>>>>>
>>>>>
>>>>> On Mon, Feb 18, 2019 at 11:20 AM Corentin <corentin.jabot_at_[hidden]>
>>>>> wrote:
>>>>>
>>>>>> Quick reply : display only, no expectation the file can be open, or
>>>>>> exists, or is a file. It's purely informative. But expectation it can be
>>>>>> displayed, the main use cases being logging. Otherwise I agree with you.
>>>>>>
>>>>>> On Mon, Feb 18, 2019, 7:16 AM Tom Honermann <tom_at_[hidden]>
>>>>>> wrote:
>>>>>>
>>>>>>>
>>>>>>> On Feb 18, 2019, at 10:04 AM, Corentin <corentin.jabot_at_[hidden]>
>>>>>>> wrote:
>>>>>>>
>>>>>>>
>>>>>>> Very good points.
>>>>>>> Wouldn't it be sufficient to specify that the strings are NTMBS
>>>>>>> encoded using the execution character set?
>>>>>>>
>>>>>>> source_location currently avoids making any assumption about how
>>>>>>> these strings are formed, including that they are derived from a source
>>>>>>> file.
>>>>>>> So since the value is implementation-defined, so should be the way
>>>>>>> it's constructed.
>>>>>>> However, it is reasonable to assume that these things are valid text
>>>>>>> and therefore have a known encoding.
>>>>>>>
>>>>>>> Adding Tom, because this is borderline SG16 territory.
>>>>>>>
>>>>>>>
>>>>>>> This isn’t borderline as we have (recently) requested review of
>>>>>>> anything involving file names.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> @Tom: Do you want to see source_location this week knowing that I'd
>>>>>>> hope it would get through LWG before the end of the week?
>>>>>>> Or do you think having function_name / filename as
>>>>>>> multi-bytes strings encoded using the execution character set is reasonable?
>>>>>>> The alternative I see are
>>>>>>>
>>>>>>> - Leave it unspecified
>>>>>>> - Force a specific character set... which the world is not ready
>>>>>>> for
>>>>>>>
>>>>>>> I think there is a higher level question to answer. Are the provided
>>>>>>> file names display only, or should one expect to be able to open the file
>>>>>>> using the provided name?
>>>>>>>
>>>>>>> If they are display only, then we can specify an encoding for them
>>>>>>> similarly to what is done for member functions of std::filesystem::path. In
>>>>>>> this case, we must explicitly acknowledge that the names do not roundtrip
>>>>>>> through the filesystem (though typically will in practice). Note that, on Windows,
>>>>>>> file names cannot be represented accurately using char based strings, so
>>>>>>> unless we want to add wchar_t support, these names will be technically
>>>>>>> display only.
>>>>>>>
>>>>>>> If they are potentially not display only, then we can’t associate an
>>>>>>> encoding and the names are bags-of-bytes. This is a limitation of POSIX.
>>>>>>> But then we need wchar_t support for Windows.
>>>>>>>
>>>>>>> In San Diego, the guidance we gave for the stacktrace proposal is
>>>>>>> that file names are implementation defined bags-of-bytes. If we
>>>>>>> advised otherwise for source location, we would be giving inconsistent
>>>>>>> guidance.
>>>>>>>
>>>>>>> I think we should discuss this in SG16 this week. Not necessarily to
>>>>>>> propose changes for the proposal, but to solidify our collective thinking
>>>>>>> around file names.
>>>>>>>
>>>>>>> Tom.
>>>>>>>
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Corentin
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Mon, 18 Feb 2019 at 03:56 Axel Naumann <Axel.Naumann_at_[hidden]>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> Hi Robert,
>>>>>>>>
>>>>>>>> Regarding your P1208R3:
>>>>>>>>
>>>>>>>> Nit: it's titled "D1208R3", it doesn't mention email addresses.
>>>>>>>>
>>>>>>>> Not-so-nit: a NB comment on the reflection TS asks to not use NTBS
>>>>>>>> but
>>>>>>>> NTMBS and "Where NTBS is mentioned in the document under ballot, the
>>>>>>>> encoding used for the string’s value is unspecified." Jens agrees
>>>>>>>> that
>>>>>>>> the proposed solution should be applied: "Specify that the strings
>>>>>>>> are
>>>>>>>> first formed using the basic source character set (with
>>>>>>>> universal-character-names as necessary) then mapped in the manner
>>>>>>>> applied to string literals with no encoding prefix in phases 5 and
>>>>>>>> 6 of
>>>>>>>> translation."
>>>>>>>>
>>>>>>>> I would very much hope that both changes are also applied to
>>>>>>>> P1208R3. I
>>>>>>>> call this out explicitly in our recommended NB comment response
>>>>>>>> paper.
>>>>>>>>
>>>>>>>> Cheers, Axel.
>>>>>>>>
>>>>>>>
>>>>>
>>>



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