[cfe-dev] [libcxx] Policy with respect to language extensions

Tue Aug 18 18:35:06 PDT 2015

libc++ has an interesting relationship with extensions. Libc++ was
designed as  a C++11 library with C++03 compatibility.
For this reason C++11 extensions are pervasive in C++03 mode. I don't
think it would be possible to provide a strict C++03 compatibility
mode because the
libc++ C++03 internals are so dependent on C++11 library and language
features such as decltype, nullptr, std::unique_ptr, std::move,
std::forward, etc.

For newer C++ dialects it should be *possible* to provide a strict
conformance mode via a switch. Unfortunately every switch added to
libc++ increases the maintenance burden and code complexity. In my
opinion the less switches libc++ has the better.

However unlike C++03 mode, newer libc++ dialects should probably only
provide *conforming* extensions, which should somewhat reduce the need
for a "strict conformance mode". In the coming week I'll be trying
to document all of the current libc++ extensions. Hopefully that will
give us a better understanding of the problem. Please feel free to
send me any libc++ extensions you have noticed beyond the ones already
mentioned.

Anyway I think the best way to make sure your code is portable across
standard libraries then you should test multiple standard libraries.

/Eric

PS. To address the extensions you listed:

1. "return {...}; into a std::tuple" is now standard in C++1Z. It is
still an extensions in C++11 and C++14 but turning it off by default
could break user code. Also I'm not willing to make tuple any more
complex unless the need is great.

2. I'm not sure what your referring to. libc++'s implementation of
std::invoke is not constexpr. I mistakenly made __invoke constexpr a
while back but that change
has been reverted.

3. std::bitset::const_reference seems fairly innocuous and conforming.

On Wed, Aug 12, 2015 at 7:04 PM, Hal Finkel via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> ----- Original Message -----
>> From: "Hal Finkel via cfe-dev" <cfe-dev at lists.llvm.org>
>> To: "Richard Smith" <richard at metafoo.co.uk>
>> Cc: "Marshall Clow" <mclow.lists at gmail.com>, "Gonzalo BG" <gonzalobg88 at gmail.com>, "clang developer list"
>> <cfe-dev at lists.llvm.org>
>> Sent: Wednesday, August 12, 2015 7:23:30 PM
>> Subject: Re: [cfe-dev] [libcxx] Policy with respect to language extensions
>>
>> ----- Original Message -----
>> > From: "Richard Smith" <richard at metafoo.co.uk>
>> > To: "Hal Finkel" <hfinkel at anl.gov>
>> > Cc: "Marshall Clow" <mclow.lists at gmail.com>, "clang developer list"
>> > <cfe-dev at lists.llvm.org>, "Gonzalo BG"
>> > <gonzalobg88 at gmail.com>, "David Majnemer"
>> > <david.majnemer at gmail.com>
>> > Sent: Wednesday, August 12, 2015 7:09:22 PM
>> > Subject: Re: [cfe-dev] [libcxx] Policy with respect to language
>> > extensions
>> >
>> > On Wed, Aug 12, 2015 at 4:01 PM, Hal Finkel < hfinkel at anl.gov >
>> > wrote:
>> >
>> > ----- Original Message -----
>> > > From: "Marshall Clow" < mclow.lists at gmail.com >
>> > > To: "David Majnemer" < david.majnemer at gmail.com >
>> > > Cc: "Richard Smith" < richard at metafoo.co.uk >, "clang developer
>> > > list" < cfe-dev at lists.llvm.org >, "Gonzalo BG"
>> > > < gonzalobg88 at gmail.com >
>> > > Sent: Wednesday, August 12, 2015 5:05:05 PM
>> > > Subject: Re: [cfe-dev] [libcxx] Policy with respect to language
>> > > extensions
>> > >
>> > > On Wed, Aug 12, 2015 at 11:32 AM, David Majnemer <
>> > > david.majnemer at gmail.com > wrote:
>> > >
>> > >
>> > > On Wed, Aug 12, 2015 at 2:05 PM, Richard Smith <
>> > > richard at metafoo.co.uk > wrote:
>> > >
>> > >
>> > >
>> > > I think it would also be useful to have a "strictly conforming"
>> > > (or
>> > > as close as we can reasonably get) mode, controlled by a macro.
>> > >
>> > >
>> > > Wouldn't we have to be extra careful or we could get ODR
>> > > violations
>> > > across object/shared object boundaries if one wanted extra
>> > > conformance but another did not?
>> > >
>> > > Yes.
>> > >
>> > >
>> > > -- Marshall
>> >
>> > Having a macro to control this seems like a recipe for bad things
>> > (ODR violation being only one problem).
>> >
>> >
>> >
>> > What sort of actual bad things do you envisage? People will, of
>> > course, be able to shoot themselves in the foot by having an
>> > inconsistent definition in different translation units, but we
>> > don't
>> > necessarily need to support that. And it seems like we should be
>> > able to avoid this having an ABI impact on sane code.
>
> Also, a few questions:
>
>  1. Are these all confirming extensions?
>
>  2. Would this be an opt-in or opt-out macro?
>
> Having thought about this for an additional few minutes, I think the relevant question is: Is there any sane code that would need to turn these things off in order to function properly? If not, then the header-file library issue won't arise in practice, and the macro is likely harmless. If so, it is not good. Regardless, I think being able to issue a warning would be better.
>
>  -Hal
>
>>
>> For these particular extensions, I can't say. However, I tend to find
>> the general technique problematic. The issue generally arises for
>> header-only libraries that define (or undefined) these kinds of
>> macros, causing very odd interactions (because such things don't
>> compose). Tracking down why including one library header breaks some
>> other library header because of conflicting desires to define
>> _XOPEN_SOURCE, _GNU_SOURCE, etc. is annoying. Doing the same for
>> features of the C++ standard library that could affect uses of
>> SFINAE, etc., I suspect, would be especially annoying.
>>
>>  -Hal
>>
>> >
>> > I think the best we can do at this point is have the compiler issue
>> > a
>> > warning in pedantic mode. Maybe we could add some kind of extension
>> > attribute for this purpose (although we'd need to think carefully
>> > about when to actually generate the warning - every time some
>> > extension participates in some overload set might be problematic).
>> >
>> > -Hal
>> >
>> >
>> >
>> > >
>> > > _______________________________________________
>> > > cfe-dev mailing list
>> > > cfe-dev at lists.llvm.org
>> > > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>> > >
>> >
>> > --
>> > Hal Finkel
>> > Assistant Computational Scientist
>> > Leadership Computing Facility
>> > Argonne National Laboratory
>> >
>> >
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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