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Re: Mandates explicit -std=c++XY for c++ projects




On 10.10.2017 12:38, Mathieu Malaterre wrote:
> Mathias,
> 
> On Tue, Oct 10, 2017 at 12:25 PM, Matthias Klose <doko@xxxxxxxxxx> wrote:
>> On 10.10.2017 11:42, Mathieu Malaterre wrote:
>>> On Tue, Oct 10, 2017 at 11:38 AM, Matthias Klose <doko@xxxxxxxxxx> wrote:
>>>> On 10.10.2017 08:45, Mathieu Malaterre wrote:
>>>>> Dear all,
>>>>>
>>>>> Since the GCC 6 release [1], the default mode for C++ is now
>>>>> -std=gnu++14 instead of -std=gnu++98. What this means is that upon
>>>>> (re)compilation a library written for c++98 will be recompiled using a
>>>>> different c++ standard (c++14 in this case), unless of course the
>>>>> upstream package explicitly set the -std= flags with the appropriate
>>>>> c++ version.
>>>>>
>>>>> The ISO committee generally describe the change in between different
>>>>> standards [2] and in some case, one can find examples of subtle change
>>>>> in behaviors [3] and [4].
>>>>>
>>>>> With this mind I'd like to make mandatory the -std=c++XY flags when
>>>>> compiling either a c++ library or a stand-alone c++ program:
>>>>>
>>>>> 1. Either upstream define the explicit -std=c++XY flags by mean of its
>>>>> build system,
>>>>> 2. Or the package maintainers needs to explicit change the CXXFLAGS to
>>>>> pass the appropriate version of the c++ standard. In which case this
>>>>> should be documented in the README.Debian file.
>>>>> 3. As a fallback, dh should initialize the CXXFLAGS with -std=gnu++98
>>>>>
>>>>> If there is a consensus on the following change, I'll go ahead and
>>>>> also file a bug for lintian to scan the compilation logs in search for
>>>>> missing -std=c++ expression when g++ command line are issued.
>>>>
>>>> I don't think this is a good idea, and I'm still trying to understand what
>>>> problem you are trying to solve.  It took a while until GCC had stable c++11
>>>> support, and now you want to fallback to a 20 year old standard by default?
>>>
>>> As said above, this is simply a fallback, I am fine with any value as
>>> long as I can see it in the log clearly.
>>>
>>> My point was about making the flag *explicit*, whatever value is
>>> chosen, so that upon recompilation we gets the same symbols, the same
>>> behavior, no FTBFS (because of deprecated feature).
>>
>> Various libraries do error out on deprecated functions/macros, which you can
>> override by preprocessor macros.  This is usually done in the package.  Why
>> should that be different for the compiler?
>>
>>>> It would be better to spend some time to prepare for -std=gnu++17 instead ;)
>>>
>>> So you have no major objection against my proposal,
>>
>> No, I have objections, because after a while this will become a debt to maintain ...
> 
> Thanks for taking the time to answer.
> 
> As with Gert's answer, I fail to understand your point: You would
> prefer to see packages being (re)compiled *without* an explicit -std=
> flag, so that it always gets recompiled using whatever is the current
> c++ standard used by gcc, correct ?

yes, that's what we do with every package, so why do you want to have an
exception here?

>>> I feared you may
>>> have an issue with mixed combination of stdc++ runtime lib (not sure
>>> exactly how this is handled at low level).
>>
>> There's only one runtime library in use. Yes, there are more or less subtle
>> issues which we address by library package renames for issues we cannot or do
>> not want to handle by renaming libstdc++ itself.  But it's always the same
>> runtime library used, independent of the standard.
> 
> Ah nice.
> 
> I may be slamming open door, but could you confirm that you see no
> objection in maintaining a complex stdc++ package where the c++98 ABI
> (eg. std::string) will not be used by any packages in the near future.

yes, if you either make sure that everything using this library is using c++98,
or that this library provides a layer that it can interact with both
std::string[c++89] and std::string[c++11].  However afaik no upstream besides
libstdc++ has done that.  Again, I don't mind doing that on a package level, but
it shouldn't be any fallback to use an old standard.

Matthias