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




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 ?

>> 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.