On Tue, Oct 10, 2017 at 3:05 PM, Matthias Klose <d...@debian.org> wrote: > On 10.10.2017 12:38, Mathieu Malaterre wrote: >> Mathias, >> >> On Tue, Oct 10, 2017 at 12:25 PM, Matthias Klose <d...@debian.org> wrote: >>> On 10.10.2017 11:42, Mathieu Malaterre wrote: >>>> On Tue, Oct 10, 2017 at 11:38 AM, Matthias Klose <d...@debian.org> 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?
True. That was the root of my confusion, a change in the c++ language is simply just like a change in the API (or ABI). >>>> 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. Great, this means users can compile a project using a mixture of both ABI. All these defeat the reason for my initial proposal :) Thanks all for your comments, -M
