Handling C++11

[Chris Jones]

> On 18 Jan 2017, at 4:15 pm, Mojca Miklavec <mojca at macports.org> wrote:
> 
>> On 18 January 2017 at 04:43, Marcus Calhoun-Lopez wrote:
>> I think perhaps I am not being clear in what I am proposing.
>> 
>> Currently, if cxx_stdlib is equal to libstdc++ then the cxx11PortGroup returns an error and the port does not build.
>> I am simply proposing that instead of returning an error, the port build instead.
>> In other words, the change is meant to respect whatever value cxx_stdlib is.
>> 
>> I am not proposing any changes to anyone’s configuration.
>> I am not proposing any changes to the buildbots.
>> 
>> If cxx_stdlib is equal to libc++, the end user would see no changes, not even revbumps.
>> If cxx_stdlib is equal to libstdc++, the only change would be that ports that didn’t build before would suddenly start (hopefully).
>> 
>> Please forgive me if I am not understanding the concerns, but this change is meant to affect *only* ports that are currently not building at all
>> for users who have elected to have cxx_stdlib be equal to libstdc++.
> 
> The only thing that most of us (that is: me included) don't understand
> is whether all "C++11 ports" would actually work flawlessly under such
> configuration.
> 
> To add to explanation on one particular example.
> 
> Octave currently requires C++11 to build. Let's assume that the user
> has < 10.9 without switching to libc++ globally. Marcus now decided to
> build octave with gcc6 and that apparently works. I think his belief
> is that if we add
>    "-D_GLIBCXX_USE_CXX11_ABI=0"
> (or something similar?) to the build flags, then the binaries won't
> crash like it would generally happen when mixing different stdlibs.
> 
> I was strongly convinced that mixing stdlibs (the default one and the
> one that comes with gcc6) was absolutely forbidden, but if adding a
> few flags can make the stdlibs "compatible", this might be a huge
> advantage that would let us build quite some ports on older OSes
> without switching to libc++. (I'm not 100% sure if that is in fact the
> case, but it might be worth investigating.)

I might be wrong but I do not think this will work. Setting the above option forces the ABI back to the old one, which is not strictly c++11 standards compliant. Maybe it might seem to help in some specific cases, but in general it is not a way of getting a completely C++11 compliant compilation and compatible stdlibs. Its not possible to have both.

Chris

> 
> 
> (That said, I would still love to see libc++ buildbots in action as one
> of our highest priorities. But that's a different issue anyway.)
> 
> Mojca