> On Jun 14, 2018, at 9:22 AM, Jonathan Wakely <c...@kayari.org> wrote: > > > > On 14 June 2018 at 14:08, John Spicer <j...@edg.com <mailto:j...@edg.com>> > wrote: > > > On Jun 13, 2018, at 9:12 PM, Richard Smith <rich...@metafoo.co.uk > > <mailto:rich...@metafoo.co.uk>> wrote: > > > > P0722R3 (wg21.link/p0722r3, just voted into the standard) does not specify > > a feature test macro, but I think it would benefit from one. However, it's > > not completely clear how we should arrange this: it needs both compiler > > support and library support, and is unusable without both. > > > > Should we add two feature test macros for it (one for compiler, one for > > library)? Should we recommend that the library macro be defined only if the > > language macro is defined, so that users need only check one, or should we > > keep them separate, to allow the library functionality to be discovered > > despite the language functionality being absent? (In the latter case, a > > library could be built using an old compiler and a new library, and provide > > functionality to clients that are built using a new compiler and a new > > library.) > > I think the normal case is that the compiler and library will be supplied > together, so that only the language macro should be needed. > > It's common for Clang and ICC to be used with Libstdc++, in which case we > need both macros. The compiler might support the feature and define the > macro, but unless a sufficiently-new version of Libstdc++ is used the library > won't support it. > > > > In the case where you are using a library from somewhere else, and the > library does not include the feature, I think the language feature would need > to be disabled in the compiler (e.g., by a command-line option) and that > would turn off the language macro. > > That requires knowing a priori which version of the std::lib you're using and > which features that version supports. That's one of the annoyances feature > test macros are supposed to remove :-)
So, is the implementation supposed to test the library flag to decide how to set its flag? The compatibility of a compiler and a library does not seem like it can, in general, be solved by feature test macros. As an example, an implementation can generate a call to the aligned operator new without the <new> header ever being included, but it will fail to link if the library doesn’t include the function. John. > > > > > > > > > Where should such discussions occur these days (now that the feature test > > macros have been merged into the standard)? I'm assuming this is still the > > right place. > > I’m not sure, but I think maybe these should start being discussed on the > other reflectors. > > I think we want to continue maintaining sd-6, and things related to that > should be discussed here. > > John. > _______________________________________________ > Features mailing list > Features@isocpp.open-std.org <mailto:Features@isocpp.open-std.org> > http://www.open-std.org/mailman/listinfo/features > <http://www.open-std.org/mailman/listinfo/features> >
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