On 14 June 2018 at 14:08, John Spicer <jhs_at_[hidden]> wrote:

>
> > On Jun 13, 2018, at 9:12 PM, Richard Smith <richard_at_[hidden]>
> 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 :-)





>
> >
> > 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.
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