[cfe-dev] __cpp_lib library feature test macros

[David Blaikie]

On Sun, Sep 21, 2014 at 11:44 AM, Seth Cantrell <seth.cantrell at gmail.com>
wrote:

> clang implements feature testing macros for language features but I notice
> that libc++ does not implement the library testing macros. Is this
> deliberate or would patches to fix this be welcome?
>
> Attached is an initial attempt at implementing
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4030.htm based
> on http://libcxx.llvm.org/cxx1y_status.html. No tests are included yet.
>
> Also, about the language feature testing, I notice that they don't take
> into account features being disabled via -Werror=.


Which, if any, features can we disable via -Werror? I don't think we have a
warning for constexpr, for example - we have a generic warning for all
C++11 features (so you can build a codebase as C++11 without allowing the
codebase to become C++98-incompatible), so arguably all the language
features could be "not available" when built with that warning and -Werror,
but that would change the behavior of your program, which would be
problematic if you wanted to make sure that when you build as C++11 the
project works (because then if you disabled the warning, you'd end up with
different behavior - so your experimental results wouldn't be
representative of the end state you were aiming for)


> It seems like this might matter in cases such as a third party library's
> headers doing feature testing, but where the local build environment has
> disabled features for whatever reason. It seems reasonable that a header
> testing #if __has_extension(__cxx_constexpr__) or #ifdef __cpp_constexpr
> would use whatever fallback code it's got when the user disables constexpr.
> Feature testing does take into account features disabled with -fno-
> arguments.
>
> - Seth
>
>
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