Re: [PATCH] C++11 keyword fallback

[Roger Leigh]

On Sun, Feb 03, 2013 at 01:02:04PM -0800, Paul Eggert wrote:
> On 02/02/2013 06:01 PM, Roger Leigh wrote:
> > The attached patch introduces three macros:
> >   AC_CXX_CONSTEXPR
> >   AC_CXX_FINAL
> >   AC_CXX_NULLPTR
> > which behave like AC_C_CONST et al but are for the new
> > C++11 constextr, final and nullptr keywords.
> 
> Sorry, I don't know C++, so I'm not really qualified to judge
> the utility of these macros or of Miles's qualms about them,
> but would his comments be addressed by Autoconf macros that
> cause config.h to #define HAVE_CONSTEXPR rather than #defining
> constexpr, etc.?  Or would that just be too awkward?  I guess
> I don't know the usage scenario here.

My initial intention was that my substituting the keyword with
nothing, it would transparently allow code to build on non
C++11 compilers.  HAVE_* would permit the user to do this
themselves if appropriate.

However, after looking into this a bit more, I'm becoming less
sure of the utility.  In the case of nullptr, it's a type in
its own right (std::nullptr_t) which can be used to influence
template specialisation.  So the substitution could easily
break code.  Likewise for constexpr and final.

> > Would including macros such as AC_CXX_MEMORY, AC_CXX_TUPLE,
> > AC_CXX_REGEX etc. be acceptable?
> 
> Is the pattern the same for all these?  If so, it sounds
> like it'd be better to have one macro AC_CXX_STD and invoke
> it via AC_CXX_STD([memory]), AC_CXX_STD([tuple]), etc.
> Even if there are slight differences it still may be better
> to have one "smart" macro rather than lots of macros with
> repetitive innards.

The pattern is roughly the same.  However... each needs a
bunch of different types importing into the std namespace, so
it's not /that/ slight.  And for some (e.g. regex) the headers
are currently bust in GCC libstdc++ so need to be coupled with
a proper test instatiating and using the types to make sure they
work.  We could certainly have a generic function like
AC_C_STD_TRY we pass all the details to.


Regards,
Roger

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