Re: [boost] [config] C++0x macros

From: Beman Dawes (bdawes_at_[hidden])
Date: 2008-05-25 16:49:38

David Abrahams wrote:
> on Sun May 25 2008, Beman Dawes <bdawes-AT-acm.org> wrote:
> 
>> There was some past discussion of changing the C++0x BOOST_HAS_* macros 
>> to BOOST_NO_*. Several participants thought that was better for the long 
>> run. Who needs to do what to make this happen?
> 
> IIRC the convention is BOOST_HAS_* when it's describing a nonstandard
> feature and BOOST_NO_* when it's describing a standard feature that is
> not supported be the compiler.  In other words, IIUC, BOOST_NO_* is
> reserved for describing nonconformances.

Yes, that's my understanding too.

>> The motivation for asking is that I'm starting to update some Boost code 
>> that could benefit from C++0x features and so would like to start using 
>> these macros.
> 
> of course, the fact that nonstandard features are now becoming standard
> makes things somewhat awkward.

Yes. Doug and I had a discussion of this, and decided that in the long 
run we were better off with the BOOST_NO_* approach.

>> Also, a C++03 workaround for the lack of the C++0x keyword "constexpr" 
>> is to simply omit it. Should we have a BOOST_CONSTEXPR macro defined as 
>> "constexpr" for platforms that support constexpr, otherwise as null?
> 
> That's not a bad idea.  I wonder whether it might not be better just to
> 
>        #define constexpr
> 
> in that case, and use that symbol.

I thought of that, and decided it wasn't a good idea to define a symbol 
that non-boost code might also be using and/or redefining. But that's 
only a weak opinion, and I'm open to reconsidering it. What do others think?

--Beman