Re: [boost] [preprocessor] [variadic] BOOST_PP_VARIADICS vs BOOST_NO_VARIADIC_MACROS

Subject: Re: [boost] [preprocessor] [variadic] BOOST_PP_VARIADICS vs BOOST_NO_VARIADIC_MACROS
From: lcaminiti (lorcaminiti_at_[hidden])
Date: 2012-01-14 09:07:13

Edward Diener-3 wrote
> 
> On 1/14/2012 5:42 AM, Lorenzo Caminiti wrote:
>> On Sun, Dec 18, 2011 at 10:18 AM, Paul Mensonides<pmenso57@> 
>> wrote:
>>> On Sat, 17 Dec 2011 12:45:26 -0800, lcaminiti wrote:
>>>> Why does Boost.Preprocessor redefines BOOST_PP_VARIAIDCS instead of
>>>> just
>>>> using BOOST_NO_VARIADIC_MACROS from Boost.Config?
>>>
>>> The pp-lib requires more than just superficial variadic macro support.
>>> I.e. BOOST_PP_VARIADICS macro is stronger.
>>>
>>>> Furthermore, GCC supports variadics also when C++11 extensions are not
>>>> enabled so I think the following should not&&
>>>> __GXX_EXPERIMENTAL_CXX0X__:
>>>>
>>>> // File: boost/preprocessor/config/config.hpp #    /* Wave (C/C++), GCC
>>>> (C++) */
>>>> #    elif defined __WAVE__&&  __WAVE_HAS_VARIADICS__ || defined
>>>> __GNUC__
>>>> &&  __GXX_EXPERIMENTAL_CXX0X__
>>>> #        define BOOST_PP_VARIADICS 1
>>>
>>> Because it isn't standard C++98.  It can be enabled via a command line
>>> definition of BOOST_PP_VARIADICS.  It is enabled by default (on GCC)
>>> when
>>> in C++11 mode.  I believe that GCC 4.7 actually finally defines
>>> __cplusplus
>>> correctly as well.
>>
>> It is annoying to manually -DBOOST_PP_VARIADICS. That is especially
>> true for libraries like Boost.ScopeExit and Boost.Closure that use
>> variadics in their public APIs so the end user is required to manually
>> define BOOST_PP_VARIADICS when Boost.Preprocessor does not
>> automatically detect it... (e.g., on GCC 4.5.3 without -std=c++0x).
> 
> You can just add:
> 
> #define BOOST_PP_VARIADICS
> 
> in your header files before using Boost PP headers. That isn't really 
> that annoying to do if you want to force it.
> 

Yes, in scope_exit.hpp, I could do:

#include <config.hpp>
#ifndef BOOST_NO_VARIADIC_MACROS
#define BOOST_PP_NO_VARIADICS
#endif
#include <boost/preprocessor/variadic/size.hpp>
...

But that way ScopeExit will make BOOST_PP_VARIADICS equivalent to
BOOST_NO_VARIADIC_MACROS... that seems bad practice... then why having two
macros in the first place?

The story is different if the forced definition of BOOST_PP_VARIADICS is
left to the end user- the user is hen on his own in overriding the default
detection of the Boost.Preprocessor configuration. However, that adds extra
configuration effort on the user side so Boost.Preprocessor should be as
accurate as possible in defining BOOST_PP_VARIADICS on all compilers that
actually support Boost.Preprocessor variadics.

>>
>> Can we define what "extra" features are required by BOOST_PP_VARIADICS
>> so I can write tests and run them to the Boost compilers and then
>> refine the definition of BOOST_PP_VARIADICS to automatically detect
>> all compilers where Boost.Preprocessor variadic support work.
> 
> Look at preprocessor/config/config.h for the internal logic for defining 
> BOOST_PP_VARIADICS.
> 

Yes, I found that logic (see below). But I was asking for exactly why this
logic needs to be more conservative than BOOST_NO_VARIADIC_MACROS? In other
words, what are the actual pp features that are required to define
BOOST_PP_VARIADICS but not to define BOOST_NO_VARIADIC_MACROS.

GCC 4.5.x without -std=c++0x is a good example. Why is BOOST_PP_VARIADICS
not automatically defined for this compiler? What are the specific issuers?
I was able to use at least VARIADIC_SIZE and VARIADIC_TO_SEQ on that
compiler without any issue at all...

# /* BOOST_PP_VARIADICS */
#
# if !defined BOOST_PP_VARIADICS
#    /* variadic support explicitly disabled for all untested compilers */
#    if defined __GCCXML__ || defined __CUDACC__ || defined __PATHSCALE__ ||
defined __clang__ || defined __DMC__ || defined __CODEGEARC__ || defined
__BORLANDC__ || defined __MWERKS__ || defined __SUNPRO_CC || defined
__HP_aCC && !defined __EDG__ || defined __MRC__ || defined __SC__ || defined
__IBMCPP__ || defined __PGI
#        define BOOST_PP_VARIADICS 0
#    /* VC++ (C/C++) */
#    elif defined _MSC_VER && _MSC_VER >= 1400 && !defined __EDG__
#        if _MSC_VER >= 1400
#            define BOOST_PP_VARIADICS 1
#            define BOOST_PP_VARIADICS_MSVC 1
#        else
#            define BOOST_PP_VARIADICS 0
#        endif
#    /* Wave (C/C++), GCC (C++) */
#    elif defined __WAVE__ && __WAVE_HAS_VARIADICS__ || defined __GNUC__ &&
__GXX_EXPERIMENTAL_CXX0X__
#        define BOOST_PP_VARIADICS 1
#    /* EDG-based (C/C++), GCC (C), and unknown (C/C++) */
#    elif !defined __cplusplus && __STDC_VERSION__ >= 199901L || __cplusplus
>= 201103L
#        define BOOST_PP_VARIADICS 1
#    else
#        define BOOST_PP_VARIADICS 0
#    endif
# elif !BOOST_PP_VARIADICS + 1 < 2
#    undef BOOST_PP_VARIADICS
#    define BOOST_PP_VARIADICS 1
#    if defined _MSC_VER && _MSC_VER >= 1400 && !(defined __EDG__ || defined
__GCCXML__ || defined __CUDACC__ || defined __PATHSCALE__ || defined
__clang__ || defined __DMC__ || defined __CODEGEARC__ || defined
__BORLANDC__ || defined __MWERKS__ || defined __SUNPRO_CC || defined
__HP_aCC || defined __MRC__ || defined __SC__ || defined __IBMCPP__ ||
defined __PGI)
#        define BOOST_PP_VARIADICS_MSVC 1
#    endif
# else
#    undef BOOST_PP_VARIADICS
#    define BOOST_PP_VARIADICS 0
# endif

Edward, thanks again for providing the variadic support which I found very
useful in all my libraries.

--Lorenzo

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