Re: [Boost-users] [preprocessor] Undeclared identifier BOOST_PP_IIF_0 under Clang only

Subject: Re: [Boost-users] [preprocessor] Undeclared identifier BOOST_PP_IIF_0 under Clang only
From: Edward Diener (eldiener_at_[hidden])
Date: 2014-04-03 16:39:01

On 4/3/2014 3:38 PM, Pierre Talbot wrote:
> On 04/03/2014 05:31 PM, Edward Diener wrote:
>> On 4/2/2014 3:48 PM, Pierre Talbot wrote:
>>> Hi,
>>>
>>> I defined a macro using Boost.Preprocessor that compiles and works fine
>>> under gcc but fails to compile under Clang. The macro is defined here:
>>> https://github.com/ptal/Boost.Expected/blob/master/include/boost/functional/do_yield.hpp
>>>
>>
>> As a quick shot, without actual testing, define BOOST_PP_VARIADICS to 1.
> It worked, thanks, but the problem is that it won't work if the user has
> previously included preprocessor without having defined this macro.

Agreed for any given translation unit which includes any of your headers.

I would just explain to end-users of your library using Boost 1.55 or 
lower that they need to define BOOST_PP_VARIADICS to 1 if they are using 
clang and Boost PP. And/or you could also add to your library ( in maybe 
a setup.hpp header ) something like:

#if defined(BOOST_PP_VARIADICS)
#if !BOOST_PP_VARIADICS
#error BOOST_PP_VARIADICS cannot be defined as 0 when using Expected
#endif
#else
#define BOOST_PP_VARIADICS 1
#endif

> Maybe it should be defined by default if the language version is C++11?

In the latest modular-boost BOOST_PP_VARIADICS is defined as 1 for all 
versions of clang in the Boost PP config.hpp header. This was a 
conservative oversight regarding clang by Paul Mensonides when variadic 
macro support was added to pplib. I have since confirmed with clang 
developers that clang always supported variadic macros even in their 
earliest implementations and even without C++11 support.

>>
>>>
>>>
>>> And the error message follows this message, the file using the macro is
>>> https://github.com/ptal/Boost.Expected/blob/master/example/monadic_calculator/monadic_calculator.cpp
>>>
>>>
>>>
>>> Thanks for any helps,
>>> Pierre Talbot
>>>
>>> Boost.Expected/example/monadic_calculator$ b2
>>> ...patience...
>>> ...found 1246 targets...
>>> ...updating 2 targets...
>>> clang-linux.compile.c++.without-pth
>>> bin/clang-linux-3.5.0/debug/monadic_calculator.o
>>> monadic_calculator.cpp:141:15: error: use of undeclared identifier
>>> 'BOOST_PP_IIF_0'
>>>      etokens = DO(
>>>                ^
>>> ../../include/boost/functional/do_yield.hpp:100:17: note: expanded from
>>> macro 'DO'
>>> #define DO(...) DO_SEQ(BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__))
>>>                  ^
>>> ../../include/boost/functional/do_yield.hpp:97:3: note: expanded from
>>> macro 'DO_SEQ'
>>>    BOOST_PP_SEQ_FOR_EACH_I(DO_ELEM, seq, seq) \
>>>    ^
>>> /usr/local/boost_1_55_0/boost/preprocessor/seq/for_each_i.hpp:27:55:
>>> note: expanded from macro 'BOOST_PP_SEQ_FOR_EACH_I'
>>> #    define BOOST_PP_SEQ_FOR_EACH_I(macro, data, seq)
>>> BOOST_PP_FOR((macro, data, seq (nil), 0), BOOST_PP_SEQ_FOR_EACH_I_P,
>>> BOOST_PP_SEQ_FOR_EACH_I_O, BOOST_PP_SEQ_FOR_EACH_I_M)
>>>                                                        ^
>>> /usr/local/boost_1_55_0/boost/preprocessor/repetition/for.hpp:27:23:
>>> note: expanded from macro 'BOOST_PP_FOR'
>>> # define BOOST_PP_FOR BOOST_PP_CAT(BOOST_PP_FOR_,
>>> BOOST_PP_AUTO_REC(BOOST_PP_FOR_P, 256))
>>>                        ^
>>> /usr/local/boost_1_55_0/boost/preprocessor/cat.hpp:22:32: note: expanded
>>> from macro 'BOOST_PP_CAT'
>>> #    define BOOST_PP_CAT(a, b) BOOST_PP_CAT_I(a, b)
>>>                                 ^
>>> /usr/local/boost_1_55_0/boost/preprocessor/cat.hpp:29:34: note: expanded
>>> from macro 'BOOST_PP_CAT_I'
>>> #    define BOOST_PP_CAT_I(a, b) a ## b
>>>                                   ^
>>> <scratch space>:130:1: note: expanded from here
>>> BOOST_PP_FOR_1
>>> ^
>>> /usr/local/boost_1_55_0/boost/preprocessor/repetition/detail/for.hpp:22:37:
>>>
>>> note: expanded from macro 'BOOST_PP_FOR_1'
>>> # define BOOST_PP_FOR_1(s, p, o, m) BOOST_PP_FOR_1_C(BOOST_PP_BOOL(p(2,
>>> s)), s, p, o, m)
>>>                                      ^
>>> /usr/local/boost_1_55_0/boost/preprocessor/repetition/detail/for.hpp:279:42:
>>>
>>> note: expanded from macro 'BOOST_PP_FOR_1_C'
>>> # define BOOST_PP_FOR_1_C(c, s, p, o, m) BOOST_PP_IIF(c, m,
>>> BOOST_PP_TUPLE_EAT_2)(2, s) BOOST_PP_IIF(c, BOOST_PP_FOR_2,
>>> BOOST_PP_TUPLE_EAT_4)(BOOST_PP_EXPR_IIF(c, o)(2, s), p, o, m)
>>>                                           ^
>>> /usr/local/boost_1_55_0/boost/preprocessor/control/iif.hpp:18:37: note:
>>> expanded from macro 'BOOST_PP_IIF'
>>> #    define BOOST_PP_IIF(bit, t, f) BOOST_PP_IIF_I(bit, t, f)
>>>                                      ^
>>> /usr/local/boost_1_55_0/boost/preprocessor/control/iif.hpp:25:39: note:
>>> expanded from macro 'BOOST_PP_IIF_I'
>>> #    define BOOST_PP_IIF_I(bit, t, f) BOOST_PP_IIF_ ## bit(t, f)
>>>                                        ^
>>> <scratch space>:136:1: note: expanded from here
>>> BOOST_PP_IIF_0
>>> ^