Re: [Boost-users] [Preprocessor] Compile error after change from Boost 1.55 to 1.57

Subject: Re: [Boost-users] [Preprocessor] Compile error after change from Boost 1.55 to 1.57
From: Edward Diener (eldiener_at_[hidden])
Date: 2015-02-27 09:56:44

On 2/27/2015 5:04 AM, Franz Alt wrote:
> Hi,
>
> I've written some preprocessor directives to generate functions for a
> class. With Boost 1.55 everything works fine with gcc and clang. When I
> try to change to Boost 1.57 I've got some strange compile errors.
>
> I'm happy if someone could help.

In your FUNCTION_DECL macro you have this line:
 
BOOST_PP_IF(BOOST_PP_LIST_IS_NIL(BOOST_PP_TUPLE_TO_LIST(BOOST_PP_TUPLE_ELEM(1, 
tup))), \

but with your second function of '(b)' there is only one tuple element 
so attempting BOOST_PP_TUPLE_ELEM(1,tup) on it is undefined behavior. 
Instead test the size of the tuple with BOOST_PP_TUPLE_SIZE and if it is 
only 1 then you know you don't have a second tuple element and can 
convert it to '()'. In other words instead of:

BOOST_PP_IF(BOOST_PP_LIST_IS_NIL(BOOST_PP_TUPLE_TO_LIST(BOOST_PP_TUPLE_ELEM(1, 
tup))), \
    (), \

have

BOOST_PP_IIF(BOOST_PP_EQUAL(BOOST_PP_TUPLE_SIZE(tup),1)), \
    (), \

You can't rely on undefined behavior and your version relies on 
BOOST_PP_TUPLE_ELEM on an element which does not exist return nothing 
and on BOOST_PP_TUPLE_TO_LIST when presented with nothing for a tuple 
expanding to an empty list.

The fact that this once worked is just a coincidence of the 
implementation details, and these have changed from 1.55 to 1.57.

>
> Thanks
>
> Franz
>
>
> gcc 4.9.1 failed with error:
>
>      test.cpp:33:1: error: macro "BOOST_PP_SEQ_ELEM_III" requires 2
> arguments, but only 1 given
>         )
>       ^
>      test.cpp:33:1: error: macro "BOOST_PP_VARIADIC_ELEM_2" requires 4
> arguments, but only 2 given
>      test.cpp:60:1: error: macro "BOOST_PP_SEQ_ELEM_III" requires 2
> arguments, but only 1 given
>       MY_FUNCTION_BEGIN((b))
>       ^
>      test.cpp:60:1: error: macro "BOOST_PP_VARIADIC_ELEM_2" requires 4
> arguments, but only 2 give
>
>
> clang 3.5.0 failed with error:
>
>      test.cpp:30:3: error: too few arguments provided to function-like
> macro invocation
>        MY_FUNCTIONS( \
>        ^
>      test.cpp:25:25: note: expanded from macro 'MY_FUNCTIONS'
>        BOOST_PP_SEQ_FOR_EACH(FUNCTION_DECL, _, seq)
>                              ^
>      ..../boost.1_57_0/boost/preprocessor/seq/for_each.hpp:26:67: note:
> expanded from macro 'BOOST_PP_SEQ_FOR_EACH'
>      #    define BOOST_PP_SEQ_FOR_EACH(macro, data, seq)
> BOOST_PP_FOR((macro, data, seq (nil)), BOOST_PP_SEQ_FOR_EACH_P,
> BOOST_PP_SEQ_FOR_EACH_O, BOOST_PP_SEQ_FOR_EACH_M)
>                                                                        ^
>
> ..../boost.1_57_0/boost/preprocessor/repetition/detail/for.hpp:22:78:
> 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)
>                                                                                   ^
>      note: (skipping 18 expansions in backtrace; use
> -fmacro-backtrace-limit=0 to see all)
>      packages/boost/boost_i4/boost/preprocessor/seq/elem.hpp:22:39:
> note: expanded from macro 'BOOST_PP_SEQ_ELEM'
>      #    define BOOST_PP_SEQ_ELEM(i, seq) BOOST_PP_SEQ_ELEM_I(i, seq)
>                                            ^
>      packages/boost/boost_i4/boost/preprocessor/seq/elem.hpp:41:45:
> note: expanded from macro 'BOOST_PP_SEQ_ELEM_I'
>      #        define BOOST_PP_SEQ_ELEM_I(i, seq)
> BOOST_PP_SEQ_ELEM_II(BOOST_PP_SEQ_ELEM_ ## i seq)
>                                                  ^
>      packages/boost/boost_i4/boost/preprocessor/seq/elem.hpp:43:62:
> note: expanded from macro 'BOOST_PP_SEQ_ELEM_II'
>      #    define BOOST_PP_SEQ_ELEM_II(im) BOOST_PP_SEQ_ELEM_III(im)
>                                                                   ^
>      packages/boost/boost_i4/boost/preprocessor/seq/elem.hpp:44:13:
> note: macro 'BOOST_PP_SEQ_ELEM_III' defined here
>      #    define BOOST_PP_SEQ_ELEM_III(x, _) x
>
>
> The program is:
>
>      #include <iostream>
>
>      #ifndef BOOST_PP_VARIADICS
>        #define BOOST_PP_VARIADICS
>      #endif
>
>      #include <boost/preprocessor.hpp>
>
>      #define ADD_CONST_REF(op, data, elem) elem const &
>
>      #define FUNCTION_DECL(r, data, tup) \
>        double BOOST_PP_TUPLE_ELEM(0, tup) \
>
> BOOST_PP_IF(BOOST_PP_LIST_IS_NIL(BOOST_PP_TUPLE_TO_LIST(BOOST_PP_TUPLE_ELEM(1,
> tup))), \
>            (), \
>            BOOST_PP_SEQ_TO_TUPLE( \
>              BOOST_PP_SEQ_TRANSFORM( \
>                ADD_CONST_REF, \
>                _, \
>
> BOOST_PP_TUPLE_TO_SEQ(BOOST_PP_LIST_TO_TUPLE(BOOST_PP_TUPLE_TO_LIST(BOOST_PP_TUPLE_ELEM(1,
> tup)))) \
>              ) \
>            ) \
>          ) \
>        const;
>
>      #define MY_FUNCTIONS(seq) \
>        BOOST_PP_SEQ_FOR_EACH(FUNCTION_DECL, _, seq)
>
>      class Foo
>      {
>      public:
>        MY_FUNCTIONS( \
>          ((a, (int))) \
>          ((b)       ) \
>        )
>      };
>
>      #define ADD_ARG(op, data, elem) elem const & BOOST_PP_CAT(a_, elem)
>
>      #define MY_FUNCTION_BEGIN(tup) \
>        double Foo::BOOST_PP_TUPLE_ELEM(0, tup) \
>
> BOOST_PP_IF(BOOST_PP_LIST_IS_NIL(BOOST_PP_TUPLE_TO_LIST(BOOST_PP_TUPLE_ELEM(1,
> tup))), \
>            (), \
>            BOOST_PP_SEQ_TO_TUPLE( \
>              BOOST_PP_SEQ_TRANSFORM( \
>                ADD_ARG, \
>                _, \
>
> BOOST_PP_TUPLE_TO_SEQ(BOOST_PP_LIST_TO_TUPLE(BOOST_PP_TUPLE_TO_LIST(BOOST_PP_TUPLE_ELEM(1,
> tup)))) \
>              ) \
>            ) \
>          ) const \
>        {
>
>      #define MY_FUNCTION_END() }
>
>      MY_FUNCTION_BEGIN((a, (int)))
>      {
>        std::cout << "Foo::a(" << a_int << ")\n";
>      }
>      MY_FUNCTION_END()
>
>      MY_FUNCTION_BEGIN((b))
>      {
>        std::cout << "Foo::b()\n";
>      }
>      MY_FUNCTION_END()
>
>      int main()
>      {
>        Foo f;
>        f.a(42);
>        f.b();
>
>        return 0;
>      }
>
>
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