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

27 Feb 2015

      On 2/27/2015 9:56 AM, Edward Diener wrote:
...
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)), \
    (), \
This should be:

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

Also your

            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)))) \
              ) \
            ) \

has to be moved to another macro since it is evaluated even when your 
'tup' only has a single element.

Correct is:

      #define FUNCTION_DECL_EMPTY(tup) ()

      #define FUNCTION_DECL_NOT_EMPTY(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)))) \
              ) \
            )

      #define FUNCTION_DECL(r, data, tup) \
        double BOOST_PP_TUPLE_ELEM(0, tup) \
          BOOST_PP_IIF(BOOST_PP_EQUAL(BOOST_PP_TUPLE_SIZE(tup),1), \
            FUNCTION_DECL_EMPTY, \
            FUNCTION_DECL_NOT_EMPTY \
          ) (tup) \
        const;
...
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 
#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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