BOOST_PP_SEQ_FOLD_RIGHT completely broken

[ookami1@…]

The following program is adapted from the macro BOOST_PP_SEQ_CAT that concatenates the elements of a sequence to a string:

(a)(b)(c)(d) ==> "abcd"

BOOST_PP_SEQ_CAT is built upon BOOST_SEQ_FOLD_LEFT; by analogy, one would expect that macro REVCAT, written after BOOST_PP_SEQ_CAT, but using BOOST_PP_SEQ_FOLD_RIGHT, builds a reversed string "dcba". This is not the case, in fact, the program does not even compile, because BOOST_PP_FOLD_RIGHT is currently completely broken.

#include <boost/preprocessor.hpp>
# define REVCAT(seq) \
    BOOST_PP_IF( \
        BOOST_PP_DEC(BOOST_PP_SEQ_SIZE(seq)), \
        REVCAT_I, \
        BOOST_PP_SEQ_HEAD(seq) BOOST_PP_TUPLE_EAT_1 \
    )(seq) \
    /**/
# define REVCAT_I(seq) BOOST_PP_SEQ_FOLD_RIGHT(REVCAT_O, BOOST_PP_SEQ_HEAD(seq), BOOST_PP_SEQ_TAIL(seq))
#
# define REVCAT_O(s, st, elem) REVCAT_O_I(st, elem)
# define REVCAT_O_I(a, b) a ## b

const char x[] = BOOST_PP_STRINGIZE(BOOST_PP_SEQ_CAT((a)(b)(c)(d)));
# pragma warning just a marker
const char revx[] = BOOST_PP_STRINGIZE(REVCAT((a)(b)(c)(d)));

int main() { return *x == 0; }

Here is the message from the compiler:

Invoking: GCC C++ Compiler g++ -I"/home/wolf/workspace/booster" -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"booster.d" -MT"booster.d" -o"booster.o" "../booster.cpp" ../booster.cpp:15: warning: ignoring #pragma warning just ../booster.cpp:16:1: error: macro "BOOST_PP_SEQ_ELEM_III" requires 2 arguments, but only 1 given

Note that the first call succeeds.

Cheers

Wolf Lammen

[ookami1@…]

Today a had a look again at fold_right.hpp to see why it failed a few days ago. To my great surprise, I was unable to reproduce the error. I do not know what has happened, but right now, everything works fine.

I am going to mark this as invalid.

Cheers

Wolf Lammen