"Eric Niebler" <eric@boost-consulting.com> wrote
I must be doing something stupid. How can I process a sequence of sequences? I expected to be able to use nested invocations of SEQ_FOR_EACH, but I'm not getting the results I expect. Here's my code:
#include <boost/preprocessor.hpp>
#define INNER(R, DATA, ELEM)\ **ELEM**,
#define OUTER(R, DATA, ELEM)\ << BOOST_PP_SEQ_FOR_EACH_R(R, INNER, ~, ELEM) >>,
BOOST_PP_SEQ_FOR_EACH(OUTER, ~, ((1)(2)) ((3)(4)) )
I expected something like: << **1**, **2**, >>, << **3**, **4**, >>,
But instead I'm getting: << BOOST_PP_SEQ_FOR_EACH_M(3, (INNER, ~, (1)(2) (nil))) BOOST_PP_SEQ_FOR_EACH_M(4, (INNER, ~, (2) (nil))) >>, << BOOST_PP_SEQ_FOR_EACH_M(4, (INNER, ~, (3)(4) (nil))) BOOST_PP_SEQ_FOR_EACH_M(5, (INNER, ~, (4) (nil))) >>,
I've tried both msvc-8.0 and gcc-3.4. Can anybody see my mistake?
Unfortunately not every macro can be nested. IIUC, by default the preprocessor does not allow it. For some of macros special code is implemented inside Boost PP to allow nesting. If you use BOOST_PP_REPEAT, for instance, you should not have any problems. Regards, Arkadiy