[preprocessor] error: macro "BOOST_PP_SEQ_ELEM_III"
I'm having a problem with the preprocessor and currently I'm stuck. I tried to reduce the problem from my program as far as possible. Why does this example compile with an error?: #include <boost/preprocessor/seq/subseq.hpp> #include <boost/preprocessor/seq/seq.hpp> #include <boost/preprocessor/control/if.hpp> #define SEQ (b)(o)(o)(s)(t) #define i 0 int main( int argc, char* argv[]){ BOOST_PP_IF(i,BOOST_PP_SEQ_HEAD(BOOST_PP_SEQ_SUBSEQ(SEQ, 0, i)),0); return 0; } The error is the following: ./test.cxx:23:1: error: macro "BOOST_PP_SEQ_ELEM_III" requires 2 arguments, but only 1 given I'm using gcc 4.3.1 under Linux. As the condition is 0, the preprocessor shouldn't expand BOOST_PP_SEQ_HEAD(BOOST_PP_SEQ_SUBSEQ(SEQ, 0, i)). The preprocessed file (g++ -P -E) has expands to 0; but the preprocessor still throws an error. Could also someone give me a hint how to debug efficiently such a problem, as I'm a newbie to boost. --- Reimund
Quoting Reimund Klemm <klemm@ifn.et.tu-dresden.de>:
Why does this example compile with an error?:
#include <boost/preprocessor/seq/subseq.hpp> #include <boost/preprocessor/seq/seq.hpp> #include <boost/preprocessor/control/if.hpp>
#define SEQ (b)(o)(o)(s)(t) #define i 0
int main( int argc, char* argv[]){ BOOST_PP_IF(i,BOOST_PP_SEQ_HEAD(BOOST_PP_SEQ_SUBSEQ(SEQ, 0, i)),0); return 0; }
You need to delay the evaluation of the branches. E.g. #include <boost/preprocessor/seq/subseq.hpp> #include <boost/preprocessor/seq/seq.hpp> #include <boost/preprocessor/control/if.hpp> #define SEQ (b)(o)(o)(s)(t) #define i 0 #define TRUE_BRANCH(SEQ)\ BOOST_PP_SEQ_HEAD(BOOST_PP_SEQ_SUBSEQ(SEQ,0,i))\ /**/ #define FALSE_BRANCH(SEQ) 0 int main( int argc, char* argv[]){ BOOST_PP_IF(i,TRUE_BRANCH,FALSE_BRANCH)(SEQ); return 0; } should work. Pete
Now it works, Thanks Pete!
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Peter Bartlett -
Reimund Klemm