Hello, I tried to use the BOOST_PP_SEQ_CAT with a sequence of parenthesized comma-separated pair of token. It works fine with visual 2010 but fails with gcc. I first think that there were a problem in the specific gcc implementation part. That's why I opened the issue 9633 : https://svn.boost.org/trac/boost/ticket/9633 But, it has recently been closed for invalidity, the corrector telling that it is not a valid use. I have not read the entire help of the PP library but as far as I understand the sequence definition given in A.4.5.1. (http://www.boostpro.com/mplbook/preprocessor.html) as "any string of nonempty parenthesized /macro arguments/." and the definition of macro arguments in A.2.3, I fail to see why the sequence could be incorrect. Specially when I read the end of the paragraph : " It /is/ possible to pass either string of tokens above as part of a single macro argument, provided it is parenthesized: FOO(*(*std::pair<int,int>*)*) // one argument FOO(*(*{ int x = 1, y = 2; return x+y; }*)*) // one argument " I fail to see where in the dedicated entry for BOOST_PP_SEQ_CAT (http://www.boost.org/doc/libs/1_55_0/libs/preprocessor/doc/ref/seq_cat.html) there is a rule I violated. So my questions are : => why the code I proposed is invalid ? => Is the workaround proposed with BOOST_PP_SEQ_FOR_EACH is also a missused and may fail according to evolutions of the library ? Thanks in advance for your answers. The code : #include <boost\preprocessor\seq\cat.hpp> #define SOME_SEQUENCE (('a',1)) (('b',2)) (('c',3)) (('d',4)) (('e',5)) struct dummy { dummy& operator()(char,int) {return *this;} }; int main() { dummy() BOOST_PP_SEQ_CAT(SOME_SEQUENCE); // Visual 2010 generates dummy() ('a',1)('b',2)('c',3)('d',4)('e',5); // gcc fails with compiler errors due to the comma I suppose return 0; }