I've attached two (tiny) files: a.cpp and b.cpp . They compile fine, but attempting to link them causes the following error: b.o(.text+0x0):b.cpp: multiple definition of `boost::foreach_detail_::cheap_copy(...)' a.o(.text+0x0):a.cpp: first defined here I'm using gcc 3.4.2 with boost 1.32 and foreach.hpp revision 1.29 from the sandbox. Making the non-template cheap_copy() in foreach.hpp at line 322 inline solves the problem. Regards, Eelis #include <boost/foreach.hpp> int main () {} #include <boost/foreach.hpp>