I'm using Boost version 1.43, perhaps this has been fixed. std::stringstream ss; (boost::phoenix::arg_names::arg1<<1<<2<<3)(ss); BOOST_CHECK_EQUAL(ss.str(),"123"); // check ss.str() == "123" failed [1 != 123] I think what's going on is that the shift operator is returning the int and not the stream. This shouldn't compile, but does: int o=(boost::phoenix::arg_names::arg1<<1<<2<<3)(ss); My use case is a log library which does the actual streaming in non-critical space. Chris