Hi, consider a library featuring code like template <..., unsigned Rank, ...> struct Foo { ... static unsigned const rank = Rank; .... }; as well as boost::format(...) % Foo::rank Such code will link if and only if a definition for Foo::rank is provided because the interface of operator % is template <class T> operator%(const T& x) This is not possible in the library, as I do not know which values of Rank will be used by client code. Thus the only possibility is a hat trick like boost::format(...) % (Foo::rank+0) If operator % took integral types by value instead, this would alleviate the problem. Hence the suggestion to use call_traits<T>::parameter_type instead of T const &. Of course, the same could be said of many functions other than format::operator% Best wishes, Luc Bourhis