Hi, I am wondering how to create a 'type trait' such as 'is_ostreamable', ie: class A {}; class B {}; std::ostream &operator<<(std::ostream &os, const A &in) { return (os << in); } // Note, no operator<< for class B // --- somewhere else .. ;) class X { std::ostream out; template<typename T> void Impl(const T &in, boost::mpl::bool_<true>) { out << "(" << typeid(in) << ") " << in; } template<typename T> void Impl(const T &in, boost::mpl::bool_<false) { out << "(" << typeid(in) << ") 0x" << std::hex << (unsigned int) ∈ } public: X(std::ostream &os) : out(os) {} template<typename T> void DoIt(const T &in) { Impl(in, boost::mpl::bool_<boost::is_ostreamable<T> >()); } }; So the above would print the type name, and then the value of the entry IF the entry could be streamed to a std::ostream. Otherwise, it would print the type name and the address of its memory location (in hex). Obviously the above is just an example, but one that illustrates my need :) Question is, how to do it? Its easy enough to get a compiler error when something is not supported, but how to get the compiler to choose behavior on whether something is supported. -- PreZ :) Founder. The Neuromancy Society (http://www.neuromancy.net)