boost::log::operator<< — Log formatting operator.
// In header: <boost/log/utility/value_ref.hpp> template<typename CharT, typename TraitsT, typename AllocatorT, typename T, typename TagT> basic_formatting_ostream< CharT, TraitsT, AllocatorT > & operator<<(basic_formatting_ostream< CharT, TraitsT, AllocatorT > & strm, value_ref< T, TagT > const & val);