message: fix compile error on xcode (issue
diff: #include <iostream> // TODO: delete me
  #define THROW_FOR_INVALID_FN( X ) \
  -if (false == m_observer. X) throw std::runtime_error("observer." #X " must be valid");
  + if (nullptr == m_observer. X) throw std::runtime_error("observer." #X " must be valid");
  namespace wampcc {
  