enum class MessageType : char {
  Add = 'A',
  Modify = 'M',
  Delete = 'D',
};

void on_message(const MessageHeader& hdr, const void* payload)
{
  switch(hdr.type)
  {
    case MessageType.Add:
      return handleAdd(payload);
    case MessageType.Modify:
      return handleModify(payload);
    // no-default here to catch by at compile time new value in MessageType enumeration.
  }
}

void handleAdd(const void* payload);
void handleModify(const void* payload);