Method add

example/cpp11/invocation/prioritised_handlers.cpp:22–29

public:
  template <typename Function>
  void add(int priority, Function function)
  {
    std::unique_ptr<queued_handler_base> handler(
        new queued_handler<Function>(
          priority, std::move(function)));

    handlers_.push(std::move(handler));
  }

  void execute_all()
  {