hello_actor.cpp

class HelloBehavior {
  public:
    virtual void receive(std::string const& msg) = 0;
};

class HelloActor
    : public Actor
    , public HelloBehavior {
    
    std::string m_greeting;

  public:
    explicit HelloActor(std::string const& greeting): m_greeting(greeting) {}
    void receive(std::string const& msg) override {
        std::cout << m_greeting << " " << msg << "\n";
    }
}