shared_ptr from abstract class

Question

I have this mediator pattern:

File actor.hpp

#pragma once
#include <memory>
#include "mediator.hpp"

class actor : std::shared_from_this<actor>
{
  public:

  actor(std::shared_ptr<mediator> mediator) : mediator_(mediator)
  {
  }

  virtual ~actor() = 0;

  virtual void changed()
  {
    mediator_->actor_changed(std::make_shared<actor>(this));
  }

  private:

  std::shared_ptr<mediator> mediator_;
};

File mediator.hpp

#pragma once
#include <memory>

class actor;

class mediator : public std::shared_from_this<mediator>
{
  public:

  virtual ~mediator() = 0;

  virtual void actor_changed(std::shared_ptr<actor> actor) = 0;
}

An implementation of an actor request_handler.hpp

#pragma once
#include "actor.hpp"

class request_handler : public actor
{
  public:

  request_handler(std::shared_ptr<mediator> mediator) : actor(mediator)
  {
  }

  void handle_request()
  {
    changed();
  }
};

But I get the error message

actor.hpp:16 error: invalid new-expression of abstract class type 'actor'

because I cannot instatiate an abstract class. But what would be the correct way of doing this?

Answer 1

This code:

 mediator_->actor_changed(std::make_shared<actor>(this));

is logically equal to:

auto ptr = std::shared_ptr<actor>( new actor( this ) );
mediator_->actor_changed(ptr);

and this is most probably not what you want, so you need to call shared_from_this() instead:

mediator_->actor_changed( this->shared_from_this() );

and class you need to inherit from is called std::enable_shared_from_this not shared_from_this