Question

I am having a bit of trouble with observer pattern
I have a beverage class that uses decorator fine.
I am trying to implement observer to let an observer( say a cell phone/text message) in this case know when there order is done. i wont include the beverage/decorator classes since they work fine.

In main i was going to do something like this:

Subject mySubject = new Subject();
Observer myObserver1 = new Observer1();
Observer myObserver2 = new Observer2();

// register observers
mySubject.Register(myObserver1);
mySubject.Register(myObserver2);

mySubject.Notify("message 1");
mySubject.Notify("message 2");

I have a subject class, observer, phone behavior class and a cellhone1,cellphone2 classes..

Here is the subject class

#ifndef _SUBJECT_
#define _SUBJECT_

//#include "Starbuzz.h"
//#include "Starbuzz2.h"
#include "Observer.h"
#include <list>

namespace CoffeeHouse {
namespace Observers {


class Subject  {


private:
std::list< Observer* > observers;

public:
Subject();
~Subject();

void Subject::Register(Observer observer)
{

//if (!observers.(observer))
//{

observers.insert(observer);
}
//}

//void Unregister(Observer observer)
//{
// if observer is in the list, remove
//if (observers.Contains(observer))
//{
//observers.Remove(observer);
//}
//}

void Notify(std::string message)
{
//need loop
Observer observer;
observer.update(message);


}
//}
//}

//void Subject::registerObserver( Observer* o ) { assert( o );
    //_observers.push_front(o);
//}
//void Subject::removeObserver( Observer* o ) { assert( o );
//  _observers.remove(o);
//}
//void Subject::notifyObservers() const {
    //for( std::list< Observer* >::iterator iterator = _observers.begin();   _observers.end() != iterator; ++iterator ) {
        //Observer* observer = *iterator;
        //observer->update(message);
    //}
//}
};
} // namespace Observer
} 


#endif

Here is the observer class

#ifndef _OBSERVER_
#define _OBSERVER_

#include <string>

namespace CoffeeHouse {
namespace Observers {

class Subject;

class Observer {

//public: virtual ~Observer() = 0; 

public: virtual void Update(std::string message) = 0;
};

here is the phone behavior class

#ifndef _PHONEBEHAVIOR_
#define _PHONEBEHAVIOR_

namespace CoffeeHouse {
namespace Observer {

class PhoneBehavior {
public: virtual void Update() const = 0;
};
protected: virtual ~PhoneBehavior() = 0 {

};


};

} // namespace Observer
} //

here is cellphone 1

#ifndef _CELLPHONE1_
#define _CELLPHONE1_

namespace CoffeeHouse {
namespace Observer {
include<iostream>
class CellPhone1: public Observer, public PhoneBehavior {
public: 
    CellPhone1();
    ~CellPhone1();
virtual void Update(std::string message)
 {
   std::cout << "CellPhone1: " << message;
}
};

} // namespace Observer
} //

#endif

here re the errors i am getting..

error C2259: 'CoffeeHouse::Observers::Observer' : cannot instantiate abstract class
1>        due to following members:
1>        'void CoffeeHouse::Observers::Observer::update(std::string)' : is abstract
see declaration of 'CoffeeHouse::Observers::Observer::update'
error C2661: 'std::list<_Ty>::insert' : no overloaded function takes 1 arguments
with[_Ty=CoffeeHouse::Observers::Observer *

error C2259: 'CoffeeHouse::Observers::Observer' : cannot instantiate abstract class
due to following members:
'void CoffeeHouse::Observers::Observer::update(std::string)' : is abstract
observer.h(15) : see declaration of 'CoffeeHouse::Observers::Observer::update

When i click on error “cannot instantiate abstract class” it takes me to:

void Subject::Register(Observer observer)

I understand that Abstract classes are made such that they cannot be instantiated!

How could i do an update then?? any suggestions of a better way?

I appreciate any help!

Answer 1

One error is that Subject::Register is currently taking an Observer object, instead of a pointer to an Observer. This means that you’re trying to instantiate an abstract object, which is illegal.