fixing Observer design pattern in c++

Question

i was implementing observer design pattern in c++.It is a very simple application where observer registers and is informed about the stock prices whenever they are updated. i am getting following error while compiling , unfortunately not able to fix it.I hope you guys will help.

compiler Error :-

observer.cpp: In method `void StockGrabber::notifyAll()':
observer.cpp:54: no matching function for call to `__rb_tree_iterator<observer *
,observer *const &,observer *const *>::end ()'
observer.cpp:56: request for member `update' in `(&it)->__rb_tree_iterator<obser
ver *,observer *const &,observer *const *>::operator *<observer *, observer *con
st &, observer *const *>() const()', which is of non-aggregate type `observer *'

My code is the following :-**

#include<iostream>
#include<map>
#include<set>
#include<string>

using namespace std;

class observer{

  public:
    virtual void update(string s) = 0;
    virtual void remove();

};
class subject{

  public:
    virtual void Register(observer *o);
    virtual void UnRegister(observer *o);
    virtual void notifyAll();

};

class StockGrabber : public subject
{
   map<string,double> mymap;
   set<observer *> myset;

   string getNewPrice()
   {
      string s="";
      map<string,double>::iterator it;

      for(it=mymap.begin();it!=mymap.end();++it)
      {

          s=s+it->first+" - "+to_string(it->second)+"\n";
      }
      return s;
   }

   public:
   virtual void Register(observer *o){
        myset.insert(o);

   }
   virtual void UnRegister(observer *o){
        myset.erase(o);
   }
   virtual void notifyAll(){
        string s=getNewPrice();
        set<observer *>::iterator it;

        for(it=myset.begin();it!=it.end();++it)
        {
           (*it).update(s);
        }   
   }
   void setPrice(string stock,double price)
   {
      mymap[stock]=price;         
   }
};



class stockObserver : public observer
{
  subject *stockGrabber;
  public:
    stockObserver(subject *sobj)
    {
        stockGrabber=sobj;
        sobj->Register(this);
    }
    void remove()
    {
        stockGrabber->UnRegister(this);
    }

    virtual void update(string s){
      cout<<"New Update\n";
      cout<<s;
    }


};

void main()
{
StockGrabber *stockgrabber=new StockGrabber();
stockObserver *ob1 = new stockObserver(stockgrabber);
stockgrabber->setPrice("google",21.43);
stockgrabber->setPrice("apple",21.43); 

}

Answer 1

• end() should be called on the container, not the iterator, at line 54
• myset is a set<observer *>, so after dereferncing an iterator, you get an observer*, at line 56

Try this:

virtual void notifyAll(){
    string s=getNewPrice();
    set<observer *>::iterator it;

    for(it=myset.begin();it!=myset.end();++it)
                           //^^^^^ HERE
    {
       (*it)->update(s); // AND HERE
    }   
}

Additionally, you probably want to change your observer signature to be:

class observer {
  public:
    virtual void update(const string& s) = 0;
    virtual void remove();
};

and update the rest accordingly.