Question

I posted a question earlier on how to cast a member function to a typedef function pointer ( How to use a typedef function pointer to register a callback ) and it prompted me to consider a slightly modified solution by implementing the “classic” observer pattern.

I want to allow class A to notify observers who get a callback via the function pointer and observers who get a callback via a member function, but I can’t figure out what’s const-correct way to do this. I’ve looked at other examples online and now I’m just confused with where I’m going wrong.

Here is an sscce compliant example that I pasted in ideone.com (with the compiler errors below it):

#include <set>

typedef int (*CallbackEvent)(const char*, const char*, int);

// Observer interface
class IObserver
{
public: 
    virtual ~IObserver(){}
    virtual int CallMeBack(const char*, const char*, int);
};

class A
{
private:
    std::set<IObserver> observers;
public:
    A(){}
    ~A(){}
    void RegisterObserver(const IObserver& observer)
    {
        observers.insert(observer);
    }

    inline void UnregisterObserver(const IObserver& observer)
    {
        observers.erase(observer);
    }

    void NotifyAll()
    {
        for(std::set<IObserver>::iterator itr = observers.begin(); itr != observers.end(); itr++)
        {
            int z = 3;
            itr->CallMeBack("x","y",z); // <-- This is the part that's not working
        }
    }
};

// Has internal logic to handle the callback
class B : public IObserver
{
private:
    A myA;
public:
    B(A& a)
    {
        myA = a;
        myA .RegisterObserver(*this);
    }
    ~B()
    {
        myA .UnregisterObserver(*this);
    }

    int CallMeBack(const char* x, const char* y, int z)
    {
        return 0;
    }
};

Compiler Errors:

prog.cpp: In member function ‘void A::NotifyAll()’:
prog.cpp:38: error: passing ‘const IObserver’ as ‘this’ argument of ‘virtual int IObserver::CallMeBack(const char*, const char*, int)’ discards qualifiers
/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/include/g++-v4/bits/stl_function.h: In member function ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = IObserver]’:
/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/include/g++-v4/bits/stl_tree.h:1141:   instantiated from ‘std::pair<typename std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(const _Val&) [with _Key = IObserver, _Val = IObserver, _KeyOfValue = std::_Identity<IObserver>, _Compare = std::less<IObserver>, _Alloc = std::allocator<IObserver>]’
/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/include/g++-v4/bits/stl_set.h:381:   instantiated from ‘std::pair<typename std::_Rb_tree<_Key, _Key, std::_Identity<_Key>, _Compare, typename _Alloc::rebind<_Key>::other>::const_iterator, bool> std::set<_Key, _Compare, _Alloc>::insert(const _Key&) [with _Key = IObserver, _Compare = std::less<IObserver>, _Alloc = std::allocator<IObserver>]’
prog.cpp:25:   instantiated from here
/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/include/g++-v4/bits/stl_function.h:230: error: no match for ‘operator<’ in ‘__x < __y’

Please note that I do most of my programming in C#, so excuse my ignorance when it comes to the “nuances” of C++. Could anybody help me figure out what’s the right way to implement this?

Update

Changed my code based on Nawaz’s answer, here is the updated code: http://www.ideone.com/AH3KG

Answer 1

The only thing that you might not be able to fix easily just by reading the error messages is:

std::set<IObserver> observers;

This needs to be a collection of pointers. Making copies of the observer objects is almost certainly the wrong thing to do. Use

std::set<IObserver*> observers;

This will also fix the problem with less_than, because pointers can be compared even when the objects they point to don’t define an ordering.

Here it is, fixed: http://ideone.com/9jzpK

Since you mentioned const-correctness, here’s a variant in which the observers are const: http://ideone.com/gxsCo