C++ Event Handler With Registration Style, Notifier Method

Question

I found this code for event handler with registration style in c++. Can anyone explain me, what the below code does?

#include <algorithm>
#include <functional>
#include <list>

class Button {

    public:
        typedef void (*OnPress)(Button *, void *closure);
    
    private:
        typedef std::pair<OnPress, void*> PressCallBack;
        std::list<PressCallBack> onPress;

        struct Notify:
        public std::binary_function<PressCallBack, Button*, void> {
            void operator()( PressCallBack& c, Button* b) const {
                (*c.first)(b, c.second);
            }
        };

        void NotifyAll(void){
            std::for_each(
                onPress.begin(), onPress.end(),
                std::bind2nd(Notify(), this)
            );
        }

    public:
        void AddOnButtonPress(OnPress f, void *c){
            onPress.push_back(PressCallBack(f,c));
        }

        void RemoveOnButtonPress(OnPress f, void *c){
            onPress.remove(PressCallBack(f,c));
        }
};

Any comment will be helpful!!! Thanks in advance!

Answer 1

Somewhere there are functions with the signature:

 void function_name(Button* btn, void* ptrToSomeData);

The functions with the second parameter (ptrToSomeData) can be added into the list of event handlers with the function Button::AddOnButtonPress:

 Button::AddOnButtonPress(function_name, &SomeData);

when the function Button::NotifyAll() is called for some object SomeButton, then all added functions will be called in the order in which they were added. The first argument will be passed a pointer to the button (SomeButton), and the second argument will be the one specified when adding.

typedef void (*OnPress)(Button *, void *closure);
//< declare OnPress as pointer to function (pointer to Button, pointer to void) returning void

typedef std::pair<OnPress, void*> PressCallBack;
//< declare pair of 
//< - pointer to function (pointer to Button, pointer to void) returning void
//    and
//< - pointer to void
//This type is used to store the pointer to callback function with the second argument of the function

std::list<PressCallBack> onPress; //< list of saved callbacks

struct Notify:
public std::binary_function<PressCallBack, Button*, void> {
    void operator()( PressCallBack& c, Button* b) const {
          (*c.first)(b, c.second);
    }
};
//< the type Notify - it is functor. It is used this way: 
//  Notify()(callback, this);
//  so
//  it calls callback_function(this, store_pointer_to_some_data)

void NotifyAll(void){
    std::for_each(
        onPress.begin(), onPress.end(),
          std::bind2nd(Notify(), this)
     );
}
//< for every stored callback:
//   1) create object Notify
//   2) call Notify::operator()(PressCallBack, this)

void AddOnButtonPress(OnPress f, void *c){
    onPress.push_back(PressCallBack(f,c));
}
//< store callback function 'f' and second argument 'c' for the callback function 'f' into onPress list

    void RemoveOnButtonPress(OnPress f, void *c){
        onPress.remove(PressCallBack(f,c));
    }
//< store callback function 'f' with the second argument 'c' from onPress list