C++ Member to member function pointers OR event driven programming - c++

Question

I come from the C# world so events and event handlers are everyday stuff.
Recently i have been studying about wxWidgets
I've been googling about this for a week now and i have found that most C++ programmers hardly understand the term "event" or "callback", perhaps it's ambiguous in the world of C++.
Here's a simple example of the event model.
class A{
public:
A(){
child.MySuperClickEvent = this.HandleSuperClick;
}
private:
B child;
void HandleSuperClick(B child){
// do stuff
}
}
class B{
public:
/*TBA*/ MySuperClickEvent;
private:
void ClickPrivate(){
MySuperClickEvent(this);
}
}
class B has it's own designated purpose and when finished it triggers an event.
The idea is that class B shouldn't have any knowledge of it's domain, making it more reusable.
I have read about std::function<> and function pointers. which all seems to take examples in handling static functions but as soon as were talking "Member to Member" things starts to get greasy.
So summarize, the question is simple. How do i make a member function pointer that can be assigned to a member externally?

Answer 1

There are a great many ways to implement such things, varying in terms of memory overheads, how many callbacks are supported, thread safety, use of e.g. weak_ptrs to see if the objects involved still exist etc..
Just as a taste to get you started - here's a simple single-threaded "observer" implementation doing basically what you ask for in the question:
#include <iostream>
#include <list>
struct Observer
{
virtual ~Observer() { }
virtual void on_event_x() { }
};
class Observable
{
public:
void add_observer(Observer& o)
{
observers_.push_back(&o);
}
void do_some_stuff()
{
std::cout << "before 1st event\n";
fire_event_x();
std::cout << "between 1st & 2nd events\n";
fire_event_x();
std::cout << "after 2nd event\n";
}
private:
std::list<Observer*> observers_;
void fire_event_x()
{
for (auto& observer : observers_)
observer->on_event_x();
}
};
struct My_Observer : Observer
{
My_Observer(int id) : id_(id) { }
void on_event_x() override
{
std::cout << "My_Observer::on_event_x() id " << id_ << '\n';
}
int id_;
};
int main()
{
My_Observer my_observer_1 { 1 };
My_Observer my_observer_2 { 2 };
Observable x;
x.add_observer(my_observer_1);
x.add_observer(my_observer_2);
x.do_some_stuff();
}
Runtime output:
before 1st event
My_Observer::on_event_x() id 1
My_Observer::on_event_x() id 2
between 1st & 2nd events
My_Observer::on_event_x() id 1
My_Observer::on_event_x() id 2
after 2nd event
If that doesn't suit you functionally, please say exactly why.

Answer 2

As this question is tagged wxWidgets, let me answer the more narrow variant of it, i.e. how to use member function as event handler in wxWidgets:
This is done with the help of Bind() method which can be used with a member function directly (you pass the pointer to the function itself and the object to call it on) or any arbitrary functor, i.e. anything that can be called using the standard function call syntax, such as an std::function<> object which, in turn, can be used to store any callable object.