How do I implement a callback in C++?

Question

I want to implement a class in c++ that has a callback.

So I think I need a method that has 2 arguments:

• the target object. (let's say *myObj)
• the pointer to a member function of the target object. (so i can do *myObj->memberFunc(); )

The conditions are:

• myObj can be from any class.

• the member function that is gonna be the callback function is non-static.

I've been reading about this but it seems like I need to know the class of myObj before hand. But I am not sure how to do it. How can I handle this? Is this possible in C++?

This is something I have in mind but is surely incorrect.

class MyClassWithCallback{
public
    void *targetObj;
    void (*callback)(int number);
    void setCallback(void *myObj, void(*callbackPtr)(int number)){
        targetObj = myObj;
        callback = callbackPtr;
    };
    void callCallback(int a){
        (myObj)->ptr(a);
    };
};
class Target{
public
    int res;
    void doSomething(int a){//so something here. This is gonna be the callback function};        
};

int main(){
    Target myTarget;
    MyClassWithCallback myCaller;
    myCaller.setCallback((void *)&myTarget, &doSomething);

}

I appreciate any help.

Thank you.

UPDATE Most of you said Observing and Delegation, well that's i exactly what i am looking for, I am kind of a Objective-C/Cocoa minded guy. My current implementation is using interfaces with virtual functions. Is just I thought it would be "smarter" to just pass the object and a member function pointer (like boost!) instead of defining an Interface. But It seems that everybody agrees that Interfaces are the easiest way right? Boost seems to be a good idea, (assuming is installed)

Answer 1

The best solution, use boost::function with boost::bind, or if your compiler supports tr1/c++0x use std::tr1::function and std::tr1::bind.

So it becomes as simple as:

boost::function<void()> callback;
Target myTarget;
callback=boost::bind(&Target::doSomething,&myTarget);

callback(); // calls the function

And your set callback becomes:

class MyClassWithCallback{
public:
  void setCallback(boost::function<void()> const &cb)
  {
     callback_ = cb;
  }
  void call_it() { callback_(); }
private:
  boost::function<void()> callback_;
};

Otherwise you need to implement some abstract class

struct callback { 
 virtual void call() = 0;
 virtual ~callback() {}
};

struct TargetCallback {
 virtual void call() { ((*self).*member)()); }
 void (Target::*member)();
 Target *self;
 TargetCallback(void (Target::*m)(),Target *p) : 
       member(m),
       self(p)
 {}
};

And then use:

myCaller.setCallback(new TargetCallback(&Target::doSomething,&myTarget));

When your class get modified into:

class MyClassWithCallback{
public:
  void setCallback(callback *cb)
  {
     callback_.reset(cb);
  }
  void call_it() { callback_->call(); }
private:
  std::auto_ptr<callback> callback_;
};

And of course if the function you want to call does not change you may just implement some interface, i.e. derive Target from some abstract class with this call.