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)
<

Answer 1

You have a few basic options:

1) Specify what class the callback is going to use, so that the object pointer and member function pointer types are known, and can be used in the caller. The class might have several member functions with the same signature, which you can choose between, but your options are quite limited.

One thing that you've done wrong in your code is that member function pointers and free function pointers in C++ are not the same, and are not compatible types. Your callback registration function takes a function pointer, but you're trying to pass it a member function pointer. Not allowed. Furthermore, the type of the "this" object is part of the type of a member function pointer, so there's no such thing in C++ as "a pointer to any member function which takes an integer and returns void". It has to be, "a pointer to any member function of Target which takes an integer and returns void". Hence the limited options.

2) Define a pure virtual function in an interface class. Any class which wants to receive the callback therefore can inherit from the interface class. Thanks to multiple inheritance, this doesn't interfere with the rest of your class hierarchy. This is almost exactly the same as defining an Interface in Java.

3) Use a non-member function for the callback. The for each class which wants to use it, you write a little stub free function which takes the object pointer and calls the right member function on it. So in your case you'd have:

dosomething_stub(void *obj, int a) {
    ((Target *)obj)->doSomething(a);
}

4) Use templates:

template class MyClassWithCallback {
    CB *callback;
 public:
    void setCallback(CB &cb) { callback = &cb; }
    void callCallback(int a) {
        callback(a);
    }
};

class Target {
    void operator()(int a) { /* do something; */ }
};

int main() {
    Target t;
    MyClassWithCallback caller;
    caller.setCallback(t);
}

Whether you can use templates depends whether your ClassWithCallback is part of some big old framework - if so then it might not be possible (to be precise: might require some more tricks, such as a template class which inherits from a non-template class having a virtual member function), because you can't necessarily instantiate the entire framework once for each callback recipient.