C++ pure virtual function implementation upon instantiating object

Question

I'm not sure if this has already been answered somewhere or if what I'm asking is feasible. I'm not so familiar with C++ and OOP in general so forgive me if I'm asking something too obvious, as my mind is a bit stuck right now. Anyway, what I want to deal with is:

I'm using a framework that handles some callbacks from an app. The API has a class for these events and the callback function as pure virtual function. For example:

class A {
public:
    A(){};
    ~A(){};
    virtual void myCallback(){} = 0 
};

In my code I'm implementing this callback and I have something like:

class B : A {
public:
    B(){
        std::cout<<"I'm a ClassB object";
    };
    ~B(){};
    void myCallback(){
        std::cout<<"Hello callback";
    };
};

Now in my main suppose that I'm having something like that:

int main(){
    int x=1;
    B objB;
    //Do some stuff for a while
    return 1;
}

I can see upon instantiating the B object the "I'm a ClassB object" message and I can see upon firing the event the "Hello callback" message. So what I'm asking here is how I can get inside my callback the value of "x" coming from main ? Can I re-implement the callback inside the main somehow? Do you think that my approach on this is correct ? What I want is to implement pure virtual callback function inside my main by having an object of class B. Or even to somehow get the value of x inside the myCallback function. I'm not sure if I'm clear enough or if I messed anything up

Answer 1

If I understand the question, what you want is usually called a closure . You want myCallback to know the value of x inside main. There are several ways of doing this, the most straightforward will look something like following:

class B : public A {
public:
    B(int x) x(x) {
        std::cout<<"I'm a ClassB object";
    }

    ~B() = default;

    void myCallback() {
        std::cout << "Hello callback, x is " << x << "\n";
    }
private:
    int x;
};

int main() {
    int x = 1;
    B objB(x);
    //Do some stuff for a while
    return 0;
}