Accessing private class members in C++

I've been called Kajtek "MOTHERFU*KINGWALLOFTEXT" Rzepecki lately, so let's make this post short.

We've got this code:

#include <iostream>

class A {
    private:
    int bar;
    void foo() {
        std::cout << "In A::foo() " << bar << "\n";
    }

    public:
    //...
};

int main() {
    A* a = new A();

    //...

    delete a;
    return 0;
}

And we feel a sudden urge to call foo() or access bar. How do we do that? It's quite simple but indicates incredibly poor style¹. Seriously, you should feel ashamed just for googling this post, Reader.

Daaamn.

All we really need is a cast:

class B {
    int bar;
    void foo() {}
};

int main() {
    B* a = reinterpret_cast<B*>(new A());

    a->bar = 23;
    a->foo();

    return 0;
}

So far, so good. We can access the fields, but calling foo() yields an unexpected result, well actually expected, it does nothing, just like B::foo() is supposed to do. This is as far as we can go with non-virtual methods.

Let's now pretend that A::foo() was virtual:

class A {
    private:
    int bar;
    virtual void foo() { /* ... */ }
};

class B {
    public:
    int bar;
    virtual void foo() = 0;
};

int main() {
    B* a = reinterpret_cast<B*>(new A());

    a->bar = 23;
    a->foo();

    delete a;
    return 0;
}

This time we create an abstract class with a pure virtual function foo() so we can access whatever foo() there is in A's vtable. This time we get the expected result:

$ ./callfoo
In A::foo() 23

2016-02-20: Adjusted some links & tags.

1. Again, the private is there for a reason. Casting it away is poor style. Don't do it unless you know what you are doing, which you clearly don't. Daaaamn.↩