Is there any way I can access Private member variable of a class?

Question

Is there any way I can access Private member variable of a class?

Editing: Not from a member function or friend function but through an instance.

Answer 1

GotW #76 has fascinating language-lawyery details on how to do some of this stuff. :-)

Answer 2

Just cast it around, shift memory and cast back. (didn't compile the code, but you should get the idea).

class Bla
{
public:
    Bla() : x(15), str("bla") {}
private:
    int x;
    std::string str;
}

int main()
{
    Bla bla;

    int x = *((int*)(&bla));
    std::string str = *((std::string*)((int*)(&bla) + 1));

    std::cout << x << str;

    return 0;
}

Since this is an interview question, I won't go into why you shouldn't do that. :)

EDIT: Classes with virtual functions will have virtual table pointer somewhere there as well. I'm not sure if & will give you address of vt or address of first data member.

Alignment is 4 by default (right?), so if member you are reading does not align, shift by 2 bytes to get to the next one.

Answer 3

One of the "dirty tricks" of C++ is to do something like:

#define private public
#include "ClassHeader.h"

// now all the private members of the included class are public

I strongly do not recommend that you do this.

Answer 4

You could:

1. Place the private members in the public section
2. Make your class or function a friend of the class.
3. Provide an accessor to the data.
4. Take the address of the class, add the offset to that variable, cast, and dereference. (Yuck)

What are you trying to do? If something is private, don't mess with it. It's private for a reason.

Answer 5

Yes. You can access a private member:

• ...within other instances of the same (exact) type.
• ...within classes or functions declared to be a friend of that class.
• ...via a public accessor (getter/setter) member function.