How can I change the variable to which a C++ reference refers?

Question

If I have this:

int a = 2;
int b = 4;
int &ref = a;

How can I make ref refer to b after this code?

Answer 1

You cannot reassign a reference.

Answer 2

This is not possible, and that's by design. References cannot be rebound.

Answer 3

Formally speaking, that is impossible as it is forbidden by design. Arbitrarily speaking, that is possible.

A references is stored as a pointer, so you can always change where it points to as long as you know how to get its address. Similarly, you can also change the value of const variables, const member variables or even private member variables when you don't have access to.

For example, the following code has changed class A's const private member reference:

#include <iostream>
using namespace std;

class A{
private:
    const int &i1;
public:
    A(int &a):i1(a){}
    int geti(){return i1;}
    int *getip(){return (int*)&i1;}
};

int main(int argc, char *argv[]){
    int i=5, j=10;
    A a(i);
    cout << "before change:" << endl;
    cout << "&a.i1=" << a.getip() << " &i=" << &i << " &j="<< &j << endl;
    cout << "i=" << i << " j=" <<j<< " a.i1=" << a.geti() << endl;
    i=6; cout << "setting i to 6" << endl;
    cout << "i=" << i << " j=" <<j<< " a.i1=" << a.geti() << endl;

    *(int**)&a = &j; // the key step that changes A's member reference

    cout << endl << "after change:" << endl;
    cout << "&a.i1=" << a.getip() << " &i=" << &i << " &j="<< &j << endl;
    cout << "i=" << i << " j=" <<j<< " a.i1=" << a.geti() << endl;
    j=11; cout << "setting j to 11" << endl;
    cout << "i=" << i << " j=" <<j<< " a.i1=" << a.geti() << endl;
    return  0;
}

Program output:

before change:
&a.i1=0x7fff1b624140 &i=0x7fff1b624140 &j=0x7fff1b624150
i=5 j=10 a.i1=5
setting i to 6
i=6 j=10 a.i1=6

after change:
&a.i1=0x7fff1b624150 &i=0x7fff1b624140 &j=0x7fff1b624150
i=6 j=10 a.i1=10
setting j to 11
i=6 j=11 a.i1=11

As you can see that a.i1 initially points to i, after the change, it points to j.

However, doing so is considered as dangerous and thus unrecommended, because it defeats the original purpose of data encapsulation and OOP. It is more like memory address hacking.

Answer 4

Although its a bad idea as it defeats the purpose of using references, it is possible to change the reference directly

const_cast< int& >(ref)=b;