Redefining a corrupted reference - c++

Question

class A
{
};
int main()
{
A *a= new A(); // variable "a" must be pointer, I can't edit this line
A& b = *a; //b must be a reference
delete a; //deletion
a= new A(); //re-definition
//b is now corrupted.
}
In a piece of code that I have to work with, a pointer to an object of class A is created and a reference to this pointer too.
For some reasons, I have to delete the pointer and re-defined it. How can I get b to work again ?

Answer 1

You can do this, subject to some constraints:
A* a = new A(); // variable "a" must be pointer, I can't edit this line
A& b = *a; // b must be a reference
a->~A(); // destroy first object
new (a) A(); // create second, new object in its place
delete a; // destroy second object and release memory
The constraints are if A contains non-static const data members, you cannot use b after the reconstruction.
(Starting with C++17, you can reuse a reconstructed object even if it has const members by using some extra acrobatics: A& b2 = *std::launder(a); But you still cannot reuse b as is.)

Answer 2

in your code you are not using a reference to a pointer, to do so:
#include <iostream>
#include <string>
using namespace std;
int main()
{
int* ptrA = new int(77);
int*& rPtrA = ptrA; // this a reference to a pointer
cout << "ptrA: " << ptrA << endl;
cout << "*ptrA: " << *ptrA << endl;
cout << "rPtrA: " << rPtrA << endl;
cout << "*rPtrA: " << *rPtrA << endl;
delete ptrA;
ptrA = NULL;
cout << "ptrA: " << ptrA << endl;
// cout << "*ptrA: " << *ptrA << endl; // cause runtme error
cout << "rPtrA: " << rPtrA << endl;
// cout << "*rPtrA: " << *rPtrA << endl;// cause runtme error
ptrA = new int(100);
cout << "ptrA: " << ptrA << endl;
cout << "*ptrA: " << *ptrA << endl;
cout << "rPtrA: " << rPtrA << endl;
cout << "*rPtrA: " << *rPtrA << endl;
delete rPtrA; // delete pointer through its reference
rPtrA = NULL;
cout << "ptrA: " << ptrA << endl;
// cout << "*ptrA: " << *ptrA << endl; // runtime error
cout << "rPtrA: " << rPtrA << endl;
// cout << "*rPtrA: " << *rPtrA << endl; // runtime error
return 0;
}
&* is used for reference to pointer not just &.
a reference is a just another pseudonym to a variable so whatever the variable changes the reference changes also.
a reference to a pointer REFERS to the POINTER'S ADDRESS not the address the pointer stores (value of pointer).
in your code you used a reference to the value(what pointer stores not the pointer itself) the pointer points to