Why this code snippet said to contain error?

Question

I know normally, * and & signs. But our teacher gives us an example and she said "Problem occurs here"

int *a1;
int *a2 = new int[100];
a1=a2 //What does this line mean???
delete []a2;

k=a1[0]//she said error occurs here.

I couldn't understand what is a1 = a2 ? and why does error occur?

Answer 1

a1=a2 //What does this line mean???

= is the assignment operator. This actually assigns the value of the LHS operand to the RHS operand.

Here, this line means, assign the value of a2 to a1, i.e, this assigns the a2 pointer to a1. That is, a1 and a2 points to the same memory location.

Now, once you call delete[] with a1, it actually frees the memory. Then, accssing either a1[i] or a2[i] is same, accessing already free-d memory which invokes undefined behavior.

To help understand, consider the analogy of a glass of juice with two straws into it. Once you have drank the juice via either of the straw, you cannot get anymore juice via the other one.