Question

I don’t understand why the following does not work:

queue<int*> q;

int counter = 1;
for (int i = 0; i < 3; i++) {
    int a[1] = {counter};
    q.push(a);
    counter++;
}

while (!q.empty()) {
    int *top = q.front();
    q.pop();

    cout << top[0] << endl;
}

It should print out: 1 2 3, but instead 3 3 3 is printed out. This is because the pointers in the queue are all the same after each run through the loop. Why does that happen?

Answer 1

You are storing pointers to local variables and using those pointers after the local variables they point to have gone out of scope.

In other words: you are invoking Undefined Behavior.

Result: It should not print out “1 2 3”. It doesn’t have to do anything and is allowed to do whatever it likes. “3 3 3” seems reasonable to me, as it is also allowed to crash.