How this piece of code accessing the local variable outside the C++ function?

Question

int * addition(int arr[])
{
    int sum=0;
    for(int i=0;i<4;i++)
    sum+=arr[i];
    return &sum;
}

int main()
{
    int arr[4]{1,3,4,5}, * ptr=addition(arr);
    cout<<*ptr<<endl;
    return 0;
}

As the variable sum is local to the function addition so the variable should be destroyed as soon as the program control moves out of the function but it is still giving the output 13. Why?

Compiler: g++ 4.8.2 on Ubuntu 14.04 LTS

Answer 1

The variable sum is on the stack. When the function addition() returns it doesn't zero the released stack memory, so the address returned still contains the sum.

Does it still work if you enable optimisation with -O3 ?