Question

I am wondering, what is happening when an int is starting with a zero?

int main() {
    int myint = 01001;
    cout << myint;
    return 0;
}

Why is that outputting:

513

I have tried several compilers.

Answer 1

Then the integer is treated as an octal number. So,

01001

equals

1 * 8 ^ 0 + 0 * 8 ^ 1 + 0 * 8 ^ 2 + 1 * 8 ^ 3 = 1 + 0 + 0 + 512 = 513

No magic in there.