C/C++ the result of the uninitialized array

Question

It might be a boring question! thanks!

Here's the code:

#include <iostream>
#include <cstring>
using namespace std;

int main()
{
        int a[5] = {0};
        int b[5];
        cout << a << endl;
        cout << b << endl;
        for (int i = 0; i < 5; i++)
        {
                cout << a[i] << " ";
        }
        cout << endl;
        for (int i = 0; i < 5; i++)
        {
                cout << b[i] << " ";
        }
        cout << endl;
        return 0;
}

in Ubuntu: g++ a.cpp

In windows with DEV C++ ,MinGW GCC 4.7.2:

So the question is focused on the array b:

I know I haven't initialized the array b.

Array b is full of garbage values, but why there is always having '0' with the fixed position like "X 0 X 0 X"??

What happens inside?? Just a protection mechanism?

Answer 1

That is undefined behavior. There is no guarantee, if these zeros are there, that just accidentally is true.

The explanation is, that for some random reason at these places in memory a 0 was stored before it was reused for your purpose here. Since you allocate your arrays on the stack, these zeroes are probably from a prior function call and might be some padding. The compiler will do that as he pleases.