std::copy and std::vector problem

Question

I understand why this causes a segfault:

#include <algorithm>
#include <vector>
using namespace std;

int main()
{
    vector<int> v;
    int iArr[5] = {1, 2, 3, 4, 5};
    int *p = iArr;

    copy(p, p+5, v.begin());

    return 0;
}

But why does this not cause a segfault?

#include <algorithm>
#include <vector>
using namespace std;

int main()
{
    vector<int> v;
    int iArr[5] = {1, 2, 3, 4, 5};
    int *p = iArr;

    v.reserve(1);
    copy(p, p+5, v.begin());

    return 0;
}

Answer 1

Both are wrong as you are copying to empty vector and copy requires that you have space for insertion. It does not resize container by itself. What you probably need here is back_insert_iterator and back_inserter:

copy(p, p+5, back_inserter(v));

Answer 2

This is undefined behavior - reserve() allocates a buffer for at least one element and the element is left uninitialized.

So either the buffer is big enough and so you technically can access elements beyond the first one or it is not big enough and you just happen to not observe any problems.

The bottom line is - don't do it. Only access elements that are legally stored in the vector instance.