How can std::vector access elements with huge gaps between them?

Question

Having this code:

template <class IIt, class OIt> OIt copy2(IIt begin, IIt end, OIt dest) {    while (begin != end)    {       //make gap between element addresses       for (int i = 0; i < 99999999; i++)       {          dest++;       }       *dest++ = *begin++;    }    return dest; }  int main(int argc, char** argv) {    vector<int> vec({ 1, 2, 3 });    vector<int> vec2;    copy2(vec.begin(), vec.end(), back_inserter(vec2));    for (int i : vec2)    {       cout << i << endl;    } } 

Which takes quite a long to compile, but will finally do with the proper output

1 2 3 

The problem is (without knowing the inner implementation of std::vector, is it c-style array? or more complex structure?), how can it properly find those elements in the for(int i:vec2), when the address (pointers) of those elements are not sequential? (i.e. because of the shifting of the iterator/pointer by 99999999).

I thought there was a requirement for OutputIterator to have that property, that only one-access, one-shift could be performed on it. But when you shift(add) it more than once between accessing them, then there is a gap, which is quite huge in my case. So how does it compile?

Answer 1

You've been fooled.

The iterator returned by std::back_inserter has it++ as a no-op. So those 'gaps' you are creating? Yeah that's all doing nothing.