What is the difference between iterator with reference symbol, and without. (C++ loop)

Question

What is the difference between an iterator with the & symbol, and one without it, as seen in cases 1 and 2 in the contrived example below?

When should I use one or the other?

#include <iostream>
#include <vector>
int main() {
    std::vector<int> v;
    v.push_back(1);
    v.push_back(2);

    // case 1:
    for (auto & i = v.begin(); i != v.end(); ++i) {
        std::cout << *i << std::endl;
    }

    // case 2:
    for (auto i = v.begin(); i != v.end(); ++i) {
        std::cout << *i << std::endl;
    }
}

Is it something to do with the creation of an iterator object, and what is then available in the code block? I'm very new to iterators.

Answer 1

The difference is that the first loop will not be compiled.:)

for (auto & i = v.begin(); i != v.end(); ++i) {
    std::cout << *i << std::endl;
}

You created a temporary object returned by member function begin and binded it with non-const reference.