C++ : if iterator is an object, why can't it be initialized to a reference iterator?

Question

Supposedly we have the below:

// Example program
#include <iostream>
#include <string>
#include <vector>
using namespace std;

int main()
{
  vector <int> v {1,2,3};
  vector <int>::iterator v_i1 = v.begin();
  vector <int>::iterator &v_i2 = v.begin(); // error out - initial value of reference to non-const must be an lvalue
}

Understanding:

1. v.begin() will return an iterator - which is actually an object
2. v_i1 a variable is initialized with the iterator return from begin(). No issue
3. If v.begin() returns an iterator (object here??), why can't it be assigned to reference v_i2? Is it because v.begin() return a rvalue?

Thanks for helping me to understand better.

Answer 1

Non const reference can't bind to rvalue returnrd by std::vector::begin(). Make the reference to const.

#include <iostream>
#include <string>
#include <vector>
using namespace std;

int main() {
  vector <int> v {1,2,3};
  vector <int>::iterator v_i1 = v.begin();
  const vector <int>::iterator &v_i2 = v.begin();
}

Or, as commented, make it rvalue reference. The both prolong the temporary object' lifetime to the scope of the reference.

#include <iostream>
#include <string>
#include <vector>
using namespace std;

int main() {
  vector <int> v {1,2,3};
  vector <int>::iterator v_i1 = v.begin();
  const vector <int>::iterator &v_i2 = v.begin();
  vector <int>::iterator &&v_i3 = v.begin();
}