A pair of iterators defines a range

Now that we understand the fundamental concept of an iterator, let's put it to some practical use. We've already seen that if you have a pair of iterators as returned from begin() and end(), you can use a for-loop to iterate over all the elements of the underlying container. But more powerfully, you can use some pair of iterators to iterate over any sub-range of the container's elements! Let's say you only wanted to view the first half of a vector:

    template<class Iterator>
    void double_each_element(Iterator begin, Iterator end) 
    {
      for (auto it = begin; it != end; ++it) {
        *it *= 2;
      } 
    }

    int main() 
    {
      std::vector<int> v {1, 2, 3, 4, 5, 6};
      double_each_element(v.begin(), v.end());
        // double each element in the entire vector
      double_each_element(v.begin(), v.begin()+3);
        // double each element in the first half of the vector
      double_each_element(&v[0], &v[3...