// Calculates the number of elements between first and last.
#include <iterator> // std::distance
#include <vector> // std::vector
#include <algorithm> // Just if you use std::find
vector<int> arr = {2,5,3,8,1};
int size = std::distance(arr.begin(), arr.end()); // 5
auto it = std::find(arr.begin(), arr.end(), 8);
int position = std::distance(arr.begin(), it); // 3