#include <iterator>
#include <vector>
#include <iostream>

int main()
{
  std::vector<double> v = { 1, 2, 3, 4, 5 };
  auto m = begin(v) + distance(begin(v), end(v)) / 2; 
  std::cout << *m << '\n';
  advance(m, 1);
  std::cout << *m << '\n';
  advance(m, -2);
  std::cout << *m << '\n';
}