vector<int> a = {5, 3, 1, 3};
vector<int> b = {6, 1, 7, 2};
vector<int> ind = {0, 1, 2, 3};

sort(ind.begin(), ind.end(), [&a, &b](int x, int y) -> bool {
  return a[x] < a[y] || (a[x] == a[y] && b[x] < b[y]);
});

for (int i: ind) {
  cout << a[i] << " " << b[i] << endl;
}
==

(Often, a cleaner alternative to using lambda functions for sorting is to define a class holding all the data for each entry and to implement the $<$ operator.)
 
_Please let us know in the comments section what other $C++11$ features do you use for programming contests. Code snippets illustrating your favorite tricks would be beneficial for us all._

(Often, a cleaner alternative to using lambda functions for sorting is to define a class holding all the data for each entry and to implement the $<$ operator.)