test.cc

#include <algorithm>
#include <iostream>
#include <vector>
#include <utility>
using namespace std;

int main( int argc, char *argv[] ) {
  vector<pair<int, int>> pairs = {
    {2, 3},
    {2, 4},
    {2, 3},
    {1, 2}};
  std::sort(pairs.begin(), pairs.end(), [](const pair<int, int>& a, const pair<int, int>& b) {
    return a.second - b.second;
  });
  for (auto& p : pairs) {
    cout << p.first << ", " << p.second << endl;
  }

  return 0;
}