Page History

...

In this noncompliant code example, the objects stored in the stdthe std::set have an overloaded operator< overloaded operator< implementation, allowing the objects to be compared with stdwith std::less. However, the comparison operation does not provide a strict weak ordering. Specifically, two sets, x and y, whose i values are both 1, but have differing j values can result in a situation where compwhere comp(x, y) and comp(y, x) are both false, failing the asymmetry requirements.

...

Compliant Solution

This compliant solution uses stduses std::tie() to properly implement the strict weak ordering operator< ordering operator< predicate.

#include <iostream>
#include <set>
#include <tuple>
 
class S {
  int i, j;
 
public:
  S(int i, int j) : i(i), j(j) {}
  
  friend bool operator<(const S &lhs, const S &rhs) {
    return std::tie(lhs.i, lhs.j) < std::tie(rhs.i, rhs.j);
  }
  
  friend std::ostream &operator<<(std::ostream &os, const S& o) {
    os << "i: " << o.i << ", j: " << o.j;
    return os;
  }
};

void f() {
  std::set<S> t{S(1, 1), S(1, 2), S(2, 1)};  
  for (auto v : t) {
    std::cout << v << std::endl;
  }
}

...