A swappable pair container for STL set or map.

swappable_pair.hpp

#include <iostream>
#include <set>
#include <map>

template<typename T>
struct SwappablePair {
    T field1, field2;

    inline auto operator==(const SwappablePair &rhs) const -> bool {
        return (field1 == rhs.field1 && field2 == rhs.field2) ||
               (field1 == rhs.field2 && field2 == rhs.field1);
    }

    inline auto operator!=(const SwappablePair &rhs) const -> bool {
        return !this->operator==(rhs);
    }

    inline auto operator<(const SwappablePair &rhs) const -> bool {
        return this->operator!=(rhs) && (field1 < rhs.field1 || field2 < rhs.field2);
    }
};

int main() {
    const auto s = std::set<SwappablePair<int>>
        {{10, 20},
         {20, 10},
         {60, 20},
         {10, 10}};
    const auto m = std::map<SwappablePair<int>, bool>
        {{{10, 20}, true},
         {{20, 10}, false},
         {{60, 20}, true},
         {{10, 10}, false}};
    // Show count
    // 3
    std::cout << s.size() << std::endl;
    // Show all elements
    // 10, 10
    // 10, 20
    // 60, 20
    for (auto e : s)
        std::cout << e.field1 << ", " << e.field2 << std::endl;
    // 10, 10: 0
    // 10, 20: 1
    // 60, 20: 1
    for (auto [k, v] : m)
        std::cout << k.field1 << ", " << k.field2 << ": " << v << std::endl;
    return 0;
}