Set intersection and union, supporting std::unordered_set

result.txt

$ ./a.out 
4 5 
1 2 3 4 5 6 7 8 

set_operator.hpp

namespace setop {
template <typename Set, typename Key = typename Set::value_type>
static inline Set
set_union(const Set& lhs, const Set& rhs) {
    Set uset{lhs};
    uset.insert(rhs.begin(), rhs.end());
    return std::move(uset);
}

template <typename Set, typename Key = typename Set::value_type>
static inline Set
set_intersection(const Set& lhs, const Set& rhs) {
    if (lhs.size() <= rhs.size()) {
        Set iset;
        for (const Key& key : lhs) {
            if (rhs.count(key) > 0) {
                iset.insert(key);
            }
        }
        return std::move(iset);
    } else {
        return set_intersection(rhs, lhs);
    }
}
}  // namespace setop

test.cc

#include <iostream>
#include <unordered_set>

#include "set_operator.hpp"

int main() {
    std::unordered_set<int> lhs{1, 2, 3, 4, 5};
    std::unordered_set<int> rhs{4, 5, 6, 7, 8};

    for (const auto& e : setop::set_intersection(lhs, rhs)) {
        std::cout << e << ' ';
    }
    std::cout << '\n';
    for (const auto& e : setop::set_union(lhs, rhs)) {
        std::cout << e << ' ';
    }
    std::cout << '\n';
    return 0;
}