std::flat_set<Key,Compare,KeyContainer>::value_comp

#include <iostream>
#include <flat_set>
#include <utility>
 
// Example module 97 key compare function
struct ModCmp
{
 bool operator()(int lhs, int rhs) const
 {
 return (lhs % 97) < (rhs % 97);
 }
};
 
int main()
{
 std::flat_set <int, ModCmp> cont{1, 2, 3, 4, 5};
 
 // Same behavior as key_comp()
 auto comp_func = cont.value_comp();
 
 for (const int val{100}; const int key : cont)
 {
 const bool before = comp_func(key, val);
 const bool after = comp_func(val, key);
 
 std::cout << "Key (" << key << ") ";
 if (!before && !after)
 std::cout << "equivalent to key (" << val << ")\n";
 else if (before)
 std::cout << "goes before key (" << val << ")\n";
 else if (after)
 std::cout << "goes after key (" << val << ")\n";
 else
 std::unreachable ();
 }
}

Key (1) goes before key (100)
Key (2) goes before key (100)
Key (3) equivalent to key (100)
Key (4) goes after key (100)
Key (5) goes after key (100)