std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::key_comp

#include <iostream>
#include <flat_map>
#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_map <int, char, ModCmp> cont;
 cont = {{1, 'a'}, {2, 'b'}, {3, 'c'}, {4, 'd'}, {5, 'e'}};
 
 auto comp_func = cont.key_comp();
 
 for (const auto it : cont)
 {
 const bool before = comp_func(it.first, 100);
 const bool after = comp_func(100, it.first);
 
 std::cout << "Key (" << it.first << ',' << it.second << ") ";
 if (!before && !after)
 std::cout << "equivalent to key (100)\n";
 else if (before)
 std::cout << "goes before key (100)\n";
 else if (after)
 std::cout << "goes after key (100)\n";
 else
 std::unreachable ();
 }
}

(1,a) goes before key (100)
(2,b) goes before key (100)
(3,c) equivalent to key (100)
(4,d) goes after key (100)
(5,e) goes after key (100)