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();
    }
}

Вывод:

(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)