std::multiset<Key,Compare,Allocator>::value_comp

#include <iostream>
#include <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::multiset<int, ModCmp> cont{1, 2, 3, 4, 5};

    // Same behaviour 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();
    }
}

Вывод:

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)