std::map key_comp() method

since C++98

// Const version only
key_compare key_comp() const;

Returns the function object that compares the keys, which is a copy of this container's constructor argument comp.

Parameters

(none)

Return value

The key comparison function object.

Complexity

Constant - O(1).

Exceptions

(none)

Example

Main.cpp

#include <cassert>
#include <iostream>
#include <map>
 
// Example module 97 key compare function
struct ModCmp {
    bool operator()(const int lhs, const int rhs) const
    {
        return (lhs % 97) < (rhs % 97);
    }
};
 
int main()
{
    std::map<int, char, ModCmp> cont;
    cont = { { 1, 'a' }, { 2, 'b' }, { 3, 'c' }, { 4, 'd' }, { 5, 'e' } };
 
    auto comp_func = cont.key_comp();
 
    for (auto it : cont) {
        bool before = comp_func(it.first, 100);
        bool after = comp_func(100, it.first);
 
        std::cout << '(' << 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
            assert(0); // Cannot happen
    }
}

Output

(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