std::map<Key,T,Compare,Allocator>::key_comp

From cppreference.com
< cpp‎ | container‎ | map

 
C++
 
Containers library
 
std::map
Member functions
Element access
Iterators
(C++11)
(C++11)
(C++11)
(C++11)
Capacity
Modifiers
(C++23)
(C++17)
(C++11)
(C++11)
(C++17)
(C++17)
(C++17)
Lookup
(C++20)
Observers
map::key_comp
Non-member functions
(C++20)
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
Deduction guides (C++17)
 
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.

Example

Run this code
#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

See also

 returns the function that compares keys in objects of type value_type
(public member function)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/container/map/key_comp&oldid=135936"