map value_comp() in C++ STL

The std::map::value_comp() is a function in C++ STL. It returns a function object that compares objects of type std::map::value.

Syntax:

value_compare value_comp() const

Parameters: It does not accept any parameters.

Returns: This method returns a function object that compares objects of type std::map::value.

Time Complexity: O(1)

Below examples illustrate the map::value_comp() method:

Example 1:

// C++ program to illustrate
// map::value_comp() method

#include <iostream>
#include <map>
using namespace std;

int main()
{
    map<char, int> m = {
        { 'a', 1 },
        { 'b', 2 },
        { 'c', 3 },
        { 'd', 4 },
        { 'e', 5 },
    };

    auto last = *m.rbegin();
    auto i = m.begin();

    cout << "Map contains "
         << "following elements"
         << endl;

    do {

        cout << i->first
             << " = "
             << i->second
             << endl;
    } while (m.value_comp()(*i++, last));

    return 0;
}

Output:

Map contains following elements
a = 1
b = 2
c = 3
d = 4
e = 5

Example 2:

// C++ Program to illustrate
// map::rbegin() method

#include <iostream>
#include <map>
using namespace std;

int main()
{

    map<char, char> m = {
        { 'a', 'A' },
        { 'b', 'B' },
        { 'c', 'C' },
        { 'd', 'D' },
        { 'e', 'E' },
    };

    auto last = *m.rbegin();
    auto i = m.begin();

    cout << "Map contains "
         << "following elements"
         << endl;

    do {

        cout << i->first
             << " = "
             << i->second
             << endl;
    } while (m.value_comp()(*i++, last));

    return 0;
}

Output:

Map contains following elements
a = A
b = B
c = C
d = D
e = E