iterator multimap_name.equal_range(key)

// C++ program to illustrate the
// multimap::equal_range() function
#include <bits/stdc++.h>
using namespace std;
 
int main()
{
 
    // initialize container
    multimap<int, int> mp;
    // insert elements in random order
    mp.insert({ 2, 30 });
    mp.insert({ 1, 40 });
    mp.insert({ 3, 60 });
    mp.insert({ 1, 20 });
    mp.insert({ 5, 50 });
 
    // Stores the range of key 1
    auto it = mp.equal_range(1);
 
    cout << "The multimap elements of key 1 is : \n";
    cout << "KEY\tELEMENT\n";
 
    // Prints all the elements of key 1
    for (auto itr = it.first; itr != it.second; ++itr) {
        cout << itr->first
             << '\t' << itr->second << '\n';
    }
    return 0;
}  

The multimap elements of key 1 is : 
KEY    ELEMENT
1    40
1    20