multiset_name.clear()

// C++ program to demonstrate the
// multiset::clear() function
#include <bits/stdc++.h>
using namespace std;
int main()
{
 
    int arr[] = { 15, 10, 15, 11, 10 };
 
    // initializes the set from an array
    multiset<int> s(arr, arr + 5);
 
    // prints all elements in set
    cout << "The elements in multiset are: ";
    for (auto it = s.begin(); it != s.end(); it++)
        cout << *it << " ";
 
    cout << "\nThe size after clear() is: ";
 
    // erases all elements
    s.clear();
    cout << s.size();
 
    return 0;
}  

multiset中的元素为: 10 10 11 15 15
clear()后的大小为: 0

// C++ program to demonstrate the
// multiset::clear() function
#include <bits/stdc++.h>
using namespace std;
int main()
{
 
    int arr[] = { 15, 10, 15, 11, 10, 18, 18, 20, 20 };
 
    // initializes the set from an array
    multiset<int> s(arr, arr + 9);
 
    // prints all elements in set
    cout << "The elements in multiset are: ";
    for (auto it = s.begin(); it != s.end(); it++)
        cout << *it << " ";
 
    cout << "\nThe size after clear() is: ";
 
    // erases all elements
    s.clear();
    cout << s.size();
 
    return 0;
}  

multiset中的元素为: 10 10 11 15 15 18 18 20 20
clear()后的大小为: 0