unordered_map_name.clear()

// CPP程序，演示unordered_map clear()函数的实现
#include <bits/stdc++.h>
using namespace std;
  
int main()
{
    // 取出任意两个unordered_map
    unordered_map<int, int> ump1, ump2;
  
    // 插入值
    ump1[1] = 2;
    ump1[3] = 4;
    ump1[5] = 6;
    ump1[7] = 8;
  
    // 输出容器的大小
    cout << "在调用clear函数之前的unordered_map size: \n";
    cout << "ump1 size = " << ump1.size() << endl;
    cout << "ump2 size = " << ump2.size() << endl;
  
    // 删除元素
    ump1.clear();
    ump2.clear();
  
    // 输出容器的大小
    cout << "在调用clear函数之后的unordered_map size: \n";
    cout << "ump1 size = " << ump1.size() << endl;
    cout << "ump2 size = " << ump2.size() << endl;
  
    return 0;
}

在调用clear函数之前的unordered_map size: 
ump1 size = 4
ump2 size = 0
在调用clear函数之后的unordered_map size: 
ump1 size = 0
ump2 size = 0