multimap_name.lower_bound(key)

// 以下是 C++ 中用来说明 multimap::lower_bound() 函数的代码
#include <bits/stdc++.h>
using namespace std;
  
int main()
{
  
    // 初始化容器
    multimap<int, int> mp;
  
    // 随机插入元素
    mp.insert({ 2, 30 });
    mp.insert({ 1, 40 });
    mp.insert({ 2, 60 });
    mp.insert({ 2, 20 });
    mp.insert({ 1, 50 });
    mp.insert({ 4, 50 });
  
    // 当元素2存在时
    auto it = mp.lower_bound(2);
    cout << "The lower bound of key 2 is ";
    cout << (*it).first << " "
         << (*it).second << endl;
  
    // 当元素3不存在时
    it = mp.lower_bound(3);
    cout << "The lower bound of key 3 is ";
    cout << (*it).first << " "
         << (*it).second << endl;
  
    // 当元素5超过时
    it = mp.lower_bound(5);
    cout << "The lower bound of key 5 is ";
    cout << (*it).first << " "
         << (*it).second << endl;
    return 0;
}

输出如下：
The lower bound of key 2 is 2 30
The lower bound of key 3 is 4 50
The lower bound of key 5 is 6 0