umm.operator= ( unordered_multimap& umm )
umm.operator= ( unordered_multimap&& umm )
umm.operator= ( initializer list )

// C++代码示例
// unordered_multimap:: operator=()
#include <bits/stdc++.h>
  
using namespace std;
  
// 合并函数
template <class T>
T merge(T a, T b)
{
    T t(a);
    t.insert(b.begin(), b.end());
    return t;
}
  
int main()
{
    unordered_multimap<int, int> sample1, sample2, sample3;
  
    // 列表初始化
    sample1 = { { 1, 2 }, { 3, 4 }, { 5, 6 } };
    sample2 = { { 7, 8 }, { 9, 10 }, { 11, 12 } };
  
    // 合并两个列表
    sample3 = merge(sample1, sample2);
  
    // 复制赋值
    sample1 = sample3;
  
    // 打印 unordered_multimap 列表
    for (auto& it : sample1)
        cout << it.first << " : " << it.second << endl;
    cout << endl;
    for (auto& it : sample2)
        cout << it.first << " : " << it.second << endl;
    cout << endl;
    for (auto& it : sample3)
        cout << it.first << " : " << it.second << endl;
    return 0;
}

7 : 8
9 : 10
11 : 12
1 : 2
3 : 4
5 : 6

11 : 12
9 : 10
7 : 8

7 : 8
9 : 10
11 : 12
1 : 2
3 : 4
5 : 6