ump.operator= ( unordered_map& ump )
ump.operator= ( unordered_map&& ump )
ump.operator= ( 初始化列表 )

// C++中说明了unordered_map::operator=()方法
#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_map<int, int> sample1, sample2, sample3;
  
    //初始化列表
    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_map列表
    for (auto& it : sample1)
        cout << it.first << " : " << it.second << endl;
  
    for (auto& it : sample2)
        cout << it.first << " : " << it.second << endl;
  
    for (auto& it : sample3)
        cout << it.first << " : " << it.second << endl;
    return 0;
}

输出结果：
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