C++ 更新unordered_map中的值，其中键值对的数据类型为int-unordered_set

Question

I was solving a DSA question and observed a slightly strange behavior in the C++ standard template library:

    vector<int> v = {1, 7, 8, 3, 12};

    unordered_map<int, unordered_set<int>> ump;

    for (int i=0; i<v.size(); i++) {
        unordered_set<int> us;
        us.insert(56);
        ump.insert(mp(v[i], us));
    }
    
    for (int i=0; i<v.size(); i++) {
        unordered_set<int> us = ump.find(v[i])->second;
        us.insert(67);
    }

    for (auto it = ump.begin(); it != ump.end(); it++) {
        cout << it->first << ": ";
        for (auto ait = it->second.begin(); ait!=it->second.end(); ait++) {
            cout << *ait << ' ';
        }
        cout << '\n';
    }

Here the output is coming to be:

12: 56 
3: 56 
8: 56 
7: 56 
1: 56 

But the expected output should be:

12: 67 56 
3: 67 56 
8: 67 56 
7: 67 56 
1: 67 56

And I guess the problem is arising during the insertion of values into the set. And I also think, the problem is in someway related to the "changing values through their reference". But not exactly sure what is causing this. I experimented a bit more and found that the below code snippet works. But still not sure, what is going wrong in the above code?

In the below code snippet, instead of directly updating the set through its reference, I made a copy of it. Then inserted my values and again copied that into the map, and it worked.

    vector<int> v = {1, 7, 8, 3, 12};

    unordered_map<int, unordered_set<int>> ump;

    for (int i=0; i<v.size(); i++) {
        unordered_set<int> us;
        us.insert(56);
        ump.insert(mp(v[i], us));
    }
    
    for (int i=0; i<v.size(); i++) {
        unordered_set<int> us = ump.find(v[i])->second;
        unordered_set<int> us1;
        for (auto it = us.begin(); it!=us.end(); it++) {
            us1.insert(*it);
        }
        us1.insert(67);
        ump.find(v[i])->second = us1;
    }

    for (auto it = ump.begin(); it != ump.end(); it++) {
        cout << it->first << ": ";
        for (auto ait = it->second.begin(); ait!=it->second.end(); ait++) {
            cout << *ait << ' ';
        }
        cout << '\n';
    }

Here the output is exactly equal to the expected one:

12: 67 56 
3: 67 56 
8: 67 56 
7: 67 56 
1: 67 56 

Answer 1

unordered_set<int> us = ump.find(v[i])->second;

Makes a copy of the set so your modifications don't change the value in ump . You need to use a reference instead:

unordered_set<int>& us = ump.find(v[i])->second;

Or using auto :

auto& us = ump.find(v[i])->second;