Insertion and Deletion of Multiset Elements while Traversing through for loop using iterators

Question

Here, I am performing erase operations and an insert operation on the multiset while traversing through the multiset. The Code that I have written is:

#include <bits/stdc++.h>

using namespace std;

int main(){
    multiset<int> ms;
    ms.insert(6);
    ms.insert(7);
    ms.insert(8);
    ms.insert(9);
    ms.insert(10);
    for(auto it = ms.begin();it != ms.end();it++){
        cout << *it << endl;
        ms.erase(it);
        if(*it == 6){
            ms.insert(4);
        }
    }
}

Output of the Above Code is : 6 7 4 8 9 10

I am unable to understand the output and how 4 is printing as a part of output!!

Does anyone know the explanation to the output???

I have tried different insertion and deletion operations on set while traversing through the for loop using iterators. Always getting stuck at some point and unable to understand the output!!

Answer 1

As you can see in the multiset::erase documentation:

References and iterators to the erased elements are invalidated

Therefore after this line:

ms.erase(it);

Any attempt to derefrence it (like you do in the next line with *it) is UB (Undefined Behavior).
This means anything can happen.

Some side notes:

1. Why should I not #include <bits/stdc++.h>?
2. Why is "using namespace std;" considered bad practice?