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!!

The intended usage of `erase(iterator)` is more akin to [this](https://stackoverflow.com/questions/9167745). I'm mostly guessing though. — Michael Foukarakis, Dec 21 '22 at 17:14

wohlstad · Accepted Answer · 2022-12-21T14:43:11.240

2

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:

edited Dec 21 '22 at 14:43

answered Dec 21 '22 at 14:15

wohlstad

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OK, then instead of dereferencing the iterator it again in if statement, if storing the value of *it in int x and then using this x shouldn't give us the problem right. But the Output remains same even in that Case. – Naruto Uzumaki Dec 21 '22 at 14:24
You can store the value of `*it` in `int x` before calling `erase`. – wohlstad Dec 21 '22 at 14:25
In that Case, the output is again 6 7 4 8 9 10, can u please explain me how are we iterating over the set and why are we getting 4 in the output? according to my guess answer should be 6 7 8 9 10. – Naruto Uzumaki Dec 21 '22 at 14:31
You cannot do it like that. Even using `it++` in the loop is actually dereferencing the `it`. One way to remove elements is to keep another `vector` with key to erase. Then iterate though this `vector` and do the actual `erase`. – wohlstad Dec 21 '22 at 14:36
Can you explain more about what you are trying to do with this code ? – wohlstad Dec 21 '22 at 14:36
gotcha!! there is nothing particular I am trying here, I was exploring my ways with iterators and data structures and stuck at this point!! Now I understand that it gives us Undefined Behavior in this scenario!! Thanks for help man!! – Naruto Uzumaki Dec 21 '22 at 14:48

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

1 Answers1