Iterator Invalidation in C++

In C++, we have different containers like vector, list, set, map etc. To iterate through these containers, we can use the iterators. We should be careful when we are using iterators in C++. When we are using iterating over a container, then sometimes, it may be invalidated. If the shape, size is changed, then we can face this kind of problems. In the following example, we can identify the problem of invalidation.

Example Code

#include <iostream>
#include <vector>
using namespace std;
int main() {
   vector <int> vec{11, 55, 110, 155, 220};
   for (auto it=vec.begin(); it!=vec.end(); it++)
      if ((*it) == 110)
         vec.push_back(89); //inserting a new value while iterating the vector
      for (auto it=vec.begin();it!=vec.end();it++)
         cout << (*it) << " ";
}

Output

11 55 110 155 220 89 89

In this program we can get different kinds of results. Here the size of the vector is not defined earlier. Some values are provided for initialization. Now while iterating, we are adding one more value. In this case, if the vector has no space, it will create a new memory block at runtime, and all of the items will be copied. But the iterator will be pointed to the previous address. For this it may generate some invalidations.

Let us see some rules for the iterator invalidations.