Iterator Invalidation in std::forward_list

Iterators in a std::forward_list can become invalidated under certain circumstances:

Modifying Operations

Any operation that modifies the list, such as insert_after(), emplace_after(), erase_after(), push_front(), emplace_front(), pop_front(), or clear(), invalidates iterators pointing to the elements following the modified element.

1#include <forward_list>
2#include <iostream>
3
4int main() {
5  std::forward_list<int> list{1, 2, 3, 4};
6  auto it = list.begin();
7  ++it;                      // points to 2
8  list.insert_after(it, 5);  // 'it' invalidated
9  std::cout << *it;          // undefined behavior
10}

Reallocation

If the internal memory of the forward_list is reallocated (e.g., due to a call to resize()), all iterators are invalidated.

To avoid issues, it's best to re-obtain iterators after performing any modifying operation on the forward_list.