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# C++ Iterator invalidation

**[Iterator invalidation](https://en.cppreference.com/w/cpp/container)** occurs when a container operation (like `insert`, `erase`, `push_back`, or `resize`) makes existing iterators or references unsafe to use. Different containers have different invalidation rules: `vector` invalidates on reallocation; `list` invalidates only the erased element; `map` invalidates only the erased element.

Check container-specific iterator invalidation rules before modifying containers; prefer container algorithms like `std::erase` (C++20) or the erase-remove idiom.

## Example

This example shows how vector reallocation invalidates iterators and demonstrates safety.

```cpp
// compile: g++ -std=c++20 -o invalid invalid.cpp
// run: ./invalid
// description: iterator invalidation and how to safely modify containers

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> v{1, 2, 3, 4, 5};
    
    // Unsafe: push_back may reallocate, invalidating iterator
    // auto it = v.begin();
    // v.push_back(6);
    // it->print();  // undefined behavior
    
    // Safe: erase returns new iterator
    auto it = v.begin() + 2;
    it = v.erase(it);  // erase element at [2], it now points to next
    
    // Safe: algorithms handle invalidation
    v.erase(std::remove(v.begin(), v.end(), 3), v.end());
    
    for (int x : v) std::cout << x << " ";
    std::cout << "\n";
    
    return 0;
}
```