Aprenda Iterator Invalidation and Safety

Understanding how iterators become invalid is essential for writing reliable C++ code with STL containers and algorithms. Iterator invalidation occurs when an iterator that once pointed to a valid element becomes unsafe to use after the container is modified. This can cause undefined behavior, crashes, or hard-to-trace bugs.

Adding or removing elements from a std::vector may invalidate iterators, especially when reallocation occurs;
Operations like insert, erase, or resize affect iterator validity differently depending on the container;
Containers such as std::list often preserve iterator validity during insertions and deletions.

Iterator invalidation happens after operations that alter a container's structure or contents. The exact behavior depends on the container type and the operation performed.

main.cpp


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#include <iostream>
#include <vector>

int main()
{
    std::vector<int> numbers = {1, 2, 3, 4, 5};

    // Obtain an iterator to the second element
    auto it = numbers.begin() + 1;

    // Insert a new element at the beginning
    numbers.insert(numbers.begin(), 0);

    // 'it' may now be invalidated due to possible reallocation
    // Accessing *it is unsafe
    std::cout << "Possible invalid iterator value: ";
    std::cout << *it << std::endl; // Undefined behavior

    // Erase an element and try to use the iterator again
    it = numbers.begin() + 2;
    numbers.erase(numbers.begin() + 1);

    // 'it' still points to the old position, which may now be invalid
    std::cout << "After erase, possible invalid iterator value: ";
    std::cout << *it << std::endl; // Undefined behavior
}

Note

When using STL algorithms, be aware of operations that may invalidate iterators. Using an invalidated iterator can compromise your program's safety and correctness.

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Adding or removing elements from a std::vector may invalidate iterators, especially when reallocation occurs;
Operations like insert, erase, or resize affect iterator validity differently depending on the container;
Containers such as std::list often preserve iterator validity during insertions and deletions.