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Iterator Invalidation in C++

In this article, we will discuss the Iterator invalidation in C++ with its examples.

Iterator invalidation is a term used in C++ to describe conditions in which an iterator, a powerful tool used to traverse across containers such as vectors, lists, or maps, becomes invalid or useless as a result of actions done on the container. This idea is critical for understanding and producing strong C++ code, particularly when working with dynamic data structures.

Iterator behavior and invalidation are intrinsically connected to the underlying container and the actions performed on it. When it comes to the rules of iterator invalidation, there are mainly three groups that contain different container groups:

  • No Invalidation: Certain containers, such as std::vector, have reasonably stable iterators. Unless an action causes memory reallocation, iterators to vector items stay valid throughout most operations.
  • Invalidation of Specific Iterators: Containers such as std::list and std::deque may invalidate individual iterators after certain actions. For example, std::list ensures that only iterators pointing to the deleted entries become invalid, whereas other ones remain unaffected.
  • Complete Invalidation: Some container activities can render all iterators connected with it invalid. For example, if the capacity of a std::vector is exceeded while putting elements into it, all previously obtained iterators for that vector become invalid.

Understanding these invalidation requirements is critical for developing efficient and bug-free programs. It's critical to be aware of the type of box you've chosen to hold the result. It's also vital to understand what's occurring throughout the conversion and how it could affect other areas of your code, such as walking through a list step by step


Let us consider the following example to illustrate the iterator invalidation in C++:

Filename: Iterator.cpp


1 3 14 7 4 


In the above example code, adding an element -1 while iterating the vector may cause the vector's size to exceed the maximum size. In this case, a new memory is assigned to the vector, and all items are copied there. However, the iterator continues to refer to the prior old memory location. So, we can now declare that the iterator is invalidated. It is a type of invalidation.

Iterator Invalidation rules:

  • Vector: All iterators that point to an element prior to the insertion point are unaffected, while all others are invalidated. However, if the size of this vector increases due to insertion, all iterators that are become invalidated, as stated in the preceding example.
  • Dequeue: If the added member is at the front or back of the deque, all iterators become invalid, but the references to the elements remain valid. Otherwise, all iterators and references become invalid.
  • List: Insertion/Deletion: Only the iterators pointing to the deleted/added items are invalidated when elements in a list are erased or inserted. Other iterators that are continue to be valid.

Iterator Handling Principles:

  1. Know the Container Rules: Recognize the unique iterator invalidation rules associated with the various types of containers you interact with, such as lists, maps, and vectors. Each of the containers has its own set of rules for when iterators become invalid because of certain operations.
  2. Avoid altering Containers during Iteration: Avoid altering the container while iterating through it with iterators. It includes adding, deleting, or resizing items in a way that may invalidate the iterators.
  3. Reassign or rebuild Iterators: After executing activities that may invalidate iterators, evaluate or rebuild them to ensure their validity.
  4. Keep Iterators Short: Iterators should be used only when essential. Avoid storing iterators that are for a lengthy period to limit the possibility of invalidation.
  5. Prefer Algorithms from the Standard Library: Instead of manually maintaining iterators, use Standard Library methods (std::find, std::remove_if, etc.) that yield valid iterators. Internally, such methods are built to manage iterator invalidation.
  6. Use Iterator Traits: Use std::iterator_traits to identify the iterator category and the associated invalidation rules for individual containers.
  7. Understand Reallocations: Operations that involve memory reallocation, such as expanding a vector above its capacity or putting entries into a map that causes rehashing, can invalidate iterators.
  8. Store Indexes Rather Than Iterators: If iteration modification is required, consider using indices rather than iterators. Indexes are less likely to be invalidated since they indicate positions in a container.
  9. Avoid Nested Iterations and Container Modifications: Use caution when using nested loops that affect the same container. Make sure that inner loops do not invalidate iterators used in outside loops.

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