How to Safely Remove Elements from an `std::list` While Iterating?

Can you remove elements from an std::list while iterating through it while iterating through it?

In C , the built-in std::list class is a doubly-linked list, providing efficient insertion and removal of elements. However, when attempting to modify a list during iteration, it's crucial to approach the operation with caution.

Problem: An error occurred when iteratively checking the activity status of items in the std::list and attempting to remove inactive items immediately. The error "List iterator not incrementable" was encountered upon attempting to move the iterator after removing an element.

Solution: The key to successfully removing elements during iteration is applying the modifications to the iterator first, before performing the removal. Instead of attempting to increment the iterator and then remove the element, it's more appropriate to restructure the loop as a while loop, allowing the iterator to be incremented within the loop itself. Thus, the updated code below demonstrates the correct approach:

std::list<item*>::iterator i = items.begin();
while (i != items.end()) {
    bool isActive = (*i)->update();
    if (!isActive) {
        i = items.erase(i);  // alternatively, i = items.erase(i++);
    } else {
        other_code_involving(*i);
        ++i;
    }
}

This revised code ensures that the iterator is correctly incremented before executing the element removal, thereby avoiding the error and enabling efficient removal of inactive elements while iterating through the list.

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