How to Safely Delete Elements from a std::list During Iteration?

Deleting Elements During Iteration in a std::list

When iterating through a std::list, removing elements while maintaining a valid iterator can be challenging. Consider the following code:

for (std::list<item*>::iterator i = items.begin(); i != items.end(); i++)
{
    bool isActive = (*i)->update();
    //if (!isActive) 
    //  items.remove(*i); 
    //else
       other_code_involving(*i);
}
items.remove_if(CheckItemNotActive);

Adding the commented-out lines to remove inactive items immediately would result in a "List iterator not incrementable" error. This is because removing an element invalidates the iterator.

Avoiding Multiple Passes

To remove items efficiently while iterating, consider a while loop approach:

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

The key here is to increment the iterator before removing the element. Alternatively, "i = items.erase(i)" can be used. This allows for safe and efficient removal of elements while iterating.

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