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

Question

I've got code that looks like this:

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);

I'd like remove inactive items immediately after update them, inorder to avoid walking the list again. But if I add the commented-out lines, I get an error when I get to i++: "List iterator not incrementable". I tried some alternates which didn't increment in the for statement, but I couldn't get anything to work.

What's the best way to remove items as you are walking a std::list?

Answer 1

You have to increment the iterator first (with i++) and then remove the previous element (e.g., by using the returned value from i++). You can change the code to a while loop like so:

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;
    }
}

Answer 2

You want to do:

i= items.erase(i);

That will correctly update the iterator to point to the location after the iterator you removed.

Answer 3

Use std::remove_if algorithm.

Edit: Work with collections should be like: 1. prepare collection. 2. process collection.

Life will be easier if you won't mix this steps.

1. std::remove_if. or list::remove_if ( if you know that you work with list and not with the TCollection )
2. std::for_each

Answer 4

Removal invalidates only the iterators that point to the elements that are removed.

So in this case after removing *i , i is invalidated and you cannot do increment on it.

What you can do is first save the iterator of element that is to be removed , then increment the iterator and then remove the saved one.