Erasing while iterating an std::list

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The idiomatic way to write that loop would be:

for (auto i = list.begin(); i != list.end();) {
    if (condition)
        i = list.erase(i);
    else
        ++i;
}

You can do the same thing with a set, multiset, map, or multimap. For these containers you can erase an element without affecting the validity to any iterators to other elements. Other containers like vector or deque are not so kind. For those containers only elements before the erased iterator remain untouched. This difference is simply because lists store elements in individually allocated nodes. It’s easy to take one link out. vectors are contiguous, taking one element out moves all elements after it back one position.

Your loop is broken because you erase the element at i on some given condition. i is no longer a valid iterator after that call. Your for loop then increments i, but i is not valid. Hell upon earth ensues. This is the exact situation that is why erase returns the iterator to the element after what was erased… so you can continue traversing the list.

You could also use list::remove_if:

list.remove_if([](auto& i) { return i > 10; });

In the lambda, return true if the element should be removed. In this example, it would remove all elements greater than 10.

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