How to Efficiently Erase Elements from a Vector During Iteration Without Invalidating the Iterator?

Efficient Erasing from a Vector During Iteration

When iterating through a vector, it can be necessary to remove elements that meet certain criteria. However, straightforward removal can invalidate the iterator used for looping. This article provides a solution for efficiently erasing elements without using the v[i] method.

Maintaining the Iterator After Erase

The erase() method returns a new iterator pointing to the element after the removed element. This allows for continued iteration by updating the loop condition:

<code class="cpp">for(iterator it = begin; it != end(container); it = vec.erase(it))
{
    if (it->somecondition())
    {
        it = vec.erase(it);
    }
}</code>

Combining remove_if and erase

To optimize the removal process, consider using std::remove_if and erase together:

<code class="cpp">iterator it = std::remove_if(begin, end, pred);
vec.erase(it, vec.end());</code>

This approach separates the removal of eligible elements from their actual removal, resulting in a faster O(N) operation.

Example with Template-Based Removal

To generalize the removal process, use a template-based predicate and remove_by_caller class:

<code class="cpp">class remove_by_caller
{
public:
    remove_by_caller(AguiWidgetBase* pWidget) : mWidget(pWidget) {}

    template <typename T>
    bool operator()(const T& pX) const
    {
        return pX.getCaller() == mWidget;
    }

private:
    AguiWidgetBase* mWidget;
};

std::vector<AguiTimedEvent>::iterator it =
    std::remove_if(timedEvents.begin(), timedEvents.end(), remove_by_caller(widget));
timedEvents.erase(it, timedEvents.end());</code>

Through these approaches, it is possible to efficiently erase elements from a vector while maintaining the iterator.

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