Why Does Erasing Vector Elements in a Loop Cause a Crash, and How Can I Avoid It?

Vector Erase Iterator: Understanding the Pitfalls

When working with vectors, it's essential to use the erase iterator correctly to avoid unexpected behavior. In this article, we'll explore a common pitfall related to erasing elements with a loop.

Consider the following code:

int main()
{
    vector<int> res;
    res.push_back(1);
    vector<int>::iterator it = res.begin();
    for ( ; it != res.end(); it++)
    {
        it = res.erase(it);
        //if (it == res.end())
        //  return 0;
    }
}

According to the documentation, the erase iterator "points to the new location of the element that followed the last element erased by the function call." This implies that erasing the last element in the sequence will result in the iterator pointing to the end of the vector.

However, if we run this code without checking if it == res.end(), the program crashes. Why is this?

The Increment Trap

The key to understanding this behavior lies in the way the for loop works. After each iteration of the loop, the iterator it is automatically incremented. This means that if the last element is erased, the iterator will point to the end of the vector, which is not a valid location to increment.

By adding the if (it == res.end()) return 0; check, we handle the case where the last element is erased and the iterator points to the end. This prevents the increment operation from causing a crash.

A More Efficient Approach

While the above approach works, there's a more efficient way to erase all elements from a vector. Instead of iterating through the vector and erasing each element individually, simply calling res.clear() will clear the entire vector without requiring a loop.

Conditional Erasing

However, if you only need to erase specific elements based on a condition, you can use the following pattern:

for ( ; it != res.end(); ) {
  if (condition) {
    it = res.erase(it);
  } else {
    ++it;
  }
}

This approach allows you to iterate through the vector, check each element for a condition, and erase it only if the condition is met.

The above is the detailed content of Why Does Erasing Vector Elements in a Loop Cause a Crash, and How Can I Avoid It?. For more information, please follow other related articles on the PHP Chinese website!