Question

I’m basically looping through all the entries to check whether some entries is to be erased, but seems in a wrong way:

std::vector<HANDLE> myvector; 
for(unsigned int i = 0; i < myvector.size(); i++)
{
    if(...)
         myvector.erase(myvector.begin()+i);
}

Anyone spot the problem in it? How to do it correctly?

Answer 1

Your problem is algorithmic. What happens if two adjacent elements meet your criterion for deletion? The first will be deleted, but because i is incremented after each iteration of the loop, the second will be skipped. This is because a vector is contiguous in memory, and all elements after the deleted one are moved forwards one index.

An ugly hack would be to do the following:

std::vector<HANDLE> myvector; 
for(unsigned int i = 0; i < myvector.size();)
{
    if(...)
         myvector.erase(myvector.begin()+i);
    else
         i++;
}

I’m not sure if using iterators would work, because calling erase invalidates iterators to elements after the erased element.

The elegant solution would be to use std::remove_if, as GMan suggested. This would abstract away two things:

1. Your removal condition
2. The process by which the elements of a container are removed

Edit: I should also add, the hacked solution is O(n²) in the worst case. GMan’s solution is O(n), assuming your removal condition is O(1). I would strongly encourage you to learn and use GMan’s solution.