What is the safe way to iterate through a vector, and delete its certain elements

Question

Is it possible to iterate in one single step through elements of a vector, while deleting some of the elements, to prevent repeated processing? (I know the fragment is bogus, but illustrates what I want)

std::vector<MyRecord*>* BLV = GetVector();
for (std::vector<MyRecord*>::iterator it = BLV->begin(); it != BLV->end(); ++it)
{
  MyRecord* BL = *it;
  if(Selected)
  { 
    delete BL;
    BLV->erase(it);
  }
}

Possible duplicate of http://stackoverflow.com/questions/3938838/erasing-from-a-stdvector-while-doing-a-for-each — Edgar Rokjān, Jun 16 '16 at 08:16
Have you thought about using a vector of unique_ptr's and apply the erase-remove-idiom? — MikeMB, Jun 16 '16 at 08:18
What's the relationship between `MyRecord` and `LinkRegisterRecord`? — songyuanyao, Jun 16 '16 at 08:23
@songyuanyao LinkRegisterRecord is the real name I use in my program. I forgot to rename it to my MWE. Sorry. — katang, Jun 16 '16 at 08:34

songyuanyao · Answer 1 · 2016-06-16T08:32:56.630

Note std::vector::erase will invalidate the iterator to the element to be erased, and then ++it; will lead to UB.

Invalidates iterators and references at or after the point of the erase, including the end() iterator.

You can use the return value of erase().

Iterator following the last removed element. If the iterator pos refers to the last element, the end() iterator is returned.

then change the loop to

for (std::vector<MyRecord*>::iterator it = BLV->begin(); it != BLV->end(); )
{
  MyRecord* BL = *it;
  if(Selected)
  { 
    delete BL;
    it = BLV->erase(it);
  } else
  {
    ++it;
  }
}

score 1 · Accepted Answer · answered Jun 16 '16 at 09:09

songyuanyao already gave you a nice answer: In short, you have to reset your iterator to the value returned by erase.

However, if applicable, I'd suggest using a vector of unique_ptr's and the erase-remove idiom (which is also more efficient, as it only moves the elements once).

std::vector<std::unique_ptr<MyRecord>>* BLV = GetVector();
BLV->erase(
    std::remove_if(BLV->begin(), BLV->end(), 
        [&](const std::unique_ptr<MyRecord>& rec) { return Selected; }
    ),
    BLV->end()
);

What is the safe way to iterate through a vector, and delete its certain elements

2 Answers2