How to erase element from vector and update iterator?

Question

I am trying to delete a object from a vector at a specific index. The vector iterator keeps track of the index throughout the program. In the below code, the first IF statement works perfectly. But, if the iterator is pointing to anywhere OTHER than the last element, I erase the element from the vector and then increment the iterator. The program crashes and says "iterator not incrementable".

I ran the debugger several times and everything looks correct, so I cannot see what I am missing?

vector<Card> myVector; //container to hold collection of cards. vector<Card>::iterator myVectorIterator; //points to each "card" in the collection. Card Collection::remove() { if (myVectorIterator== myVector.end()-1) { //at the last card //erase the "current" card myVector.erase(myVectorIterator); //update to the first card. myVectorIterator= myVector.begin(); } else { myVector.erase(myVectorIterator); //crashes here! myVectorIterator++; } return *myVectorIterator;

}

The return value of erase is an iterator pointing to the location of the element that followed the last element erased. — tgmath
– tgmath, Commented Jul 14, 2014 at 15:04

Mike Seymour · Accepted Answer · 2014-07-14 15:06:07Z

4

erase invalidates the iterator, so you can't use it afterwards. But it helpfully returns an iterator to the element after the removed one:

myVectorIterator = myVector.erase(myVectorIterator);

answered Jul 14, 2014 at 15:06

Mike Seymour

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2 Comments

alig227 Over a year ago

thank you, this works. I looked at numerous vector documentation and could not find any explanation for this, sigh.

Mike Seymour Over a year ago

@astra: Did you try this one? That's generally a pretty good reference.

Stian Svedenborg · Accepted Answer · 2014-07-14 15:12:05Z

This is because the call to erase invalidates all iterators. Imagine you have something pointing to an element and that element disappears, what shall you point to?

Worse still, the behavior is highly implementation dependent. The correct approach is to store use the return value from erase which will be an iterator to the next element in the vector.

myVectorIterator = myVector.erase(myVectorIterator);

Note that you now have to remove the incrementation on the next line (or you will skip an item.)

In general, when manipulating different STL containers, you will see that in the documentation whether a given operation will have an effect on iterators. If you take a look at this link, you will see that for vector::erase this is the case:

"Iterators, pointers and references pointing to position (or first) and beyond are invalidated, with all iterators, pointers and references to elements before position (or first) are guaranteed to keep referring to the same elements they were referring to before the call."

Different containers may have different guarantees when it comes to iterator validity.

Cory Kramer · Accepted Answer · 2014-07-14 15:04:29Z

You have to do this

myVectorIterator = myVector.erase(myVectorIterator);

It will remove the current iterator then assign the next item into it.

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