Using a Set Iterator in C++

Question

When I try to use a set iterator in debug mode in C++, I get an error that says "map/set iterator not dereferencable". I don't understand because I thought dereferincing was how you are supposed to use an iterator. The code looks like this:

set<int>::iterator myIterator;
for(myIterator = mySet.begin();
    myIterator != mySet.end();
    myIterator++)
    DoSomething(*myIterator)

That is the format of all the examples I have seen online about how to use iterators. What am I doing wrond?

Answer 1

If DoSomething() changes the set - removes or inserts items, then the iterator you're holding is invalidated, which will probably cause this error.

Answer 2

That error generally means you are accessing an "end()" iterator.

Answer 3

This question was based on a false premise. I saw the error "map/set iterator not dereferencable" and thought that that was a general statement that applied to all map/set iterators, which like I said wouldn't make any sense. But I looked again and the real problem was just that the pointer I was using to access that iterator was invalid.

Answer 4

The first and biggest thing you're doing wrong is writing code like this at all. What you have above is the manually-written equivalent of:

std::for_each(mySet.begin(), mySet.end(), DoSomething);

There are relatively few really good uses of iterators outside of implementing algorithms. Once in a while it's reasonable with a map or multimap (or unordered_[multi]map), but that's mostly compensating for map and multimap using std::pair, which isn't entirely wonderful.