What happens if you increment an iterator that is equal to the end iterator of an STL container

Following is the quote from Nicolai Josuttis book:

Note that advance() does not check whether it crosses the end() of a sequence (it can't check because iterators in general do not know the containers on which they operate). Thus, calling this function might result in undefined behavior because calling operator ++ for the end of a sequence is not defined

In other words, the responsibility of maintaining the iterator within the range lies totally with the caller.

Perhaps you should have something like this:

template <typename Itr>
Itr safe_advance(Itr i, Itr end, size_t delta)
{
    while(i != end && delta--)
        i++;
    return i;
}

You can overload this for when iterator_category<Itr> is random_access_iterator to do something like the following:

return (delta > end - i)? end : i + delta;

You could use the "distance" function between your iterator (it) and the iterator at vec.begin() and compare it with the vector's size (obtained by size()).

In that case your for loop would look like this:

for (vector<int>::iterator it = vec.begin(); distance(vec.begin(), it) < vec.size(); ++it)
{
     // Possibly advance n times here.
}