Is it allowed to increment an end iterator?

Question

Is it allowed to increment an iterator variable it that already is at end(), i.e. auto it = v.end()?

• Is it allowed in general?
• If not, is it not allowed for vector?
• If yes, is ++it maybe idempotent if it==v.end()?

I ask, because I stumbled upon code like this:

std::vector<int> v{ 1, 2, 3, 4, 5, 6, 7 };
// delete every other element
for(auto it=v.begin(); it<v.end(); ++it) { // it<end ok? ++it ok on end?
  it = v.erase(it);
}

It works fine with g++-6, but that is no proof.

For one it<v.end() may only work with vectors, I suppose it should read it!=v.end() in general. But in this example that would not recognize the end of v if ++it is applied when it already is on the end.

Answer 1

No the behaviour is undefined. You are allowed to set an iterator to end(), but you must not increment it or dereference it.

You are allowed to decrement it so long as the backing container is not empty.