Why can't I use mid here?

[CodingIsHard17]

Why do I get an error when computing the following line?

vector<int>::iterator mid = (vi.begin() + vi.end()) / 2;

[jlb]

What is the error?

[dutch]

You can't add the iterators.
Try

    vector<int>::iterator mid = vi.begin() + vi.size() / 2;