Question

For a std::for_each call, is it legal to have both iterators be the end iterator? For example,

std::vector<int> some_ints;
std::for_each(ints.end(), ints.end(), [&](int i) {
    std::cout << i;
});

Answer 1

Yes this is perfectly legal and produces an empty range. The result will be that nothing happens but the behavior is well defined.

Here is a relevant quote from the SGI reference page on iterators and their use as ranges (Documentation)

Most algorithms are expressed not in terms of a single iterator but in terms of a range of iterators; the notation [first, last) refers to all of the iterators from first up to, but not including, last. Note that a range may be empty, i.e. first and last may be the same iterator.