Why does std::fill use ForwardIterator, not OutputIterator?

Question

(This question has the same "question template" as Why does std::max_element require a ForwardIterator? but the answer has to be different, because std::fill doesn't return an iterator into the output sequence.)

std::fill is defined to work only when the output range is given by a pair of ForwardIterators. However, the superficially similar std::fill_n works fine with OutputIterator.

Surely the algorithm is simply

template<class OutIt, class T>
void fill(OutIt first, OutIt last, T value)
{
    while (first != last) {
        *first = value;
        ++first;
    }
}

What about this algorithm requires ForwardIterator? What am I missing?

Answer 1

Output iterators are not comparable. The == and != operators are not defined for output iterators.

You need a forward iterator, because it

satisfies the requirements of an input iterator

which supports EqualityComparable.

With output iterators, std::fill cannot compare first with last:

while (first != last) {

Not supported, for output iterators.

std::fill_n avoids this comparison, it just uses the counter to write to the iterator, so all it needs is an output iterator.