Why does std::count return a signed integer? [duplicate]

Question

I was really surprised to see that std::count returned a iterator_traits<InputIterator>::difference_type, which in turns refers to a long int on my platform.

Why is that? A negative count elements within a container doesn’t make any sense.

Answer 1

It's actually a std::ptrdiff_t, which has to be a signed integer. It has to be signed because it can be used as the difference between two iterators, and that can of course be negative.