[mehrdadn]

Any kind of generator is basically like this. Imagine one for listing the contents of a directory... maybe it looks like this:

  class DirectoryChildren
  {
    void *dir;
    void *buffer;
    size_t buffer_size;
    size_t buffer_offset;
    ...
  public:
    bool next(FileInfo *info);
  };

Bear in mind the buffer only represents a chunk of intermediate results fetched from the OS; when we reach its end, we have to request more data from the OS.

If you had to make an iterator for this, what would you do? You'd basically need to either (a) create iterators that at least duplicate the entire state of DirectoryChildren on the stack, or (b) make iterators allocate on the heap. Both of these suck, because (1) iterators are passed around and copied all over the place on the assumption that these are fundamentally cheap operations, which is an assumption that easily breaks (like here), and (2) the iterators fundamentally do not have any meaning or utility as separate entities from the range itself.

[pumpkinpal]

This functionality already exists in the C++ standard library as std::filesystem::(recursive_)directory_iterator with the default constructor creating a sentinel value. This is solution (1) and this is due to the restriction that prior to C++20 iterators were required to be the same type. C++20 relaxes this restriction and allows the sentinel to be any type as long as it is equality comparable with the other iterator. Since on *nix systems readdir is used under the hood, the sentinel can simply be an empty type that is only equal when the actual iterator's last cached dirent pointer is equal to NULL. The sentinel itself can be an empty type.

[mehrdadn]

Why do you keep repeating the same comment? Are you reading my replies? I already addressed exactly what you're talking about: https://news.ycombinator.com/item?id=25492756