An object of type 
recursive_directory_iterator provides an iterator for
a sequence of 
directory_entry elements representing the files in a
directory or in an 
implementation-defined directory-like file
type, and its subdirectories
.namespace std::filesystem {
  class recursive_directory_iterator {
  public:
    using iterator_category = input_iterator_tag;
    using value_type        = directory_entry;
    using difference_type   = ptrdiff_t;
    using pointer           = const directory_entry*;
    using reference         = const directory_entry&;
    
    recursive_directory_iterator() noexcept;
    explicit recursive_directory_iterator(const path& p);
    recursive_directory_iterator(const path& p, directory_options options);
    recursive_directory_iterator(const path& p, directory_options options,
                                 error_code& ec);
    recursive_directory_iterator(const path& p, error_code& ec);
    recursive_directory_iterator(const recursive_directory_iterator& rhs);
    recursive_directory_iterator(recursive_directory_iterator&& rhs) noexcept;
    ~recursive_directory_iterator();
    
    directory_options  options() const;
    int                depth() const;
    bool               recursion_pending() const;
    const directory_entry& operator*() const;
    const directory_entry* operator->() const;
    
    recursive_directory_iterator&
      operator=(const recursive_directory_iterator& rhs);
    recursive_directory_iterator&
      operator=(recursive_directory_iterator&& rhs) noexcept;
    recursive_directory_iterator& operator++();
    recursive_directory_iterator& increment(error_code& ec);
    void pop();
    void pop(error_code& ec);
    void disable_recursion_pending();
    bool operator==(default_sentinel_t) const noexcept {
      return *this == recursive_directory_iterator();
    }
    
  };
}