cpp/filesystem/recursive directory iterator

is a that iterates over the  elements of a directory, and, recursively, over the entries of all subdirectories. The iteration order is unspecified, except that each directory entry is visited only once.

By default, symlinks are not followed, but this can be enabled by specifying the directory option at construction time.

The special pathnames and  are skipped.

If the reports an error or is advanced past the last directory entry of the top-level directory, it becomes equal to the default-constructed iterator, also known as the end iterator. Two end iterators are always equal, dereferencing or incrementing the end iterator is undefined behavior.

If a file or a directory is deleted or added to the directory tree after the recursive directory iterator has been created, it is unspecified whether the change would be observed through the iterator.

If the directory structure contains cycles, the end iterator may be unreachable.

Non-member functions
Additionally, provided as required by.

It is unspecified whether an equality operator is a member or non-member.

Helper templates
These specializations for make it a  and a.