Re: Iterating over a directory

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Tue, 13 Apr 2010 10:40:42 CST
On 13 Apr., 11:52, Ulrich Eckhardt <> wrote:

I'm just musing about how to write an STL iterator that walks over a
directory. There are two things that create a problem for me:
1. For iterating, I need to use OS functions that allocate a resource.
2. Iterators are generally assumed to be copyable.

Holding a resource that can't be copied poses the questions of ownership,
which in turn poses the question of how to handle copying. Currently, the
only solution that would solve this is shared ownership using some kind of
reference counting. Alternatively, I could use exclusive ownership but that
would prevent copying and make the iterator less useful.

I see that Boost.Filesystem uses a shared_ptr for its iteration metadata,
i.e. use reference counting, any other ideas?

If you don't need to support the iterator requirements, I would
consider to use exclusive ownership. If you do want/need to
support these requirements, shared ownership seems the one
and only one possible solution to me in an ownership-controlled
system. But in many situations it is possible to let a system
look like a shared-ownership control system by simply lacking
of the control ;-)

The latter approach is simply to provide a reference/pointer to
some externally controlled object in the iterator. This is basically,
what std::ostream_iterator does with the delimiter and the stream
or what the diverse insert iterators do with the referenced
container-like object: This is also a shared object, but there is
no inherent ownership control.

This is a very efficient mechanism, but requires some discipline
on the programmer's side. In regard to your resource-allocation
problem, the different problem domains can be easily separated:

1) First define some RAII class that ensures that your OS
resources will be freed at the end of the life-time of this
resource holder. This can be e.g. unique_ptr or some
user-defined class.

2) Decide, whether your iterator should completely auto-manage
the resource - this basically enforces the shared-ownership control.

If complete auto-management is not needed, provide a constructor
of the iterator that accepts a reference to the RAII-controller
(whichever you prefer - even unique_ptr or some other non-copyable
holder will work). Now the user has to take care that if the iterator
leaves the current scope, the life-time of the RAII controller is
lengthened to at least the same life-type as the leaving iterator

In fact it is possible to support both strategies within the same
iterator type. This is comparable to what std::unique_lock
does via it's different mutex/lock-accepting c'tors.

HTH & Greetings from Bremen,

Daniel Kr?gler

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