[SAGA-RG] Semantics of the ns_entry constructor

[Ceriel Jacobs]

Andre Merzky wrote:
> Quoting [Ceriel Jacobs] (Aug 17 2009):
>>> If the above works, I don't see why 
>>>
>>>  saga::file::entry e (u);
>>>
>>> should not work.  Or am I missing something?  The same then
>>> also holds for the open methods - the origin of Matijs'
>>> complain.
>> It is a major semantics change. If you do that, all methods in the
>> ns_entry implementations now have to consider the possibility that
>> the entry in fact is a directory, whereas earlier they could just
>> be hidden by ns_directory implementations.
> 
> But yes, that is how it is supposed to work, IMHO.  For
> example, the spec states what happens on entry.copy() if the
> entry happens to be a directory.  And similar for other
> calls on entry.

Yes, of course, but up until now, an implementation could do this by
redefining f.i. the copy method in its ns_directory implementation.

>> Also, I think it is confusing
>> that you can have a ns_entry object that represents a directory but cannot
>> be casted to a ns_directory. Note that I don't have an issue with the
>> change in the ns_directory.open method, only with the change in the ns_entry
>> constructor which, being a constructor, cannot deliver a ns_directory.
> 
> No, it does not deliver an ns_directory.  The following
> would *not* work:
> 
>   saga::url u ("file://localhost/etc/");
>   saga::file::dir * f = new saga::file::entry (u);
> 
> That would be technically impossible I think.
> But (again) the following should work:
> 
>   saga::url u ("file://localhost/etc/");
>   saga::file::entry * e = new saga::file::entry (u);
> 
> I still don't see a reason why that should be invalid.

It WAS invalid, because the specs say so explicitly.

If an ns_entry can represent a directory, can a SAGA file object do so as well?
IMHO, the intention of ns_entry has clearly been that it does not represent
a directory. For that, you have ns_directory. If you change that, what should
ns_directory.open() create when the requested entry does not exist?
And what happens with directory.open()? Its notes refer to the
ones from ns_directory.open(). So those are wrong as well?
and logical_directory.open()?

> Matijs' use case seems very valid to me:
> 
>   saga::url u ("file://localhost/etc/");
>   saga::file::entry * e = new saga::file::entry (u);
> 
>   if ( e->is_dir () )
>   {
>     saga::file::dir * f = new saga::file::directory (u);
>   }

I agree that the use-case is valid, I just have the impression that
allowing directories to be represented by ns_entry may open a can of
worms in the SAGA specs ...

Also, the use case could be expressed differently:

saga::url u ("file://localhost/etc/");
saga::file::entry *e;
try {
     e = new saga::file::entry (u);
} catch(BadParameter ex) {
     try {
         e = new saga::file::directory (u);
     } catch(Throwable ex2) {
         ... deal with exception
     }
}

Ceriel