On Fri, 29 Oct 2004, Simon Urbanek wrote:
> I have encountered a strange behavior of the str function - it seems to
> modify the object that is displayed. Probably I'm using something
> unsupported (objects consisting just of an external reference), but
Yes, and I think it is documented somewhere, but I can't lay my hands on
it right now.
> still I'm curious as of why this happens. I create (in C code)
> EXTPTRSXP and associate a class to it via SET_CLASS. Such objects works
> fine until it's passed to str as the following output demonstrates:
>
> > c<-.MCall("RController","getRController")
> > c
> [1] "<RController: 0x3be5d0>"
> > str(c)
> Class 'ObjCid' length 1 <pointer: 0x3be5d0>
> > c
> <pointer: 0x3be5d0>
> > str(c)
> length 1 <pointer: 0x3be5d0>
>
> The .MCall basically produces an external reference and assigns a class
> (ObjCid) to it. There's a corresponding print method and it works fine.
> However, when str is called, it strips the class information from the
> object as a repeated call to str also shows:
>
> > str(c); str(c)
> Class 'ObjCid' length 1 <pointer: 0x3be5d0>
> length 1 <pointer: 0x3be5d0>
>
> Is this behavior intentional, undocumented or simply wrong?
The issue is almost certainly that something has forgotten/decided not to
either set or respect SET_NAMED on the object, so when str does
object <- unclass(object)
or some such, the original object gets changed. Now the `something' has
to be C code: possibly yours but probably something in R itself.
I think this is intentional. External references do not get copied, and
the advice I recall is to wrap them in a list for use at R level (and
before setting a class on them). In RODBC I took another tack, and attach
the reference as an attribute to a `documentation' object.
str() probably ought to be more cautious when it encounters at external
reference or similar exotic object, since it will look at list elements
and attributes.
Brian
--
Brian D. Ripley, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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