[Rd] Destructive str(...)?

[Prof Brian Ripley]

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,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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