Destructive str(...)?

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.