[Rd] R 2.6.0 S4 data breakage, R _data_class(), class<-, etc.

[John Chambers]

Most of your problems seem related to assigning an S4 class to an 
arbitrary object--a really bad idea, since it can produce invalid objects.

Objects from S4 classes are created by calling the function new(), and 
in principal _only_ by calling that function.  Objects from one class 
are coerced to another by calling the function as().

Assigning a class to any old object is a very S3 idea (and not a good 
idea except in low-level code there, either).

At the C level there are  macros for new() (R recommends NEW_OBJECT()), 
although the safest approach when feasible is to allocate the object in 
R.  The general as() computation really needs to be done in R because of 
its special use of method dispatch; there are macros for the equivalent 
of the as.<type>() functions.

Perhaps some improvements to the documentation would make this clearer, 
although Chapter 7 and Appendix A of Programming with Data seem 
reasonably definite.

Thanks for sharing your notes.

John


Hin-Tak Leung wrote:
> Hi,
>
> (somebody would probably yell at me for not checking 2.6.0rc,
> for which I can only apologize...)
>
> Our R package (snpMatrix in 
> http://www-gene.cimr.cam.ac.uk/clayton/software/) is broken rather badly
> in 2.6.0 ; I have fixed most of it now so a new release is imminent;
> but I'd like to mention a few things, mostly to summarize my experience
> and hopefully the 'writing R extensions' document can be updated to
> reflect some of this...
>
> 1) We created and bundled some data in the past in the 2.2 to 2.5
> time frame (well, 18 months in reality);
> most of them triggers a warning 'pre-2.4.0 S4 objects detected...
> consider recreating...'
>    a) I could fix all of them with just 'a <- asS4(a)' and save()
>         (they are relatively simple objects just missing the S4 object
>          bit flag)
>    b) I am surprised one of them were actually saved from 2.5 - our buggy
>       code no doubt, see below.
>
> We never noticed we didn't do SET_S4_OBJECT() in our C code nor
> asS4() in our R code until this week. Obviously we were mistakenly
> relying on the S4 method dispatch on S3 objects, which were withdrawn in 
> 2.6.0...
>
> 2) I am surprised that 'class(a)' can read S4 class names, but 
> 'class(a)<-' does not set the S4 object bit. I suppose the correct way 
> would be to do new(...)? This needs to be written down somewhere...
> The asymmetry is somewhat surprising though.
>
> 3) We have some C code which branches depending on the S4 class.
> The R extension doc didn't explain that one needs to do R_data_class()
> rather than classgets() (or 'getAttrib(x, RClassSymbol)') to retrieve
> S4 classes; further more,
> R_data_class() is not part of the public API, and I only found it by
> looking at the C code of 'class()' (do_class()). But R_data_class()
> is part of exposed binary interface and the methods package certainly
> uses it; isn't it time to make it part of the public API? In any case, I 
> think a way of retrieving the S4 class in C is needed.
>   
Yes, or at the least instructions to handle the case of a NULL class 
attribute, but a macro would be good.
>   4) The documentation is missing a fair part - specifically,
> I need to be able to read and write the S4 class attribute...
> so R_data_class() needs to be documented and exposed as part of the 
> public API (and included in the Rinternals.h include),
> and the recommended way of making an S4 object in C? I found
> classgets() + SET_S4_OBJECT() seem to work, but I'd like an 
> authoritative answer...
>
> 5) I am finding 'class()<-' + asS4() in R and classgets()+ 
> SET_S4_OBJECT() in C combo's a bit awkward. Is there any reasons why
> class<- or classgets() (or if there is a more 'correct' API to use for
> S4) cannot automatically set the S4 bit if the name is a known S4 class?
>
> Thanks for reading so far...
>
> Hin-Tak
>
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