[Rd] S4 object type (Was: S4 object type available for testing)

[John Chambers]

The S4 type introduced for testing last month is now the standard in the 
r-devel (2.4) version of R.

Note that older binary objects, including class definitions, if restored 
will not be consistent with the new type.  They will probably work most 
of the time, but should be regenerated from source.

Also, a note about future changes:  There will likely be two more 
changes to S4 code in the next few weeks:  an internal bit to mark S4 
objects for testing, and a new version of method dispatch designed to 
fix some logical flaws and to be more efficient.

John Chambers wrote:
> The default prototype object for an S4 class up to now has been a 
> zero-length list (nothing else worked).  This is wrong in principle, 
> because it makes the objects look like vectors to low-level C code, 
> and they should not.  It may also be the cause of some inefficiency.
>
> A new low-level type has been added for these classes.  It's currently 
> available in the r-devel version, but not turned on by default.  If 
> you have code that makes use of S4 objects, or C-level code that tries 
> to handle all internal types, please try to test out the new feature.
>
> To turn the feature on, set environment variable R_S4_Type to anything 
> non-empty, for example in bash
>
> $ export R_S4_Type=TRUE
>
> before starting up R.  You can also turn the new type on and off in R 
> by calling methods:::.useS4Prototype(TRUE) or FALSE.  (This is a 
> temporary situation, eventually the type will be made the default.)
>
> Note the following points.
>
> 1.  The type affects the prototype object for the class.  Therefore 
> what matters is whether it's turned on when the call to setClass() 
> takes place.  It has NO effect on objects generated from an existing 
> class.  And turning the type off has no effect until a new setClass() 
> call.
>
> 2.  Classes that contain any of the basic R object types ("character", 
> "function", etc.) are unaffected by the change.  Their prototype 
> objects will still have the inherited basic type, as they should.
>
> 3. To see what the current prototype is, use defaultPrototype().  If 
> the new type is on you should see:
>
> > defaultPrototype()
> <S4 Type Object>
>
> 4. Not directly related, but some changes have been made to make 
> automatic printing of S4 objects correspond more closely to a call to 
> show(), as per the green book.
>
> John
>