How to add a slot to S4 class of an existing package?

Hi,

Is there a better approach to adding new slots to an S4 class without
breaking code that accesses older objects of that class than the
Bioconductor reference in the thread below? ?Thanks.

Cheers,
Seb



On Mon, 29 Dec 2008 22:53:50 +0000,
Wolfgang Huber <huber at ebi.ac.uk> wrote:

Dear Christian this post from Martin Morgan on class versioning in
Bioconductor's Biobase package might be relevant:
https://stat.ethz.ch/pipermail/bioc-devel/2006-May/000545.html and
also section 6 of this:
http://www.bioconductor.org/packages/2.4/bioc/vignettes/Biobase/inst/doc/BiobaseDevelopment.pdf

? ? ? Best wishes Wolfgang

---------------------------------------------------- Wolfgang Huber
EMBL-EBI http://www.ebi.ac.uk/huber

cstrato wrote:

Dear all,

Since my package is based on S4 classes, I would like to know how to
add a slot to an existing S4 class without breaking the code of users
of my package.

Assume the following S4 class: setClass("MyClass",
representation(name = "character", type = "character", data =
"data.frame" ), prototype(name = "", type = "Default", data =
data.frame(matrix(nr=0,nc=0)) ) )#MyClass

Assume that a user has created an object: > myclass <- new("MyClass",
name="MyName", type="MyType", data=tmp) > str(myclass)

Now I would like to add another slot "info" to MyClass:
setClass("MyClass", representation(name = "character", type =
"character", data = "data.frame", info = "data.frame" ),
prototype(name = "", type = "Default", data =
data.frame(matrix(nr=0,nc=0)), info = data.frame(matrix(nr=0,nc=0)) )
)#MyClass

Now when the user loads my package with S4 class MyClass containing a
new slot and calls: > str(myclass) Error in FUN(c("name", "type",
"data", "info")[[4L]], ...) : no slot of name "info" for this object
of class "MyClass"

My question is: Is there any possibility or special trick, which
would avoid this error message?

Are there other possibilities to access an additional data.frame from
an existing class?

Is there something like an "evolution" of S4 classes, which
distinguishes the different implementations of an S4 class, and
allows the user to keep the object of an old class?

Best regards Christian _._._._._._._._._._._._._._._._._._
C.h.r.i.s.t.i.a.n S.t.r.a.t.o.w.a V.i.e.n.n.a A.u.s.t.r.i.a
e.m.a.i.l: cstrato at aon.at _._._._._._._._._._._._._._._._._._

--
Seb

Seb That thread and the resources in Biobase assumes that you have a
class that extends "Versioned".  Doing so will help you in the long
run by providing you with updateObject (at the cost of some
complexity).  However, it does not really help you if the existing
class does not extend Versioned.

In general, adding a new slot should not break any existing code, but
you may find users who have old objects lying around that they cannot
use with the new functionality (I assume you are adding slots to
provide new functionality).