Object-oriented programming in R

Dear R-users,

I have very recently started learning about object-oriented programming in
R. I am far from being an expert in programming, although I do have an
elementary C++ background.

Please take a look at these lines of code.

some.data = data.frame(V1 = 1:5, V2 = 6:10) ;
p.plot = ggplot(data=some.data,aes(x=V1, y=V2)) ;
class(p.plot) ;
[1] "ggplot"

My understanding is that the object p.plot belongs to the "ggplot" class.
However, a new class definition like

setClass("AClass", representation(mFirst = "numeric", mSecond = "ggplot"))
;

yields the warning

Warning message:
In .completeClassSlots(ClassDef, where) :
?undefined slot classes in definition of "AClass": mSecond(class "ggplot")

The ggplot object is also a list :

is.list(p.plot)
[1] TRUE

So, I guess I could identify mSecond as being a list.

However, I don't understand why "ggplot" is not considered a valid slot
type. I thought setClass() was analogous to the class declaration in C++,
but I guess I might be wrong. Would anyone care to provide additional
explanations about this?

I decided to explore object-oriented programming in R so that I could
organize the output from my analysis in a more rigorous fashion and then
define custom methods that would yield relevant output. However, I'm
starting to wonder if this aspect is not better suited for package builders.
R lists are already very powerful and convenient templates. Although it
wouldn't be as elegant, I could define functions that would take lists
outputted by the different steps of my analysis and do what I want with
them. I'm wondering what the merits of both approaches in the context of R
would be. If anyone has any thoughts about this, I'd be most glad to read
them.

Cheers,
--
*Luc Villandr?*
/Biostatistician
McGill University Health Center -
Montreal Children's Hospital Research Institute/