[R] method dispatching vs inheritance/polymorphism (re-post)

[Duncan Murdoch]

On 25/04/2010 9:07 AM, Albert-Jan Roskam wrote:
> Hi,
>
> I'm having trouble seeing the added value over functions defined by setGeneric vis-a-vis
> methods defined by inheritance and polymorphism. setGeneric offers a 'clean' call to a generic function, ie. no need to call new(), so less typing to do for the user. But such explicit calls can also be avoided by functions like f <- function(x, y) new(Class = SomeClass, x=x, y=y). Then again, R relies heavily on method dispatching, so it must have clear advantages.
>
> *confused* Can anybody enlighten me? Thanks in advance!

I agree you are confused.  setGeneric sets up a function to use to 
dispatch methods on an object; e.g. the stats4 package defines the 
generic AIC, so that AIC(obj) will calculate the AIC for any object that 
provides an AIC method.  new() is not involved, because the object was 
already created somewhere else.

What you seem to be asking about is something like a constructor 
function.  E.g. if I have a class "SomeClass", instead of 
new("SomeClass", x=x, y=y) I might prefer to type SomeClass(x=x, y=y).  
That's possible (you have to do it yourself, like your f() above), but 
is really unrelated to setGeneric.

Duncan Murdoch