[R] S4 base class

[Martin Morgan]

On 10/17/2013 08:54 AM, Michael Meyer wrote:

> Suppose you have a base class "Base" which implements a function "Base::F"
> which works in most contexts but not in the context of "ComplicatedDerived" class
> where some preparation has to happen before this very same function can be called.
>
> You would then define
>
> void ComplicatedDerived::F(...){
>
>      preparation();
>      Base::F();
> }
>
> You can nealry duplicate this in R via
>
> setMethod("F",
> signature(this="ComplicatedDerived"),
> definition=function(this){
>
>      preparation(this)
>      F(as(this,"Base"))
> })
>
> but it will fail whenever F uses virtual functions (i.e. generics) which are only defined
> for derived classes of Base

With

   .A <- setClass("A", representation(a="numeric"))
   .B <- setClass("B", representation(b="numeric"), contains="A")

   setGeneric("f", function(x, ...) standardGeneric("f"))

   setMethod("f", "A", function(x, ...) {
       message("f,A-method")
       g(x, ...)   # generic with methods only for derived classes
   })

   setMethod("f", "B", function(x, ...) {
       message("f,B-method")
       callNextMethod(x, ...)  # earlier response from Duncan Murdoch
   })

   setGeneric("g", function(x, ...) standardGeneric("g"))

   setMethod("g", "B", function(x, ...) {
       message("g,B-method")
       x
   })

one has

 > f(.B())
f,B-method
f,A-method
g,B-method

An object of class "B"
Slot "b":
numeric(0)

Slot "a":
numeric(0)

?


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