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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