If you take an example which works with slots,
setClass("A", representation(a = "numeric")
setClass("B", contains = c("A"), representation(b = "numeric"))
a <- new("A", a = 2)
b <- new("B", a = 3, b = 2)
setClass("AB", contains = c("A", "B"))
new("AB", a = 2, b = 3)
You see, that there is only one @a slot, the one inherited from B, that B
inherits from A. If this were not the case, which slot should be taken, if we
would call @a? To avoid this kind of ambiguity, only one A class is inherited
to AB: the one B already inherits from A.
You could create a class, that contains another A object in a slot:
setClass("AandB", contains = c("B"), representation(A = "A"))
new("AandB", a = 2, b = 3, A = new("A", a = 3))
Now back to your example: as there is only one A object inside the B object
which is contained by the AB object, method dispatch works the way as it
should: It looks for a method f for an AB object. It does not find one. Then it
looks for a method f for the contained B object (as this is the only one
contained in AB) and it finds a method. Then it calls this method on the B part
of the object AB and the result is "B-B"
Best
Simon
On Aug 13, 2013, at 4:24 PM, Hadley Wickham <h.wick...@gmail.com> wrote:
>> The class AB inherits from A and from B, but B already inherits from class
>> A. So actually you only have an object of class B in your object of class
>> AB. When you call the function f R looks for a method f for AB objects. It
>> does not find such a method and looks for a method of the object inherited
>> from, B. Such a method is present and is then executed.
>>
>> The inheritance structure has to be changed. The behavior is actually
>> desired, as if this behavior weren't given a diamond class inheritance would
>> be fatal.
>
> Are you sure? That behaviour doesn't agree with the description of
> method dispatch given in ?Methods, not with getClass("AB") which shows
> that AB inherits from both A and B. (I totally agree that this is a
> bad idea, and unlikely to be useful in real life, but I'm trying to
> understand the details of S4 dispatch)
>
>> getClass("AB")
> Class "AB" [in ".GlobalEnv"]
>
> Slots:
>
> Name: .xData
> Class: NULL
>
> Extends:
> Class "B", directly
> Class "A", directly
> Class ".NULL", by class "A", distance 2
> Class "NULL", by class "A", distance 3, with explicit coerce
> Class "OptionalFunction", by class "A", distance 4, with explicit coerce
> Class "optionalMethod", by class "A", distance 4, with explicit coerce
>
> --
> Chief Scientist, RStudio
> http://had.co.nz/
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