[R] Understanding S4 method dispatch

[Simon Zehnder]

Hadley,

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. 


Best

Simon



On Aug 13, 2013, at 3:08 PM, Hadley Wickham <h.wickham at gmail.com> wrote:

> Hi all,
> 
> Any insight into the code below would be appreciated - I don't
> understand why two methods which I think should have equal distance
> from the call don't.
> 
> Thanks!
> 
> Hadley
> 
> # Create simple class hierarchy
> setClass("A", "NULL")
> setClass("B", "A")
> 
> a <- new("A")
> b <- new("B")
> 
> setGeneric("f", function(x, y) standardGeneric("f"))
> setMethod("f", signature("A", "A"), function(x, y) "A-A")
> setMethod("f", signature("B", "B"), function(x, y) "B-B")
> 
> # These work as I expect
> f(a, a)
> f(b, b)
> 
> setClass("AB", contains = c("A", "B"))
> ab <- new("AB")
> 
> # Why does this return B-B? Shouldn't both methods be an equal distance?
> f(ab, ab)
> 
> # These both return distance 1, as I expected
> extends("AB", "A", fullInfo=TRUE)@distance
> extends("AB", "B", fullInfo=TRUE)@distance
> # So why is signature("B", "B") closer than signature("A", "A")
> 
> -- 
> Chief Scientist, RStudio
> http://had.co.nz/
> 
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