[R] Question on callNextMethod

Martin Morgan mtmorgan at fhcrc.org
Tue Nov 6 17:56:19 CET 2012


On 11/06/2012 07:03 AM, Simon Knapp wrote:
> I don't understand why I get the following results. I define two classes
> 'Base' and 'Derived', the latter of which 'contains' the first. I then
> define a generic method 'test' and overload it for each of these classes. I
> call 'callNextMethod()' in the overload for Derived. From the output, it
> appears that the overload for Base gets called twice. Why is this? Test
> code follows:
>
> setClass('Base')
> setClass('Derived', contains='Base')
> setGeneric('test', function(x) standardGeneric('test'))
> setMethod('test', signature(x='Base'), function(x) print('base called'))
> setMethod('test', signature(x='Derived'), function(x) {print('derived
> called'); callNextMethod()})
>
> d = new('Derived')
> test(d)
>
>
> Produces the output:
>
> [1] "derived called"
> [1] "base called"
> [1] "base called"

Fun; I think you're seeing the print command, and also the (normally invisible) 
return value from print

 > d = new('Derived')
 > res = test(d)
[1] "derived called"
[1] "base called"
 > res
[1] "base called"

similar to

 > print("x")
[1] "x"
 > (print("x"))
[1] "x"
[1] "x"

Martin

>
>
> and I was expecting:
>
> [1] "derived called"
> [1] "base called"
>
>
> Thanx in advance,
> Simon Knapp
>
> 	[[alternative HTML version deleted]]
>
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>


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