[R] S4 non-virtual class with no slots?

[Martin Morgan]

On 04/22/2016 04:38 PM, Boylan, Ross wrote:
> It seems that if an S4 class has no slots it can't be instantiated because it is assumed to be virtual.  Is there a way around this other than adding a do-nothing slot?  A singleton would be OK, though is not essential.
>
> Problem:
> EmptyFitResult <- setClass("EmptyFitResult", representation=representation())
> # also tried it without the second argument.  same result.
>   > e <- EmptyFitResult()
>   Error in new("EmptyFitResult", ...) :
>     trying to generate an object from a virtual class ("EmptyFitResult")
>
> This in R 3.1.1.
>
> Context:
> I fit simulated data; in some simulations none survive to the second stage of fitting.  So I just need a way to record that this happened, in a way that integrates with my other non-null results.
>

A not too artificial solution is to create a base class, with derived 
classes corresponding to stateless or stateful conditions

Base = setClass("Base"); A = setClass("A", contains="Base"); A()

Martin Morgan

> Thanks.
> Ross Boylan
>
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