R Under development (unstable) (2024-10-31 r87283 ucrt) -- "Unsuffered Consequences"
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> message("TESTING: isBeingCreated()...")
TESTING: isBeingCreated()...
> 
> library("R.oo")
Loading required package: R.methodsS3
R.methodsS3 v1.8.2 (2022-06-13 22:00:14 UTC) successfully loaded. See ?R.methodsS3 for help.
R.oo v1.27.0 successfully loaded. See ?R.oo for help.

Attaching package: 'R.oo'

The following object is masked from 'package:R.methodsS3':

    throw

The following objects are masked from 'package:methods':

    getClasses, getMethods

The following objects are masked from 'package:base':

    attach, detach, load, save

> 
> setConstructorS3("Car", function(brand=NULL, nbrOfWheels=0) {
+   if(!isBeingCreated(Car)) {
+     if (is.null(brand))
+       throw("A car must have a brand")
+ 
+     if (nbrOfWheels <= 0)
+       throw("A car must have one or more wheels: ", nbrOfWheels)
+   }
+ 
+   extend(Object(), "Car",
+     .brand = brand,
+     .nbrOfWheels = nbrOfWheels
+   )
+ })
> 
> setMethodS3("as.character", "Car", function(this, ...) {
+   cat(class(this)[1], ":", this$.brand, " with ",
+                      this$.nbrOfWheels, " wheels.", sep="")
+ })
NULL
> 
> print(Car("Volvo", 4))
Car:Volvo with 4 wheels.NULL
> print(Car("BMW", 4))
Car:BMW with 4 wheels.NULL
> print(Car("Tyrrell P34", 6))
Car:Tyrrell P34 with 6 wheels.NULL
> print(Car("T-Rex", 3))
Car:T-Rex with 3 wheels.NULL
> 
> message("TESTING: isBeingCreated()...DONE")
TESTING: isBeingCreated()...DONE
> 
> proc.time()
   user  system elapsed 
   0.23    0.12    0.36