classes and methods

I have put together a test class and methods

setClass("foo",representation(x="numeric"))

[1] "foo"

setMethod("plot","foo",function(x,y,...)boxplot(x,...))

[1] "plot"

x <- rnorm(100)
class(x) <- "foo"
plot(x)

y <- new("foo",x=rnorm(50))
class(y)

[1] "foo"


My question is:  should I have a "foo" function which sets the class
to "foo", please?  or a sort of Validity checker, please?

Thanks,
Edna Bell