[R] Coercing in R and in C (R-Extensions)

[Oliver]

Hello,

in "Writing R Extensions" I found the following stuff:

  "Note that these coercion functions are not the same as calling as.numeric
   (and so on) in R code, as they do not dispatch on the class of the object.
    Thus it is normally preferable to do the coercion in the calling R code."
                 ( Writing R Extensions, 2011-04-13, page 100 )

What is meant by this?

Maybe someone can explain me the problem in other words?

What does coercion in R do, and what does coercion in C (not) do?

(And what would be needed in C to get the same effect as in R?)


Thanks,
   Oliver