R help - Jan 2016 - Strange behavior with S3 class

Hi all.
For the development of a package I created a S3 class with a "setter"
operator "<-".
Running the following code will create two objects: eem and eem2. Additionally,
the class eem has a names.eem function used to retrieve the value of the sample
field.
names.eem <- function(x, ...){  x$sample}
# First constructoreem1 <- function(sample){  eem <- list(sample = sample)
class(eem) <- "eem"  return(eem)}
# Second constructoreem2 <- function(sample){  eem <- list(sample =
sample)  class(eem) <- "eem2"  return(eem)}
test1 <- eem1("justaname")test2 <- eem2("justaname")

This is "bugged":
  > str(test1)List of 1$ justaname: chr "justaname"- attr(*,
"class")= chr "eem"
This is ok:    > str(test2)List of 1$ sample: chr "justaname"-
attr(*, "class")= chr "eem2"
It seems that override of the "<-" is causing problem since in
str(test1) the variable name is not preserved.
Any idea?
Thank you,Philippe  		 	   		  
	[[alternative HTML version deleted]]

> This is "bugged":
>   > str(test1)List of 1$ justaname: chr "justaname"- attr(*,
"class")= chr "eem"
> This is ok:    > str(test2)List of 1$ sample: chr "justaname"-
attr(*, "class")= chr
> "eem2"
> It seems that override of the "<-" is causing problem since in
str(test1) the
> variable name is not preserved.
> Any idea?
Compare 
str(test1)
# List of 1
#  $ justaname: chr "justaname"
#  - attr(*, "class")= chr "eem"

str(unclass(test1))
# List of 1
#  $ sample: chr "justaname"

That says the internal variable name is preserved but while it still has the
'eem' class the name is reported differently.
That is presumably because you defined a 'names' function for objects of
class 'eem' which returns the contents of $sample as the name, not
"sample". Looks like str is using your names.eem method to extract the
name of each component of your object, which is kind of what redefining
'names' asks for. And just to demonstrate that str does indeed use the
current class's names method:

names.eem <- function(x, ...){  "AnotherName"}
str(test1)
# List of 1
#  $ AnotherName: chr "justaname"
#  - attr(*, "class")= chr "eem"

So it isn't your '<-', it's because you overrode
'names'


S Ellison



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> That says the internal variable name is preserved but while it still has
the 'eem' class the name is reported differently.
> That is presumably because you defined a 'names' function for
objects of class 'eem' which returns the contents of $sample as the
name, not "sample". Looks like str is using your names.eem method to
extract the name of each component of your object, which is kind of what
redefining 'names' asks for. And just to demonstrate that str does
indeed use the current class's names method:
>
> names.eem <- function(x, ...){  "AnotherName"}
> str(test1)
> # List of 1
> #  $ AnotherName: chr "justaname"
> #  - attr(*, "class")= chr "eem"
>
> So it isn't your '<-', it's because you overrode
'names'
You are absolutely right I made a mistake describing the problem.

So my question is how I can implement the "names" method for my class 
without having this problem. The following piece of code reproduce the 
problem I am facing.

names.eem <- function(x, ...){
   x$sample
}

# First constructor
eem1 <- function(sample){
   eem <- list(sample = sample)
   class(eem) <- "eem"
   return(eem)
}

# Second constructor
eem2 <- function(sample){
   eem <- list(sample = sample)
   class(eem) <- "eem2"
   return(eem)
}

test1 <- eem1("justaname")
test2 <- eem2("justaname")

str(test1)
str(test2)

Thank you for your help,
Philippe