subtraction (and other internal methods) for POSIXxt class

[shoebodh]

Hi, I ran into a small issue with the new POSIXxt objects stringx introduces where the substraction method doesn't work.

For instance we know this works

aa = base::Sys.time()
bb = base::Sys.time()
bb - aa
Time difference of 0.002556086 secs

But currently the following gives an error

aa = stringx::Sys.time()
bb = stringx::Sys.time()
bb - aa
bb - aa
Error in as.POSIXct.default(x, tz = tz, ...) : 
  do not know how to convert 'x' to class “POSIXct”

I found that this occurs because the class of stringx::Sys.time() is
"POSIXxt" "POSIXct" "POSIXt" so the - doesn't know how to handle it when it finds POSIXxt instead of the internal POSIX classes.

For my case I managed this with a very simple work around by introducing the `-.POSIXxt' S3 method like the following

`-.POSIXxt` <- function(...) difftime(...)
#Now the subtraction works
bb-aa
Time difference of 0.002810955 secs

This works great but I think the internal '-.POSIXxt' is a little more comprehensive so the following also works, but i find this a litte hacky because here I simply revert the class attribute) to trick the subtraction method to find the "POSIXxt" when it looks for that attribute.

`-.POSIXxt` <- function(e1, e2) {
  
  attr(e1, 'class') <- rev(class(e1)) # 
  attr(e2, 'class') <- rev(class(e2))
  
  return( `-`(e1 , e2))
}

bb-aa
Time difference of 0.002810955 secs

These are my naive workarounds, but I am sure there is a more comprehensive and elegant way to add this method in the library.
I think that having having these internal POSIXt methods for the POSIXxt class in the library is important for improved compatibility with the base package.

Thanks

[gagolews]

Thanks for pointing this out!

Could you test if the following function does the trick?

`-.POSIXxt` <- function(e1, e2) NextMethod(.Generic)

[shoebodh]

That works wonderfully. Thank you.

[gagolews]

Thanks again!