[R-pkg-devel] S3 length method behavior

[Nathan Wendt]

Hello,

I have run into an issue while developing an R package. Specifically, my
issue relates to what happens when I define an S3 length method for my
class. Here is my basic setup:

# Here is the structure of myClass
# uses an externalptr to operate on file using some C functions
list(file = "my/file/location", handle = <externalptr>)

# this function gets an S3Method() in NAMESPACE
length.myClass <- function(x) {
  .Call("get_class_length",myPkg)
}

As you can see, myClass is just a list of two pieces of information about
the particular object. Without a defined length method, length(myClass)
would be 2. However, I defined a length method to give me the information
about the amount of binary messages contained in the file the object points
to. In one of my test files there are 82 messages and length(myObject)
correctly returns 82. What I ran into is that I was crashing RStudio when
rebuilding my package with an object of myClass in my environment. Only
removing my S3 length method caused the crash to not occur. After looking
for memory leaks and other errors I finally noticed that the str() on the
object of myClass looked odd. It returned something like this:

List of 82
 $ file  : chr "my/file/location"
 $ handle:<externalptr>
 $ NA:
Error in object[[i]] : subscript out of bounds

It seems that when I define an S3 method for length that somehow the object
of myClass then gets restructured to have the length returned by the
function. There are 82 messages in the file, but myClass objects should
only have two elements. My best guess here is that the RStudio crash was
happening because the object was misrepresented internally. Of course, I
cannot be sure. None of my tests pointed to anything useful.

My questions are, then, whether this behavior makes sense and what to do
about it. If I define my own str() method, will that fix it? I think I am
just misunderstanding what is going on with the methods I have defined.
Hopefully, someone can offer some clarity.

Thanks,

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