[R-pkg-devel] S3 length method behavior

[Jim Lemon]

Hi Nathan,
Perhaps if you defined your "length" object as "nbm" (number of binary
messages), it would not interfere with the default "length".

Jim


On Tue, Feb 2, 2016 at 12:46 AM, Georgi Boshnakov <
georgi.boshnakov at manchester.ac.uk> wrote:

> Hi,
>
> >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
>
>
> You could define methods for indexing ("[" and "[[") for objects from your
> class.
> Since you have not defined special subsetting methods for your class, R
> uses a default one,  but honours your length(), hence the mess.
>
> Regards,
> Georgi Boshnkaov
>
>
> -----Original Message-----
> From: R-package-devel [mailto:r-package-devel-bounces at r-project.org] On
> Behalf Of Nathan Wendt
> Sent: 30 January 2016 19:25
> To: r-package-devel at r-project.org
> Subject: [R-pkg-devel] S3 length method behavior
>
> 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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>
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