On Tue, Feb 2, 2016 at 3:28 PM, Hadley Wickham <h.wick...@gmail.com> wrote:
> I've found that it's a very bad idea to provide length or names
> methods for just this reason.
>> 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
>> 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.
Defining a str on your class will at least fix the out of bounds error:
Create a trivial S3 class:
> z=list(1,22)
> class(z)="foo"
length method looks at the second element:
> length.foo=function(x){x[[2]]}
> length(z)
[1] 22
and str barfs:
> str(z)
List of 22
$ : num 1
$ : num 22
$ :Error in object[[i]] : subscript out of bounds
Define a str method:
> str.foo=function(object,...){for(i in
1:length(unclass(object))){str(unclass(object[[i]]))}}
> str(z)
num 1
num 22
BUT... the real problem is that S3 classes are seriously informal and
there's no concept of what methods you need to define on a class
because there's no concept of an "interface" that new classes have to
conform to. So stuff breaks, seemingly at random, and via action at a
distance. Somewhere something is going to expect z[[1]] to
z[[length(z)]] to exist, which is what the default str is doing...
+1 on Hadley - don't override any basic R structural methods, create
new ones with new names. You can make them more meaningful too. For
your example, maybe "messageCount(myObject)"?
Barry
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