[R] Methods dispatch and inheritance R.oo

[Henrik Bengtsson]

Hi,

On Thu, Nov 6, 2008 at 2:12 AM, Yuri Volchik <yuri.volchik at gmail.com> wrote:
>
> Thanks for reply Henrik, seems obvious now.
> Can child class (B) access argument of the parent class, i.e. can i rewrite
> definition of the class B as
>
> setConstructorS3("ClassB", function() {
>  extend(ClassA(), "ClassB",
>    .size2 = A
>  );
> })
>
> it didn't work for me, so guess i'm doing smth wrong and i couldn't find any
> reference on inheritance in the help files.

Note that 'A' is only an argument (~= local variable) of the
constructor function ClassA().  There is no way the (constructor)
function ClassB() can know about that one; compare to:

foo <- function(A=1) {
}

bar <- function(B=2) {
  # Inside here you have no access to 'A' in foo(), regardless if you
call foo() or not.
}

So, I'm not sure what you are trying to do in reality.  Note, since
the value of 'A' is already assigned in ClassA() and part of its
returned object, then it will also be part of the returned object from
ClassB(), so you don't really have to assign 'A' again.

I think this is a better example.  See if you want to achive what
ClassB or ClassC do:

setConstructorS3("ClassA", function(A=0) {
 extend(Object(), "ClassA",
   A = A
 );
})

> objA <- ClassA();
> ll(objA)
  member data.class dimension objectSize
1      A    numeric         1         32
> objA$A
[1] 0


setConstructorS3("ClassB", function(A=0, B=1) {
 extend(ClassA(A=A), "ClassB",
   B = B
 );
})

> objB <- ClassB(A=1, B=2);
> ll(objB);
  member data.class dimension objectSize
1      A    numeric         1         32
2      B    numeric         1         32
> objB$A
[1] 1
> objB$B
[1] 2


If you really want to access a field from a superclass inside the
constructor function, you have to first get the object returned by the
constructor of the superclass, and then from that object retrieve the
field:

setConstructorS3("ClassC", function(B=1) {
 this <- extend(ClassA(), "ClassC",
   B = B
 );
 this$copyOfA <- this$A;
 this;
})

> objC <- ClassC(B=3);
> ll(objC);
  member data.class dimension objectSize
1      A    numeric         1         32
2      B    numeric         1         32
> objC$A
[1] 0
> objC$B
[1] 3


Note, there is nothing "magic" going on inside Object(), extend() or
any other R.oo methods.  Most of the things you can think of as
regular lists and functions.  The new think is that objects inheriting
from the Object class store their values inside an environment, which
is why the object behaves as it is passed by reference.

Hope this helps

/Henrik

>
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