R help - Oct 2013 - S4 base class

Greetings,

I have an S4 class "B" (Base) which defines a function
f=f(this="B",...)
Dervided from?B we have a derived class D which also defines a function
f=f(this="D",...)

In the definition of D::f we want to call the version B::f and could do this by
simply?calling

f(baseClassObject(this),...)

The question is the following:

How do I refer to the base class?object from the derived class?



Many thanks?

?
Michael Meyer

Greetings,

Meanwhile I have figured out?how do do it only to find out that I have more
serious problems.
Generally calling Base::f on the base class object is not what you want, instead
you want to call
Base::f on the full object for the following reasons:

If the base class is virtual, then Base::f might use virtual functions 
(not defined in Base but defined in derived classes).

If you then call Base::f on an object of class "Base" the call will
fail.

Is it possible in R to call Base::f from within Derived (when there is also
Derived::f) on the full object this?

I suspect not, which would be a serious drawback to the R class mechanism.


Thanks,


Michael Meyer

Sorry,

if the previous message seems without context.
Indeed, the first message was bounced by filtering rules (triggered by subject
heading than which nothing could be more benign or less liable to suspician). It
was:

Greetings,

I have an S4 class "B" (Base) which defines a function
f=f(this="B",...)
Dervided from?B we have a derived class D which also defines a function
f=f(this="D",...)

In the definition of D::f we want to call the version B::f and could do this by
simply?calling

f(baseClassObject(this),...)

The question is the following:

How do I refer to the base class?object from the derived class?



Many thanks?


Michael Meyer

Quote

By the way, your use of the syntax D::f and B::f suggests that you're 
thinking from a C++ point of view.? That's very likely to lead to 
frustration:? the S4 object system is very different from C++.? Methods 
don't belong to classes, they belong to generics. There is no such
thing
as D::f or B::f, only f methods with different signatures.

Duncan Murdoch?


#-------------------------------------------------------------------------------------------#

I am aware of this.
We can probably agree that?we should use?S4 classes and generic functions to
duplicate more usual object oriented architecture as far as possible while
remaining conscious of the regrettable differences.

For example we can pretend we are defining a virtual function in class Base by
writing:

setGeneric("F",
function(this) standardGeneric("F")
)

where the code for Base? is, even though it has nothing to do with the class
Base.
We can even use it in other functions "defined?in class Base" by
writing


setGeneric("G",
function(this) standardGeneric("G")
)
setMethod("G",
signature(this="Base"),
definition=function(this){

????F(this)
})

which will work on all derived classes which implement F in some fashion:

setMethod("F",
signature(this="Derived"),
definition=function(this){

????# do something appropriate for derived.
})

With this we can reproduce some semblance of object oriented programming
However, apparently we cannot solve in this manner a common problem of object 
oriented programming (from now on C++ parlance):

Suppose you have a base class "Base" which implements a function
"Base::F"
which works in most contexts but not in the context of
"ComplicatedDerived" class
where some preparation has to happen before this very same function can be
called.

You would then define

void ComplicatedDerived::F(...){

????preparation();
????Base::F();
}

You can nealry?duplicate this in R via 

setMethod("F",
signature(this="ComplicatedDerived"),
definition=function(this){

????preparation(this)
????F(as(this,"Base"))
})

but it will fail whenever F uses virtual functions (i.e. generics) which are
only defined
for derived classes of Base, whereas this is not a problem at all in normal
object oriented
languages.

This is not a contrived problem but is rather basic.
I wonder if you can do it in R in some other way.


Many thanks,

Michael

@Martin Morgan, Duncan Murdoch:

OK Thanks.
I did not understand the callNextMethod.
I will investigate this in detail.
This is great!

Thanks again,

?
Michael Meyer