[Rd] common base functions stripping S3 class

[Murat Tasan]

Hi all --- this is less a specific question and more general regarding
S3 classes.
I've noticed that quite a few very common default implementations of
generic functions (e.g. `unique`, `[`, `as.data.frame`) strip away
class information.
In some cases, it appears conditionals have been created to re-assign
the class, but only for a few special types.
For example, in `unique.default`, if the argument inherits (_only_)
from "POSIXct" or "Date", the initial class is re-assigned to the
returned object.
But for any other custom S3 classes, it means we have to catch these
frequent cases and write a lot relatively plain wrappers, e.g.:

unique.MyClass <- function(x, incomparables = FALSE, ...) {
    structure(unique(unclass(x)), class = class(x))
}

It's certainly nice to be able to create a very simple wrapper class
on a base type, so that we can override common functions like plot(x).
(An example is a simple class attribute that dictates a particular
plot style for a vector of integers.)
But it would be even nicer to not have to detect and override all the
un-class events that occur when manipulating these objects with
everyday functions, e.g. when adding that 'classed' integer vector to
a data frame.

Apart from moving to S4 classes, how have most dealt with this?
Might there be a list of common functions for which the default
implementation strips class information?
(Such a list could be a handy "consider overriding _this_" guide for
implementors of any new classes.)

Cheers and thanks for any tips!

-murat