R possesses a simple generic function mechanism which can be used for an object-oriented style of programming. Method dispatch takes place based on the class of the first argument to the generic function.
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a R object
a character vector naming classes.
logical affecting return value: see ‘Details’.
Here, we describe the so called “S3” classes (and methods). For “S4” classes (and methods), see ‘Formal classes’ below.
Many R objects have a
class attribute, a character vector
giving the names of the classes from which the object inherits.
If the object does not have a class attribute, it has an implicit
"array" or the result of
mode(x) (except that integer vectors have implicit class
oldClass<- get and set the attribute, which can also be done
NULL objects cannot have attributes (hence not
classes) and attempting to assign a class is an error.
When a generic function
fun is applied to an object with class
c("first", "second"), the system searches for a
fun.first and, if it finds it, applies it to
the object. If no such function is found, a function called
fun.second is tried. If no class name produces a suitable
function, the function
fun.default is used (if it exists). If
there is no class attribute, the implicit class is tried, then the
class prints the vector of names of classes an
object inherits from. Correspondingly,
class<- sets the
classes an object inherits from. Assigning
NULL removes the
unclass returns (a copy of) its argument with its class
attribute removed. (It is not allowed for objects which cannot be
copied, namely environments and external pointers.)
inherits indicates whether its first argument inherits from any
of the classes specified in the
what argument. If
TRUE then an integer vector of the same length as
what is returned. Each element indicates the position in the
class(x) matched by the element of
what; zero indicates
no match. If
inherits if any of the names in
inherits are primitive functions.
An additional mechanism of formal classes, nicknamed
“S4”, is available in package methods which is attached
by default. For objects which have a formal class, its name is
class as a character vector of length one and
method dispatch can happen on several arguments, instead of
only the first. However, S3 method selection attempts to treat objects
from an S4 class as if they had the appropriate S3 class attribute, as
inherits. Therefore, S3 methods can be defined for S4
classes. See the ‘Introduction’ and ‘Methods_for_S3’
help pages for basic information on S4 methods and for the relation
between these and S3 methods.
The replacement version of the function sets the class to the value
provided. For classes that have a formal definition, directly
replacing the class this way is strongly deprecated. The expression
as(object, value) is the way to coerce an object to a
The analogue of
inherits for formal classes is
is. The two functions behave consistently
with one exception: S4 classes can have conditional
inheritance, with an explicit test. In this case,
test the condition, but
inherits ignores all conditional
oldClass<- behave in the same way
as functions of those names in S-PLUS 5/6, but in R
UseMethod dispatches on the class as returned by
class (with some interpolated classes: see the link) rather
oldClass. However, group generics dispatch
oldClass for efficiency, and internal generics
only dispatch on objects for which
is.object is true.
In older versions of R, assigning a zero-length vector with
class removed the class: it is now an error (whereas it
still works for
oldClass). It is clearer to always assign
to remove the class.
‘group generic’, ‘internal generic’
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