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Object Classes

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.

Keywords
classes, methods
Usage
class(x)
class(x) <- value
unclass(x)
inherits(x, what, which = FALSE)oldClass(x)
oldClass(x) <- value
Arguments
x

a R object

what, value

a character vector naming classes. value can also be NULL.

which

logical affecting return value: see ‘Details’.

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. (Functions oldClass and oldClass<- get and set the attribute, which can also be done directly.)

If the object does not have a class attribute, it has an implicit class, notably "matrix", "array", "function" or "numeric" or the result of typeof(x) (which is similar to mode(x)), but for type "language" and mode "call", where the following extra classes exist for the corresponding function calls: if, while, for, =, <-, (, {, call.

Note that 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 attribute c("first", "second"), the system searches for a function called 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 default method.

The function 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 class attribute.

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 which is 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 which is FALSE then TRUE is returned by inherits if any of the names in what match with any class.

All but inherits are primitive functions.

Note

Functions oldClass and 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 than oldClass. However, group generics dispatch on the 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 NULL to remove the class.

Formal classes

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 returned by 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 does 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 particular class.

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, is will test the condition, but inherits ignores all conditional superclasses.

UseMethod, NextMethod, ‘group generic’, ‘internal generic

• class
• class<-
• oldClass
• oldClass<-
• unclass
• inherits
Examples
library(base) # NOT RUN { x <- 10 class(x) # "numeric" oldClass(x) # NULL inherits(x, "a") #FALSE class(x) <- c("a", "b") inherits(x,"a") #TRUE inherits(x, "a", TRUE) # 1 inherits(x, c("a", "b", "c"), TRUE) # 1 2 0 class( quote(pi) ) # "name" ## regular calls class( quote(sin(pi*x)) ) # "call" ## special calls class( quote(x <- 1) ) # "<-" class( quote((1 < 2)) ) # "(" class( quote( if(8<3) pi ) ) # "if" # } 
Documentation reproduced from package base, version 3.6.0, License: Part of R 3.6.0

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