Class names including the corresponding package

The function className() generates a valid references to a class, including the name of the package containing the class definition. The object returned, from class "className", is the unambiguous way to refer to a class, for example when calling setMethod, just in case multiple definitions of the class exist.

Function "multipleClasses" returns information about multiple definitions of classes with the same name from different packages.

programming , classes
className(class, package)

multipleClasses(details = FALSE)

class, package

The character string name of a class and, optionally, of the package to which it belongs. If argument package is missing and the class argument has a package slot, that is used (in particular, passing in an object from class "className" returns itself in this case, but changes the package slot if the second argument is supplied).

If there is no package argument or slot, a definition for the class must exist and will be used to define the package. If there are multiple definitions, one will be chosen and a warning printed giving the other possibilities.


If FALSE, the default, multipleClasses() returns a character vector of those classes currently known with multiple definitions.

If TRUE, a named list of those class definitions is returned. Each element of the list is itself a list of the corresponding class definitions, with the package names as the names of the list. Note that identical class definitions will not be considered “multiple” definitions (see the discussion of the details below).


The table of class definitions used internally can maintain multiple definitions for classes with the same name but coming from different packages. If identical class definitions are encountered, only one class definition is kept; this occurs most often with S3 classes that have been specified in calls to setOldClass. For true classes, multiple class definitions are unavoidable in general if two packages happen to have used the same name, independently.

Overriding a class definition in another package with the same name deliberately is usually a bad idea. Although R attempts to keep and use the two definitions (as of version 2.14.0), ambiguities are always possible. It is more sensible to define a new class that extends an existing class but has a different name.


A call to className() returns an object from class "className".

A call to multipleClasses() returns either a character vector or a named list of class definitions. In either case, testing the length of the returned value for being greater than 0 is a check for the existence of multiply defined classes.

Objects from the Class

The class "className" extends "character" and has a slot "package", also of class "character".

  • className
  • multipleClasses
  • className-class
library(methods) # NOT RUN { className("vector") # will be found, from package "methods" className("vector", "magic") # OK, even though the class doesn't exist className("An unknown class") # Will cause an error # }
Documentation reproduced from package methods, version 3.6.1, License: Part of R 3.6.1

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