Classes Corresponding to Basic Data Types

Description

Formal classes exist corresponding to the basic R object types, allowing these types to be used in method signatures, as slots in class definitions, and to be extended by new classes.

Usage

### The following are all basic vector classes.
### They can appear as class names in method signatures,
### in calls to as(), is(), and new().
"character"
"complex"
"double"
"expression"
"integer"
"list"
"logical"
"numeric"
"single"
"raw"

### the class
"vector"
### is a virtual class, extended by all the above

### the class
"S4"
### is an object type for S4 objects that do not extend
### any of the basic vector classes.  It is a virtual class.

### The following are additional basic classes
"NULL"     #  NULL objects
"function" #  function objects, including primitives
"externalptr" # raw external pointers for use in C code

"ANY"  # virtual classes used by the methods package itself
"VIRTUAL"
"missing"

"namedList" # the alternative to "list" that preserves
            # the names attribute

Objects from the Classes

If a class is not virtual (see section in Classes_Details), objects can be created by calls of the form new(Class, ...), where Class is the quoted class name, and the remaining arguments if any are objects to be interpreted as vectors of this class. Multiple arguments will be concatenated.

The class "expression" is slightly odd, in that the ... arguments will not be evaluated; therefore, don't enclose them in a call to quote().

Note that class "list" is a pure vector. Although lists with names go back to the earliest versions of S, they are an extension of the vector concept in that they have an attribute (which can now be a slot) and which is either NULL or a character vector of the same length as the vector. If you want to guarantee that list names are preserved, use class "namedList", rather than "list". Objects from this class must have a names attribute, corresponding to slot "names", of type "character". Internally, R treats names for lists specially, which makes it impractical to have the corresponding slot in class "namedList" be a union of character names and NULL.

Classes and Types

The basic classes include classes for the basic R types. Note that objects of these types will not usually be S4 objects (isS4 will return FALSE), although objects from classes that contain the basic class will be S4 objects, still with the same type. The type as returned by typeof will sometimes differ from the class, either just from a choice of terminology (type "symbol" and class "name", for example) or because there is not a one-to-one correspondence between class and type (most of the classes that inherit from class "language" have type "language", for example).

Extends

The vector classes extend "vector", directly.

Methods

coerce

Methods are defined to coerce arbitrary objects to the vector classes, by calling the corresponding basic function, for example, as(x, "numeric") calls as.numeric(x).