slot {methods}R Documentation

The Slots in an Object from a Formal Class

Description

These functions return or set information about the individual slots in an object.

Usage

object@name
object@name <- value

slot(object, name)
slot(object, name, check = TRUE) <- value

slotNames(x)

Arguments

object An object from a formally defined class.
name The character-string name of the slot. The name must be a valid slot name: see Details below.
value A new value for the named slot. The value must be valid for this slot in this object's class.
x Either the name of a class or an object from that class. Print getClass(class) to see the full description of the slots.
check If TRUE, check the assigned value for validity as the value of this slot. You should never set this to FALSE in normal use, since the result can create invalid objects.

Details

The "@" operator and the slot function extract or replace the formally defined slots for the object. The operator takes a fixed name, which can be unquoted if it is syntactically a name in the language. A slot name can be any non-empty string, but if the name is not made up of letters, numbers, and ., it needs to be quoted (by backticks or single or double quotes).

In the case of the slot function, name can be any expression that evaluates to a valid slot in the class definition. Generally, the only reason to use the functional form rather than the simpler operator is because the slot name has to be computed.

The definition of the class contains the names of all slots directly and indirectly defined. Each slot has a name and an associated class. Extracting a slot returns an object from that class. Setting a slot first coerces the value to the specified slot and then stores it.

Unlike attributes, slots are not partially matched, and asking for (or trying to set) a slot with an invalid name for that class generates an error.

Note that currently, slotNames() behaves particularly for class representation objects – this is considered bogus and likely to be changed.

Currently the @ extraction operator and slot function do no checking, neither that object has a formal class nor that name is a valid slot name for the class. (They will extract the attribute of the given name if it exists from any R object.) The replacement forms do check (at least if check=TRUE).

References

The R package methods implements, with a few exceptions, the programming interface for classes and methods in the book Programming with Data (John M. Chambers, Springer, 1998), in particular sections 1.6, 2.7, 2.8, and chapters 7 and 8.

While the programming interface for the methods package follows the reference, the R software is an original implementation, so details in the reference that reflect the S4 implementation may appear differently in R. Also, there are extensions to the programming interface developed more recently than the reference. For a discussion of details see ?Methods and the links from that documentation.

See Also

@, Classes, Methods, getClass

Examples



setClass("track", representation(x="numeric", y="numeric"))
myTrack <- new("track", x = -4:4, y = exp(-4:4))
slot(myTrack, "x")
slot(myTrack, "y") <- log(slot(myTrack, "y"))
str(myTrack)

slotNames("track") # is the same as
slotNames(myTrack)



[Package methods version 2.5.0 Index]