slot                 package: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)
     getSlots(x)

Arguments:

  object: An object from a formally defined class.

    name: The name of the slot. 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. 

   value: A new value for the named slot.  The value must be valid for
          this slot in this object's class.

   check: In the replacement version of 'slot', a flag.  If 'TRUE',
          check the assigned value for validity as the value of this
          slot.  User's coded should not set this to 'FALSE' in normal
          use, since the resulting object can be invalid. 

       x: Either the name of a class or a class definition. If given an
          argument that is neither a character string nor a class
          definition, 'slotNames' (only) uses 'class(x)' instead.

Details:

     The definition of the class specifies all slots directly and
     indirectly defined for that class.  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 general 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.

     The '@' extraction operator and 'slot' function themselves do no
     checking against the class definition, simply matching the name in
     the object itself. The replacement forms do check (except for
     'slot' in the case 'check=FALSE').  So long as slots are set
     without cheating, the extracted slots will be valid.

     Be aware that there are two ways to cheat, both to be avoided but
     with no guarantees.  The obvious way is to assign a slot with
     'check=FALSE'.  Also, slots in R are implemented as attributes,
     for the sake of some back compatibility.  The current
     implementation does not prevent attributes being assigned, via
     'attr<-', and such assignments are not checked for legitimate slot
     names.

Value:

     The '"@"' operator and the 'slot' function extract or replace the
     formally defined slots for the object.  

     Functions 'slotNames' and 'getSlots' return respectively the names
     of the slots and the classes associated with the slots in the
     specified class definition.  Except for its extended
     interpretation of 'x' (above), 'slotNames(x)' is just
     'names(getSlots(x))'.

References:

     Chambers, John M. (2008) _Software for Data Analysis: Programming
     with R_ Springer.  (For the R version.)

     Chambers, John M. (1998) _Programming with Data_ Springer (For the
     original S4 version.)

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"))
     utils::str(myTrack)

     getSlots("track") # or
     getSlots(getClass("track"))
     slotNames(class(myTrack)) # is the same as
     slotNames(myTrack)

