R: The Slots in an Object from a Formal Class

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
    .hasSlot(object, name)

    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 (as character string), 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.

Note that the `"@"` operators for extraction and replacement are primitive and actually reside in the base package.

The replacement versions of `"@"` and `slot()` differ in the computations done to coerce the right side of the assignment to the declared class of the slot. Both verify that the value provided is from a subclass of the declared slot class. The `slot()` version will go on to call the coerce method if there is one, in effect doing the computation `as(value, slotClass, strict = FALSE)`. The `"@"` version just verifies the relation, leaving any coerce to be done later (e.g., when a relevant method is dispatched).

In most uses the result is equivalent, and the `"@"` version saves an extra function call, but if empirical evidence shows that a conversion is needed, either call `as()` before the replacement or use the replacement version of `slot()`.

### 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_Details`, `Methods_Details`, `getClass`, `names`.

### Examples

    setClass("track", slots = c(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)

[Package methods version 3.5.1 Index]