R: Create a Class Definition

setClass {methods} | R Documentation

## Create a Class Definition

### Description

Create a class definition, specifying the representation (the slots) and/or the classes contained in this one (the superclasses), plus other optional details.

### Usage

    setClass(Class, representation, prototype, contains=character(),
             validity, access, where, version, sealed, package)

### Arguments

`Class` | character string name for the class.
`representation` | a named list of the slots that the new class should have, the names giving the names of the slots and the corresponding elements being the character string names of the corresponding classes. Usually a call to the `representation` function. Backward compatibility and compatibility with S-Plus allows unnamed elements for superclasses, but the recommended style is to use the `contains=` argument instead.
`prototype` | an object providing the default data for the slots in this class. Usually and preferably the result of a call to `prototype`.
`contains` | what classes does this class extend? (These are called superclasses in some languages.) When these classes have slots, all their slots will be contained in the new class as well.
`where` | the environment in which to store or remove the definition. Defaults to the top-level environment of the calling function (the global environment for ordinary computations, and the environment or name space of a package in the source code for that package).
`validity` | if supplied, should be a validity-checking method for objects from this class (a function that returns `TRUE` if its argument is a valid object of this class and one or more strings describing the failures otherwise). See `validObject` for details.
`access, version` | access and version, included for compatibility with S-Plus, but currently ignored.
`sealed` | if `TRUE`, the class definition will be sealed, so that another call to `setClass` will fail on this class name.
`package` | an optional package name for the class. By default (and usually) the name of the package in which the class definition is assigned.

### Basic Use: Slots and Inheritance

The two essential arguments, other than the class name are `representation` and `contains`, defining the explicit slots and the inheritance (superclasses). Together, these arguments define all the information in an object from this class; that is, the names of all the slots and the classes required for each of them.

The name of the class determines which methods apply directly to objects from this class. The inheritance information specifies which methods apply indirectly, through inheritance. See Methods.

The slots in a class definition will be the union of all the slots specified directly by `representation` and all the slots in all the contained classes. There can only be one slot with a given name; specifically, the direct and inherited slot names must be unique. That does not, however, prevent the same class from being inherited via more than one path.

One kind of element in the `contains=` argument is special, specifying one of the R vector types or one of a few other special R types (`matrix` and `array`). A class can contain at most one of these, directly or indirectly. When it does, that contained class determines the “data part” of the class, and objects from the new class inherit all the built in behavior of the contained type. Two direct consequences are: for any object `x` from the class, `typeof(x)` will be the contained basic type; and a special pseudo-slot, `.Data`, will be shown with the corresponding class.

For an object from any class that does not contain such a type, `typeof(x)` will be `"S4"` and the class will have no `.Data` slot.

### Prototypes

Defining new classes that inherit from (‘extend’) other classes is a powerful technique, but has to be used carefully and not over-used. Otherwise, you will often get unintended results when you start to compute with objects from the new class.

As shown in the examples below, the simplest and safest form of inheritance is to start with an explicit class, with some slots, that does not extend anything else. It only does what we say it does.

Then extensions will add some new slots and new behavior.

Another variety of extension starts with one of the built-in objects, perhaps with the intention of modifying R's standard behavior for that class. In this case, the new class inherits the built-in data type as its ‘data’ part. See the `"numWithId"` example below.

When such a class definition is printed, the data part shows up as a pseudo-slot named `.Data`.

### Classes and Packages

Class definitions normally belong to packages (but can be defined in the global environment as well, by evaluating the expression on the command line or in a file sourced from the command line). The corresponding package name is part of the class definition; that is, part of the `classRepresentation` object holding that definition. Thus, two classes with the same name can exist in different packages, for most purposes.

When a class name is supplied for a slot or a superclass, a corresponding class definition will be found, looking from the name space or environment of the current package, assuming the call to `setClass` in question appears directly in the source for the package. That's where it should appear, to avoid ambiguity.

In particular, if the current package has a name space then the class must be found in the current package itself, in the imports defined by that name space, or in the base package.

When this rule does not identify a class uniquely (because it appears in more than one imported package) then the `packageSlot` of the character string name needs to be supplied with the name. This should be a rare occurrence.

### Note

Certain slot names are not allowed in the current implementation, as they correspond to attributes which are treated specially. These are `class`, `comment`, `dim`, `dimnames`, `names`, `row.names` and `tsp`.

### 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` for a general discussion of classes, `Methods` for an analogous discussion of methods, `makeClassRepresentation`

### Examples

    ## A simple class with two slots
    setClass("track",
             representation(x="numeric", y="numeric"))
    ## A class extending the previous, adding one more slot
    setClass("trackCurve",
        representation(smooth = "numeric"),
        contains = "track")
    ## A class similar to "trackCurve", but with different structure
    ## allowing matrices for the "y" and "smooth" slots
    setClass("trackMultiCurve",
             representation(x="numeric", y="matrix", smooth="matrix"),
             prototype = list(x=numeric(), y=matrix(0,0,0),
                              smooth= matrix(0,0,0)))
    ##
    ## Suppose we want trackMultiCurve to be like trackCurve when there's
    ## only one column.
    ## First, the wrong way.
    try(setIs("trackMultiCurve", "trackCurve",
        test = function(obj) {ncol(slot(obj, "y")) == 1}))

    ## Why didn't that work?  You can only override the slots "x", "y",
    ## and "smooth" if you provide an explicit coerce function to correct
    ## any inconsistencies:

    setIs("trackMultiCurve", "trackCurve",
      test = function(obj) {ncol(slot(obj, "y")) == 1},
      coerce = function(obj) {
         new("trackCurve",
             x = slot(obj, "x"),
             y = as.numeric(slot(obj,"y")),
             smooth = as.numeric(slot(obj, "smooth")))
      })

    ## A class that extends the built-in data type "numeric"

    setClass("numWithId", representation(id = "character"),
             contains = "numeric")

    new("numWithId", 1:3, id = "An Example")

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