R: Create a Class Definition

setClass {methods} | R Documentation

## Create a Class Definition

### Description

Create a formally defined class with specified slots and/or relationships to other classes. Also functions to remove a class definition, to test whether a class has been defined, to test whether an object is a class definition, and to reset the internal definition of a class.

### Usage

    setClass(Class, representation, prototype,
             contains=character(), validity, access, where=1, version=FALSE)

    removeClass(Class, where=-1)

    isClass(Class, formal=TRUE)
    isClassDef(object)

    getClasses(where)

    resetClass(Class)

### Arguments

`Class` | character string name for the class
`representation` | the slots that the new class should have and/or other classes that this class extends. Usually a call to the `representation` function.
`prototype` | an object (usually a list) providing the default data for the slots specified in the representation.
`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` | What environment to use to store or remove the definition (as metadata). By default, uses the global environment for `setClass` and searches for a definition to remove, for `removeClass`.
`validity, access, version` | Control arguments included for compatibility with the S-Plus API, but not currently used.
`x` | an arbitrary object.
`formal` | Should a formal definition be required?
`object` | any R object.

### Details

These are the functions that create and manipulate formal class definitions. Brief documentation is provided below. See the references for an introduction and for more details.

`setClass`:
  Define Class to be an S-style class. The effect is to create an object, of class `"classRepEnvironment"`, and store this (hidden) in the specified environment or database. Objects can be created from the class (e.g., by calling `new`), manipulated (e.g., by accessing the object's slots), and methods may be defined including the class name in the signature (see `setMethod`).
`removeClass`:
  Remove the definition of this class. Calling this always resets the version of the class cached for the session. If `where=0`, that's all it does. Otherwise, it removes the version from the specified environment or database (from the global environment by default).
`isClass`:
  Is this a the name of a formally defined class? (Argument `formal` is for compatibility and is ignored.)
`isClassDef`:
  Is this object a class definition (it will be, for example, if it is the value of a call to `getClass`, the complete definition of a class with its extensions, or to `getClassDef`, the local definition of the class).
`getClasses`:
  The names of all the classes formally defined on `where'. If called with no argument, all the classes currently known in the session (which does not include classes that may be defined on one of the attached libraries, but have not yet been used in the session).
`unclass`:
  Returns the object containing the values of all the slots in this object's class definition (specifically, ithe returned object has attributes corresponding to each slot), in the case that the object's class is formally defined with slots.

  For classes that extend a single other class (e.g., a basic class such as `"numeric"`) the result is an object of that class.

`resetClass`:
  Reset the internal definition of a class. The effect is that the next time the definition of this class is needed, it will be recomputed from the information on the attached databases.

  This function is called when aspects of the class definition are changed. You would need to call it explicitly if you changed the definition of a class that this class extends (but doing that in the middle of a session is living dangerously, since it may invalidate existing objects).

### Inheritance and 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 basic classes, perhaps with the intension of modifying R's standard behavior for that class. Perfectly legal and sometimes quite helpful, but you may need to be more careful in this case: your new class will inherit much of the behavior of the basic (informally defined) class, and the results can be surprising. Just proceed with caution and plenty of testing.

As an example, the class `"matrix"` is included in the pre-defined classes, to behave essentially as matrices do without formal class definitions. Suppose we don't like all of this; in particular, we want the default matrix to have 0 rows and columns (not 1 by 1 as it is now).

`setClass("myMatrix", "matrix", prototype = matrix(0,0,0))`

The arguments above illustrate two short-cuts relevant to such examples. We abbreviated the `representation` argument to the single superclass, because the new class doesn't add anything to the representation of class `"matrix"`. Also, we provided an object from the superclass as the prototype, not a list of slots.

### Author(s)

John Chambers

### References

The web page http://www.omegahat.org/RSMethods/index.html is the primary documentation.

The functions in this package emulate the facility for classes and methods described in Programming with Data, (John M. Chambers, Springer, 1998). See this book for further details and examples.

### See Also

`Methods`, `setSClass`

### 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("track", smooth = "numeric"))
    ## 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")))})

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