R: Generate an Object from a Class

new {methods} | R Documentation

## Generate an Object from a Class

### Description

Given the name or the definition of a class, plus optionally data to be included in the object, `new` returns an object from that class.

### Usage

    new(Class, ...)

    initialize(.Object, ...)

### Arguments

`Class` | either the name of a class, a `character` string, (the usual case) or the object describing the class (e.g., the value returned by `getClass`).
`...` | data to include in the new object. Named arguments correspond to slots in the class definition. Unnamed arguments must be objects from classes that this class extends.
`.Object` | An object: see the Details section.

### Details

The function `new` begins by copying the prototype object from the class definition. Then information is inserted according to the `...` arguments, if any. As of version 2.4 of R, the type of the prototype object, and therefore of all objects returned by `new()`, is `"S4"` except for classes that extend one of the basic types, where the prototype has that basic type. User functions that depend on `typeof(object)` should be careful to handle `"S4"` as a possible type.

Note that the name of the first argument, `"Class"` entails that `"Class"` is an undesirable slot name in any formal class: `new("myClass", Class = <value>)` will not work.

The interpretation of the `...` arguments can be specialized to particular classes, if an appropriate method has been defined for the generic function `"initialize"`. The `new` function calls `initialize` with the object generated from the prototype as the `.Object` argument to `initialize`.

By default, unnamed arguments in the `...` are interpreted as objects from a superclass, and named arguments are interpreted as objects to be assigned into the correspondingly named slots. Thus, explicit slots override inherited information for the same slot, regardless of the order in which the arguments appear.

The `initialize` methods do not have to have `...` as their second argument (see the examples). Initialize methods are often written when the natural parameters describing the new object are not the names of the slots. If you do define such a method, note the implications for future subclasses of your class. If these have additional slots, and your `initialize` method has `...` as a formal argument, then your method should pass such arguments along via `callNextMethod`. If your method does not have this argument, then either a subclass must have its own method or else the added slots must be specified by users in some way other than as arguments to `new`.

For examples of `initialize` methods, see `initialize-methods` for existing methods for classes `"traceable"` and `"environment"`, among others. See the comments there on subclasses of `"environment"`; any `initialize` methods for these should be sure to allocate a new environment.

Methods for `initialize` can be inherited only by simple inheritance, since it is a requirement that the method return an object from the target class. See the `simpleInheritanceOnly` argument to `setGeneric` and the discussion in `setIs` for the general concept.

Note that the basic vector classes, `"numeric"`, etc. are implicitly defined, so one can use `new` for these classes.

### 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 an overview of defining class, and `setOldClass` for the relation to S3 classes.

### Examples

    ## using the definition of class "track" from \link{setClass}

    ## a new object with two slots specified
    t1 <- new("track", x = seq_along(ydata), y = ydata)

    # a new object including an object from a superclass, plus a slot
    t2 <- new("trackCurve", t1, smooth = ysmooth)

    ### define a method for initialize, to ensure that new objects have
    ### equal-length x and y slots.

    setMethod("initialize",
              "track",
              function(.Object, x = numeric(0), y = numeric(0)) {
                if(nargs() > 1) {
                  if(length(x) != length(y))
                    stop("specified x and y of different lengths")
                  .Object@x <- x
                  .Object@y <- y
                }
                .Object
              })

    ### the next example will cause an error (x will be numeric(0)),
    ### because we didn't build in defaults for x,
    ### although we could with a more elaborate method for initialize

    try(new("track", y = sort(stats::rnorm(10))))

    ## a better way to implement the previous initialize method.
    ## Why?  By using callNextMethod to call the default initialize method
    ## we don't inhibit classes that extend "track" from using the general
    ## form of the new() function.  In the previous version, they could only
    ## use x and y as arguments to new, unless they wrote their own
    ## initialize method.

    setMethod("initialize", "track", function(.Object, ...) {
        .Object <- callNextMethod()
        if(length(.Object@x) != length(.Object@y))
         stop("specified x and y of different lengths")
        .Object
      })

[Package methods version 3.1.2 Index]