new {methods} | R Documentation

## Generate an Object from a Class

### Description

Given the 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 (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.

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 there second argument (see the examples), and generally it is better design not to have `...` as a formal argument, if only a fixed set of arguments make sense.

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

### 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

Classes

### Examples

    ## using the definition of class "track" from Classes

    ## 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(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
    ## intialize 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 Contents]