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\name{initialize-methods}
\docType{methods}
\title{Methods to Initialize New Objects from a Class}
\alias{initialize-methods}
\alias{initialize,ANY-method}
\alias{initialize,traceable-method}
\alias{initialize,signature-method}
\alias{initialize,environment-method}
\description{The arguments to function \code{\link{new}} to create an
object from a particular class can be interpreted specially for that
class, by the definition of a method for function \code{initialize}
for the class.  This documentation describes some existing
methods, and also outlines how to write new ones.
}
\section{Methods}{\describe{

\item{.Object = "ANY"}{The default method for \code{initialize} takes
    either named or unnamed arguments.  Argument names must be the
    names of slots in this class definition, and the corresponding
    arguments must be valid objects for the slot (that is, have the
    same class as specified for the slot, or some superclass of that
    class). If the object comes from a superclass, it is not coerced
    strictly, so normally it will retain its current class (specifically,
    \code{\link{as}(object, Class, strict = FALSE)}).

    Unnamed arguments must be objects of this class, of one of its
    superclasses, or one of its subclasses (from the class, from a
    class this class extends, or from a class that extends this
    class). If the object is from a superclass, this normally defines
    some of the slots in the object.  If the object is from a
    subclass, the new object is that argument, coerced to the current
    class.

    Unnamed arguments are processed first, in the order they appear.
    Then named arguments are processed.  Therefore, explicit values
    for slots always override any values inferred from superclass or
    subclass arguments.
}

\item{.Object = "traceable"}{ Objects of a class that extends
 \code{traceable} are used to implement debug tracing (see
 \link{traceable-class} and \code{\link[base]{trace}}).

The \code{initialize} method for these classes takes special arguments
\code{def, tracer, exit, at, print}.  The first of these is the object
to use as the original definition (e.g., a function).  The others
correspond to the arguments to \code{\link[base]{trace}}.
 }

\item{.Object = "environment"}{The \code{initialize} method for
    environments takes a named list of objects to be used to
    initialize the environment. }

\item{.Object = "signature"}{ This is a method for internal use only.
 It takes an optional \code{functionDef} argument to provide a generic
 function with a \code{signature} slot to define the argument names.
 See \link{Methods} for details.
 }


}}
\section{Writing Initialization Methods}{Initialization methods
  provide a general mechanism corresponding to generator functions in
  other languages.

  The arguments to \code{\link{initialize}} are \code{.Object} and
  \dots. Nearly always, \code{initialize} is called from \code{new},
  not directly.  The \code{.Object} argument is then the
  prototype object from the class.

  Two techniques are often appropriate for \code{initialize} methods:
  special argument names and \code{callNextMethod}.

  You may want argument names that are more natural to your users than
  the (default) slot names.  These will be the formal arguments to
  your method definition, in addition to \code{.Object} (always) and
  \dots (optionally).  For example, the method for class
  \code{"traceable"} documented above would be created by a call to
  \code{\link{setMethod}} of the form:

\code{setMethod("initialize", "traceable",}
\cr
\code{    function(.Object, def, tracer, exit, at, print) } \dots
\cr
\code{)}

In this example, no other arguments are meaningful, and the resulting
method will throw an error if other names are supplied.

When your new class extends another class, you may want to call the
initialize method for this superclass (either a special method or the
default).  For example, suppose you want to define a method for your
class, with special argument \code{x}, but you also want users to be
able to set slots specifically.  If you want \code{x} to override the
slot information, the beginning of your method definition might look
something like this:

\code{function(.Object, x, ...) \{}
\cr
\code{    Object <- callNextMethod(.Object, ...) }
\cr
\code{    if(!missing(x)) \{ # do something with x }

You could also choose to have the inherited method override, by first
interpreting \code{x}, and then calling the next method.

}
\keyword{methods}
\keyword{programming}