\Header{initialize-methods}{Methods to Initialize New Objects from a Class}
\alias{initialize,ANY-method}{initialize-methods}
\alias{initialize,environment-method}{initialize-methods}
\alias{initialize,signature-method}{initialize-methods}
\alias{initialize,traceable-method}{initialize-methods}
\keyword{methods}{initialize-methods}
\keyword{programming}{initialize-methods}
\begin{Description}\relax
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.
\end{Description}
\begin{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{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{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.

}
\end{Section}
\begin{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:

\begin{verbatim}
    setMethod("initialize", "traceable",
      function(.Object, def, tracer, exit, at, print) ...
    )
\end{verbatim}

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:

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

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

