\HeaderA{getClass}{Get Class Definition}{getClass}
\aliasA{getClassDef}{getClass}{getClassDef}
\keyword{programming}{getClass}
\keyword{classes}{getClass}
\begin{Description}\relax
Get the definition of a class.
\end{Description}
\begin{Usage}
\begin{verbatim}
getClass(Class, .Force = FALSE, where)
getClassDef(Class, where, package, inherits = TRUE)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{Class}] the character-string name of the class, often with a
\code{"package"} attribute as noted below under \code{package}.
\item[\code{.Force}] if \code{TRUE}, return \code{NULL} if the class is
undefined; otherwise, an undefined class results in an error.
\item[\code{where}] environment from which to begin the search for the definition; by default,
start at the top-level (global) environment and proceed through
the search list.
\item[\code{package}] the name of the package asserted to hold the
definition.  If it is a non-empty string it is used instead of
\code{where}, as the first place to look for the class.
Note that the package must be loaded but need not be attached.  By
default, the package attribute of the \code{Class} argument is
used, if any.  There will usually be a package attribute if
\code{Class} comes from \code{class(x)} for some object.

\item[\code{inherits}] Should the class definition be retrieved from any
enclosing environment and also from the cache?  If \code{FALSE}
only a definition in the environment \code{where} will be returned.

\end{ldescription}
\end{Arguments}
\begin{Details}\relax
Class definitions are stored in metadata objects in a package
namespace or other environment where they are defined.  When
packages are loaded, the class definitions in the package are cached in an internal
table.  Therefore, most calls to \code{getClassDef} will find the
class in the cache or fail to find it at all, unless \code{inherits}
is \code{FALSE}, in which case only the environment(s) defined by
\code{package} or \code{where} are searched.

The class cache allows for multiple definitions of the
same class name in separate environments, with of course the
limitation that the package attribute or package name must be
provided in the call to
\end{Details}
\begin{Value}
The object defining the class. If the class definition is not found,
\code{getClassDef} returns \code{NULL}, while \code{getClass}, which
calls \code{getClassDef}, either generates an error or, if
\code{.Force} is \code{TRUE}, returns a simple definition for the
class.  The latter case is used internally, but is not typically
sensible in user code.

The non-null returned value is an object of class
\code{\LinkA{classRepresentation}{classRepresentation.Rdash.class}}.  For all reasonable
purposes, use this object only to extract information, rather than trying
to modify it: Use functions such as \code{\LinkA{setClass}{setClass}} and
\code{\LinkA{setIs}{setIs}} to create or modify class definitions.
\end{Value}
\begin{References}\relax
Chambers, John M. (2008)
\emph{Software for Data Analysis: Programming with R}
Springer.  (For the R version.)

Chambers, John M. (1998)
\emph{Programming with Data}
Springer (For the original S4 version.)
\end{References}
\begin{SeeAlso}\relax
\LinkA{Classes}{Classes},
\code{\LinkA{setClass}{setClass}},
\code{\LinkA{isClass}{isClass}}.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
getClass("numeric") ## a built in class

cld <- getClass("thisIsAnUndefinedClass", .Force = TRUE)
cld ## a NULL prototype
## If you are really curious:
utils::str(cld)
## Whereas these generate errors:
try(getClass("thisIsAnUndefinedClass"))
try(getClassDef("thisIsAnUndefinedClass"))
\end{ExampleCode}
\end{Examples}

