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  there is no class attribute, the implicit class is tried, then the
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  there is no class attribute, the implicit class is tried, then the
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  default method.
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  default method.
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  The function \code{class} prints the vector of names of classes an
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  The function \code{class} prints the vector of names of classes an
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  object inherits from.  Correspondingly, \code{class<-} sets the
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  object inherits from.  Correspondingly, \code{class<-} sets the
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  classes an object inherits from.  Assigning a zero-length vector or
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  classes an object inherits from.  Assigning \code{NULL} removes the
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  \code{NULL} removes the class attribute.
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  class attribute.
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  \code{unclass} returns (a copy of) its argument with its class
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  \code{unclass} returns (a copy of) its argument with its class
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  attribute removed.  (It is not allowed for objects which cannot be
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  attribute removed.  (It is not allowed for objects which cannot be
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  copied, namely environments and external pointers.)
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  copied, namely environments and external pointers.)
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  \code{\link{UseMethod}} dispatches on the class as returned by
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  \code{\link{UseMethod}} dispatches on the class as returned by
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  \code{class} (with some interpolated classes: see the link) rather
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  \code{class} (with some interpolated classes: see the link) rather
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  than \code{oldClass}.  \emph{However}, \link{group generic}s dispatch
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  than \code{oldClass}.  \emph{However}, \link{group generic}s dispatch
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  on the \code{oldClass} for efficiency, and \link{internal generic}s
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  on the \code{oldClass} for efficiency, and \link{internal generic}s
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  only dispatch on objects for which \code{\link{is.object}} is true.
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  only dispatch on objects for which \code{\link{is.object}} is true.
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  In some versions of \R, assigning a zero-length vector with
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  \code{class} removes the class: in others it is an error (whereas it
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  works for \code{oldClass}.  It is clearer to always assign \code{NULL}
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  to remove the class.
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}
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}
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\section{Formal classes}{
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\section{Formal classes}{
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  An additional mechanism of \emph{formal} classes, nicknamed
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  An additional mechanism of \emph{formal} classes, nicknamed
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  \dQuote{S4}, is available in
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  \dQuote{S4}, is available in package \pkg{methods} which is attached
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  packages \pkg{methods} which is attached by default.  For objects
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  by default.  For objects which have a formal class, its name is
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  which have a formal class, its name is returned by \code{class}
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  returned by \code{class} as a character vector of length one and
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  as a character vector of length one and method dispatch can happen on
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  method dispatch can happen on \emph{several} arguments, instead of
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  \emph{several} arguments, instead of only the first. However, S3 method selection
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  only the first.  However, S3 method selection attempts to treat objects
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  attempts to treat objects from an S4 class as if they had the
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  from an S4 class as if they had the appropriate S3 class attribute, as
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  appropriate S3 class attribute, as does \code{inherits}.  Therefore,
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  does \code{inherits}.  Therefore, S3 methods can be defined for S4
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  S3 methods can be defined for S4 classes.  See the \sQuote{\link{Classes}}
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  classes.  See the \sQuote{\link{Classes}} and \sQuote{\link{Methods}}
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  and \sQuote{\link{Methods}} help pages for details.
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  help pages for details.
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  The replacement version of the function sets the class to the value
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  The replacement version of the function sets the class to the value
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  provided.  For classes that have a formal definition, directly
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  provided.  For classes that have a formal definition, directly
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  replacing the class this way is strongly deprecated.  The expression
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  replacing the class this way is strongly deprecated.  The expression
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  \code{\link{as}(object, value)} is the way to coerce an object to a
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  \code{\link{as}(object, value)} is the way to coerce an object to a