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functions as described in the section on \dQuote{Reference Objects}.
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functions as described in the section on \dQuote{Reference Objects}.
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}
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}
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} %% end of \describe
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} %% end of \describe
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} %% end of \section
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} %% end of \section
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\section{Implementation}{
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\section{Implementation; Reference Classes as S4 Classe}{
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Reference classes are implemented as S4 classes with a data part of
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Reference classes are implemented as S4 classes with a data part of
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type \code{"environment"}.
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type \code{"environment"}.
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Fields correspond to named objects in the environment.
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Fields correspond to named objects in the environment.
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A field associated with a function is implemented as an
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A field associated with a function is implemented as an
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\link[=bindenv]{active binding}.
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\link[=bindenv]{active binding}.
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of fields as data within the object.
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of fields as data within the object.
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A field, say \code{data}, can be accessed generally by an expression
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A field, say \code{data}, can be accessed generally by an expression
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of the form \code{x$data} %$}
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of the form \code{x$data} %$}
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for any object from the relevant class.
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for any object from the relevant class.
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In a method for this class, the field can be accessed by the name
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In an internal method for this class, the field can be accessed by the name
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\code{data}.
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\code{data}.
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A field that is not locked can be set by an expression of the form
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A field that is not locked can be set by an expression of the form
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\code{x$data <- value}.%$
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\code{x$data <- value}.%$
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Inside a method, a field can be assigned by an expression of the form
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Inside an internal method, a field can be assigned by an expression of the form
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\code{x <<- value}.
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\code{x <<- value}.
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Note the \link[=assignOps]{non-local assignment} operator.
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Note the \link[=assignOps]{non-local assignment} operator.
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The standard \R interpretation of this operator works to assign it in
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The standard \R interpretation of this operator works to assign it in
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the environment of the object.
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the environment of the object.
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If the field has an accessor function defined, getting and setting
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If the field has an accessor function defined, getting and setting
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When a method is invoked on an object, the function defining the method is
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When a method is invoked on an object, the function defining the method is
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installed in the object's environment, with the same environment as the
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installed in the object's environment, with the same environment as the
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environment of the function.
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environment of the function.
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- 
 
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Reference classes can have validity methods in the same sense as any
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S4 class (see \code{\link{setValidity}}).
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Such methods are often a good idea; they will be called by calling
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\code{\link{validObject}} and a validity method, if one is defined,
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will be called when a reference object is created (from version 3.4 of
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\R on).
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Just remember that these are S4 methods.  The function will be called
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with the \code{object} as its argument.  Fields and methods must be
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accessed using \code{$}.%$
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\emph{Note: Slots.} Because of the implementation, new reference classes can inherit from
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\emph{Note: Slots.} Because of the implementation, new reference classes can inherit from
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non-reference S4 classes as well as reference classes, and can include
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non-reference S4 classes as well as reference classes, and can include
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class-specific slots in the definition.
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class-specific slots in the definition.
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This is usually a bad idea, if the slots from the non-reference
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This is usually a bad idea, if the slots from the non-reference
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class are thought of as alternatives to fields.
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class are thought of as alternatives to fields.