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Clean up doc on class redefinition and instance class changing

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* doc/ref/goops.texi (Class Redefinition): Deleted, with its material
  all merged into later `Redefining a Class' and `Changing the Class of
  an Instance' sections.
This commit is contained in:
Neil Jerram 2011-02-18 22:15:11 +00:00
parent b0fc1b9f37
commit ed478161f3
1 changed files with 46 additions and 107 deletions
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153
doc/ref/goops.texi
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@ -1811,7 +1811,6 @@ customizing the behaviour of GOOPS itself.
* Instance Creation Protocol::
* Class Definition Protocol::
* Customizing Class Definition::
* Class Redefinition::
* Method Definition::
* Method Definition Internals::
* Generic Function Internals::
@ -2515,82 +2514,6 @@ typically it would perform additional class initialization steps before
and/or after calling @code{(next-method)} for the standard behaviour.
@node Class Redefinition
@subsection Class Redefinition
@itemize @bullet
@item
@code{class-redefinition @var{old-class} @var{new-class}} (generic)
@code{define-class} calls @code{class-redefinition} if the variable
specified by its first argument already held a GOOPS class definition.
@var{old-class} and @var{new-class} are the old and new class metaobjects.
The applied method should perform whatever is necessary to handle the
redefinition, and should return the class metaobject that is to be bound
to @code{define-class}'s variable. The default class redefinition
protocol is described in @ref{Class Redefinition}.
@end itemize
The default @code{class-redefinition} method, specialized for classes
with the default metaclass @code{<class>}, has the following internal
protocol.
@code{class-redefinition (@var{old <class>}) (@var{new <class>})}
(method)
@itemize @bullet
@item
@code{remove-class-accessors! @var{old}} (generic)
@item
@code{update-direct-method! @var{method} @var{old} @var{new}} (generic)
@item
@code{update-direct-subclass! @var{subclass} @var{old} @var{new}} (generic)
@end itemize
This protocol cleans up things that the definition of the old class
once changed and modifies things to work with the new class.
The default @code{remove-class-accessors!} method removes the
accessor methods of the old class from all classes which they
specialize.
The default @code{update-direct-method!} method substitutes the new
class for the old in all methods specialized to the old class.
The default @code{update-direct-subclass!} method invokes
@code{class-redefinition} recursively to handle the redefinition of
subclasses.
When a class is redefined, any existing instance of the redefined class
will be modified for the new class definition before the next time that
any of the instance's slot is referenced or set. GOOPS modifies each
instance by calling the generic function @code{change-class}.
The default @code{change-class} method copies slot values from the old
to the modified instance, and initializes new slots, as described in
@ref{Changing the Class of an Instance}. After doing so, it makes a
generic function invocation that can be used to customize the instance
update algorithm.
@code{change-class (@var{old-instance <object>}) (@var{new <class>})} (method)
@itemize @bullet
@item
@code{update-instance-for-different-class @var{old-instance} @var{new-instance}} (generic)
@code{change-class} invokes @code{update-instance-for-different-class}
as the last thing that it does before returning. The applied method can
make any further adjustments to @var{new-instance} that are required to
complete or modify the change of class. The return value from the
applied method is ignored.
The default @code{update-instance-for-different-class} method does
nothing.
@end itemize
@node Method Definition
@subsection Method Definition
@ -2892,8 +2815,8 @@ be customized@dots{}
@node Customizing Class Redefinition
@subsection Customizing Class Redefinition
When @code{define-class} notices that a class is being redefined,
it constructs the new class metaobject as usual, and then invokes the
When @code{define-class} notices that a class is being redefined, it
constructs the new class metaobject as usual, then invokes the
@code{class-redefinition} generic function with the old and new classes
as arguments. Therefore, if the old or new classes have metaclasses
other than the default @code{<class>}, class redefinition behaviour can
@ -2912,6 +2835,26 @@ Implements GOOPS' default class redefinition behaviour, as described in
for the new class definition.
@end deffn
The default @code{class-redefinition} method, for classes with the
default metaclass @code{<class>}, calls the following generic functions,
which could of course be individually customized.
@deffn generic remove-class-accessors! old
The default @code{remove-class-accessors!} method removes the accessor
methods of the old class from all classes which they specialize.
@end deffn
@deffn generic update-direct-method! method old new
The default @code{update-direct-method!} method substitutes the new
class for the old in all methods specialized to the old class.
@end deffn
@deffn generic update-direct-subclass! subclass old new
The default @code{update-direct-subclass!} method invokes
@code{class-redefinition} recursively to handle the redefinition of
subclasses.
@end deffn
An alternative class redefinition strategy could be to leave all
existing instances as instances of the old class, but accepting that the
old class is now ``nameless'', since its name has been taken over by the
@ -2935,34 +2878,18 @@ is specialized for this metaclass:
When customization can be as easy as this, aren't you glad that GOOPS
implements the far more difficult strategy as its default!
Finally, note that, if @code{class-redefinition} itself is not customized,
the default @code{class-redefinition} method invokes three further
generic functions that could be individually customized:
@itemize @bullet
@item
(remove-class-accessors! @var{old-class})
@item
(update-direct-method! @var{method} @var{old-class} @var{new-class})
@item
(update-direct-subclass! @var{subclass} @var{old-class} @var{new-class})
@end itemize
and the default methods for these generic functions invoke further
generic functions, and so on@dots{} The detailed protocol for all of these
is described in @ref{MOP Specification}.
@node Changing the Class of an Instance
@section Changing the Class of an Instance
You can change the class of an existing instance by invoking the
generic function @code{change-class} with two arguments: the instance
and the new class.
When a class is redefined, any existing instance of the redefined class
will be modified for the new class definition before the next time that
any of the instance's slots is referenced or set. GOOPS modifies each
instance by calling the generic function @code{change-class}.
@deffn generic change-class
@end deffn
More generally, you can change the class of an existing instance at any
time by invoking the generic function @code{change-class} with two
arguments: the instance and the new class.
The default method for @code{change-class} decides how to implement the
change of class by looking at the slot definitions for the instance's
@ -2973,6 +2900,9 @@ discarded. Slots that are present only in the new class are initialized
using the corresponding slot definition's init function (@pxref{Classes,,
slot-init-function}).
@deffn generic change-class instance new-class
@end deffn
@deffn {method} change-class (obj <object>) (new <class>)
Modify instance @var{obj} to make it an instance of class @var{new}.
@ -2984,11 +2914,20 @@ pre-existing slots are initialized according to @var{new}'s slot definitions'
init functions.
@end deffn
The default @code{change-class} method also invokes another generic
function, @code{update-instance-for-different-class}, as the last thing
that it does before returning. The applied
@code{update-instance-for-different-class} method can make any further
adjustments to @var{new-instance} that are required to complete or
modify the change of class. The return value from the applied method is
ignored.
@deffn generic update-instance-for-different-class old-instance new-instance
A generic function that can be customized to put finishing touches to an
instance whose class has just been changed. The default
@code{update-instance-for-different-class} method does nothing.
@end deffn
Customized change of class behaviour can be implemented by defining
@code{change-class} methods that are specialized either by the class
of the instances to be modified or by the metaclass of the new class.
When a class is redefined (@pxref{Redefining a Class}), and the default
class redefinition behaviour is not overridden, GOOPS (eventually)
invokes the @code{change-class} generic function for each existing
instance of the redefined class.