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Customizing Instance Creation to metaobject protocol section

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* doc/ref/goops.texi (Customizing Instance Creation): Moved from
  `Creating Instances' to `The Metaobject Protocol'.

* doc/ref/goops.texi (Basic Instance Creation): Flattened into parent
  `Creating Instances', refs updated accordingly.
This commit is contained in:
Neil Jerram 2010-10-02 16:12:32 +01:00
parent 3dc071e483
commit 9e57815ddf
1 changed files with 70 additions and 77 deletions
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147
doc/ref/goops.texi
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@ -495,14 +495,6 @@ If the @code{#:name} option is absent, GOOPS uses the first argument to
@node Creating Instances
@section Creating Instances
@menu
* Basic Instance Creation::
* Customizing Instance Creation::
@end menu
@node Basic Instance Creation
@subsection Basic Instance Creation
To create a new instance of any GOOPS class, use the generic function
@code{make} or @code{make-instance}, passing the required class and any
appropriate instance initialization arguments as keyword and value
@ -537,73 +529,6 @@ instance's class. Any unprocessed keyword value pairs are ignored.
@code{make-instance} is an alias for @code{make}.
@end deffn
@node Customizing Instance Creation
@subsection Customizing Instance Creation
@code{make} itself is a generic function. Hence the @code{make}
invocation itself can be customized in the case where the new instance's
metaclass is more specialized than the default @code{<class>}, by
defining a @code{make} method that is specialized to that metaclass.
Normally, however, the method for classes with metaclass @code{<class>}
will be applied. This method calls two generic functions:
@itemize @bullet
@item
(allocate-instance @var{class} . @var{initargs})
@item
(initialize @var{instance} . @var{initargs})
@end itemize
@code{allocate-instance} allocates storage for and returns the new
instance, uninitialized. You might customize @code{allocate-instance},
for example, if you wanted to provide a GOOPS wrapper around some other
object programming system.
To do this, you would create a specialized metaclass, which would act as
the metaclass for all classes and instances from the other system. Then
define an @code{allocate-instance} method, specialized to that
metaclass, which calls a Guile primitive C function, which in turn
allocates the new instance using the interface of the other object
system.
In this case, for a complete system, you would also need to customize a
number of other generic functions like @code{make} and
@code{initialize}, so that GOOPS knows how to make classes from the
other system, access instance slots, and so on.
@code{initialize} initializes the instance that is returned by
@code{allocate-instance}. The standard GOOPS methods perform
initializations appropriate to the instance class.
@itemize @bullet
@item
At the least specialized level, the method for instances of type
@code{<object>} performs internal GOOPS instance initialization, and
initializes the instance's slots according to the slot definitions and
any slot initialization keywords that appear in @var{initargs}.
@item
The method for instances of type @code{<class>} calls
@code{(next-method)}, then performs the class initializations described
in @ref{Customizing Class Definition}.
@item
and so on for generic functions, method, operator classes @dots{}
@end itemize
Similarly, you can customize the initialization of instances of any
application-defined class by defining an @code{initialize} method
specialized to that class.
Imagine a class whose instances' slots need to be initialized at
instance creation time by querying a database. Although it might be
possible to achieve this a combination of @code{#:init-thunk} keywords
and closures in the slot definitions, it is neater to write an
@code{initialize} method for the class that queries the database once
and initializes all the dependent slot values according to the results.
@node Accessing Slots
@section Accessing Slots
@ -1307,7 +1232,7 @@ default method calls @code{goops-error} with an appropriate message.
Suppose that a class @code{<my-class>} is defined using @code{define-class}
(@pxref{Defining New Classes,, define-class}), with slots that have
accessor functions, and that an application has created several instances
of @code{<my-class>} using @code{make} (@pxref{Basic Instance Creation,,
of @code{<my-class>} using @code{make} (@pxref{Creating Instances,,
make}). What then happens if @code{<my-class>} is redefined by calling
@code{define-class} again?
@ -1326,7 +1251,7 @@ GOOPS' default answer to this question is as follows.
All existing direct instances of @code{<my-class>} are converted to be
instances of the new class. This is achieved by preserving the values
of slots that exist in both the old and new definitions, and initializing the
values of new slots in the usual way (@pxref{Basic Instance Creation,,
values of new slots in the usual way (@pxref{Creating Instances,,
make}).
@item
@ -1874,6 +1799,7 @@ GOOPS' power, by customizing the behaviour of GOOPS itself.
* Class Definition Internals::
* Customizing Class Definition::
* Instance Creation::
* Customizing Instance Creation::
* Class Redefinition::
* Method Definition::
* Generic Function Invocation::
@ -2604,6 +2530,73 @@ instance in whatever sense is appropriate for its class. The method's
return value is ignored.
@end itemize
@node Customizing Instance Creation
@subsection Customizing Instance Creation
@code{make} itself is a generic function. Hence the @code{make}
invocation itself can be customized in the case where the new instance's
metaclass is more specialized than the default @code{<class>}, by
defining a @code{make} method that is specialized to that metaclass.
Normally, however, the method for classes with metaclass @code{<class>}
will be applied. This method calls two generic functions:
@itemize @bullet
@item
(allocate-instance @var{class} . @var{initargs})
@item
(initialize @var{instance} . @var{initargs})
@end itemize
@code{allocate-instance} allocates storage for and returns the new
instance, uninitialized. You might customize @code{allocate-instance},
for example, if you wanted to provide a GOOPS wrapper around some other
object programming system.
To do this, you would create a specialized metaclass, which would act as
the metaclass for all classes and instances from the other system. Then
define an @code{allocate-instance} method, specialized to that
metaclass, which calls a Guile primitive C function, which in turn
allocates the new instance using the interface of the other object
system.
In this case, for a complete system, you would also need to customize a
number of other generic functions like @code{make} and
@code{initialize}, so that GOOPS knows how to make classes from the
other system, access instance slots, and so on.
@code{initialize} initializes the instance that is returned by
@code{allocate-instance}. The standard GOOPS methods perform
initializations appropriate to the instance class.
@itemize @bullet
@item
At the least specialized level, the method for instances of type
@code{<object>} performs internal GOOPS instance initialization, and
initializes the instance's slots according to the slot definitions and
any slot initialization keywords that appear in @var{initargs}.
@item
The method for instances of type @code{<class>} calls
@code{(next-method)}, then performs the class initializations described
in @ref{Customizing Class Definition}.
@item
and so on for generic functions, method, operator classes @dots{}
@end itemize
Similarly, you can customize the initialization of instances of any
application-defined class by defining an @code{initialize} method
specialized to that class.
Imagine a class whose instances' slots need to be initialized at
instance creation time by querying a database. Although it might be
possible to achieve this a combination of @code{#:init-thunk} keywords
and closures in the slot definitions, it is neater to write an
@code{initialize} method for the class that queries the database once
and initializes all the dependent slot values according to the results.
@node Class Redefinition
@subsection Class Redefinition