(guile.info.gz) Object Properties
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24.2 Object Properties
======================
It's often useful to associate a piece of additional information with a
Scheme object even though that object does not have a dedicated slot
available in which the additional information could be stored. Object
properties allow you to do just that.
An object property is most commonly used to associate one kind of
additional information with each instance of a class of similar Scheme
objects. For example, all procedures have a `name' property, which
stores the name of the variable in which the procedure was stored by a
`define' expression, or `#f' if the procedure wasn't created by that
kind of expression.
Guile's representation of an object property is a
procedure-with-setter ( Procedures with Setters) that can be
used with the generalized form of `set!' (REFFIXME) to set and retrieve
that property for any Scheme object. So, setting a property looks like
this:
(set! (my-property obj1) value-for-obj1)
(set! (my-property obj2) value-for-obj2)
And retrieving values of the same property looks like this:
(my-property obj1)
=>
value-for-obj1
(my-property obj2)
=>
value-for-obj2
To create an object property in the first place, use the
`make-object-property' procedure:
(define my-property (make-object-property))
-- Scheme Procedure: make-object-property
Create and return an object property. An object property is a
procedure-with-setter that can be called in two ways. `(set!
(PROPERTY OBJ) VAL)' sets OBJ's PROPERTY to VAL. `(PROPERTY OBJ)'
returns the current setting of OBJ's PROPERTY.
A single object property created by `make-object-property' can
associate distinct property values with all Scheme values that are
distinguishable by `eq?' (including, for example, integers).
Internally, object properties are implemented using a weak key hash
table. This means that, as long as a Scheme value with property values
is protected from garbage collection, its property values are also
protected. When the Scheme value is collected, its entry in the
property table is removed and so the (ex-) property values are no longer
protected by the table.
Menu
* Property Primitives Low level property implementation.
* Old-fashioned Properties An older approach to properties.
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