Node:Object Properties, Next:Sorting, Previous:Equality, Up:Utility Functions
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
(see Procedures with Setters) that can be used with the generalized
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
(define my-property (make-object-property))
Create and return an object property. An object property is a
procedure-with-setter that can be called in two ways. |
A single object property created by
associate distinct property values with all Scheme values that are
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.