property-value

Arguments: object property-or-name

Returns the current value of a specified property of object. property-or-name should be either a property object or the symbol that names the property. When property-or-name is a property object, it should be one that was returned by calling properties on the same object that is now being passed to property-value. Passing the property object is more efficient than passing the property name.

To retrieve the values of particular properties when the names of the property-reader functions are known, it is more efficient to call the reader functions directly. But calling property-value may be useful for meta-level uses such as finding the values of all of the properties of an object. For example, the following function returns a list of the names and values of all of the properties of an object.

(defun list-properties (object)
  (mapcar #'(lambda (property)
              (list (name property)
                    (property-value object property)))
    (properties object)))

If property-or-name is a symbol but no property of object is found by that name, then an error is signaled. Otherwise the property-reader of the property is called, and the value that it returns is returned. If an error occurs in the call to the property-reader function, the error is trapped and the keyword :property-read-error is returned.

See defproperties for information on defining properties on any type of object.

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Created 2012.5.30.