An Ephemeron is an object which refers strongly to its contents as long as the Ephemeron's key is not garbage collected, and weakly from then on. Ephemerons solve a problem which is commonly found when trying to "attach" properties to objects by using a registry. When some property should be attached to an object, the property should (in terms of GC behavior) typically have the life-time that an instance variable of this object would have. However, this is complicated by having an external association between the object and its property such as:
property --------- registry --------- association --------- object
Here, the registry (a third party) will hold onto the association itself which would require manual removal from the registry (instead of automated garbage collection). While this problem can be always be solved in any given concrete situation by using one of the various weak association types, choosing the 'right' kind of association depends on a variety of factors some of which can change dynamically.
Ephemerons solve this problem by defining that the 'contents' (value) of an Ephemeron will be held strongly until the key is known to be garbage collected. From then on, the contents of the ephemeron will be held weakly. Therefore, the contents of an ephemeron can only become eligible for garbage collection if and only if the key is garbage collectable which is the exact behavior which we would observe for an instance variable of the object.
Ephemerons were first invented by Geroge Bosworth while he worked at Digitalk. They were used as the finalization mechanism in Visual Smalltalk Enterprise. Today Ephemerons are available in most Smalltalk dialects as well as many other languages with automatic garbage collection.