Ephemerons and weak hash tables.
Ephemerons and weak hash tables are useful when one wants to cache
or memorize the computation of a function, as long as the
arguments and the function are used, without creating memory leaks
by continuously keeping old computation results that are not
useful anymore because one argument or the function is freed. An
Hashtbl.t is not suitable because all
associations would keep the arguments and the result in memory.
Ephemerons can also be used for "adding" a field to an arbitrary boxed OCaml value: you can attach some information to a value created by an external library without memory leaks.
Ephemerons hold some keys and one or no data. They are all boxed OCaml values. The keys of an ephemeron have the same behavior as weak pointers according to the garbage collector. In fact OCaml weak pointers are implemented as ephemerons without data.
The keys and data of an ephemeron are said to be full if they
point to a value, or empty if the value has never been set, has
been unset, or was erased by the GC. In the function that accesses
the keys or data these two states are represented by the
The data is considered by the garbage collector alive if all the full keys are alive and if the ephemeron is alive. When one of the keys is not considered alive anymore by the GC, the data is emptied from the ephemeron. The data could be alive for another reason and in that case the GC will not free it, but the ephemeron will not hold the data anymore.
The ephemerons complicate the notion of liveness of values, because it is not anymore an equivalence with the reachability from root value by usual pointers (not weak and not ephemerons). With ephemerons the notion of liveness is constructed by the least fixpoint of: A value is alive if:
output_valueor the functions of the
Ephemerons are defined in a language agnostic way in this paper: B. Hayes, Ephemerons: A New Finalization Mechanism, OOPSLA'97
Ephemerons with one key.
Ephemerons with two keys.
Ephemerons with arbitrary number of keys of the same type.