15.2 Ephemerons

An ephemeron is similar to a weak box (see Weak Boxes), except that

• an ephemeron contains a key and a value; the value can be extracted from the ephemeron, but the value is replaced by #f when the automatic memory manager can prove that either the ephemeron or the key is reachable only through weak references (see Weak Boxes); and

• nothing reachable from the value in an ephemeron counts toward the reachability of an ephemeron key (whether for the same ephemeron or another), unless the same value is reachable through a non-weak reference, or unless the value’s ephemeron key is reachable through a non-weak reference (see Weak Boxes for information on weak references).

In particular, an ephemeron can be combined with a weak hash table (see Hash Tables) to produce a mapping where the memory manager can reclaim key–value pairs even when the value refers to the key.