On this page:
Version: 4.2.1

15.1 Weak Boxes

A weak box is similar to a normal box (see Boxes), but when the garbage collector (see Garbage Collection) can prove that the content value of a weak box is only reachable via weak references, the content of the weak box is replaced with #f. A weak reference is a reference through a weak box, through a key reference in a weak hash table (see Hash Tables), through a value in an ephemeron where the value can be replaced by #f (see Ephemerons), or through a custodian (see Custodians).

(make-weak-box v)  weak-box?
  v : any/c
Returns a new weak box that initially contains v.

(weak-box-value weak-box)  any
  weak-box : weak-box?
Returns the value contained in weak-box. If the garbage collector has proven that the previous content value of weak-box was reachable only through a weak reference, then #f is returned.

(weak-box? v)  boolean?
  v : any/c
Returns #t if v is a weak box, #f otherwise.