Module Weak

module Weak: sig .. end

Arrays of weak pointers and hash sets of weak pointers.

Low-level functions

type !'a t 

The type of arrays of weak pointers (weak arrays). A weak pointer is a value that the garbage collector may erase whenever the value is not used any more (through normal pointers) by the program. Note that finalisation functions are run before the weak pointers are erased, because the finalisation functions can make values alive again (before 4.03 the finalisation functions were run after).

A weak pointer is said to be full if it points to a value, empty if the value was erased by the GC.


  • Integers are not allocated and cannot be stored in weak arrays.
  • Weak arrays cannot be marshaled using output_value nor the functions of the Marshal module.
val create : int -> 'a t

Weak.create n returns a new weak array of length n. All the pointers are initially empty.

val length : 'a t -> int

Weak.length ar returns the length (number of elements) of ar.

val set : 'a t -> int -> 'a option -> unit

Weak.set ar n (Some el) sets the nth cell of ar to be a (full) pointer to el; Weak.set ar n None sets the nth cell of ar to empty.

val get : 'a t -> int -> 'a option

Weak.get ar n returns None if the nth cell of ar is empty, Some x (where x is the value) if it is full.

val get_copy : 'a t -> int -> 'a option

Weak.get_copy ar n returns None if the nth cell of ar is empty, Some x (where x is a (shallow) copy of the value) if it is full. In addition to pitfalls with mutable values, the interesting difference with get is that get_copy does not prevent the incremental GC from erasing the value in its current cycle (get may delay the erasure to the next GC cycle).

val check : 'a t -> int -> bool

Weak.check ar n returns true if the nth cell of ar is full, false if it is empty. Note that even if Weak.check ar n returns true, a subsequent Weak.get ar n can return None.

val fill : 'a t -> int -> int -> 'a option -> unit

Weak.fill ar ofs len el sets to el all pointers of ar from ofs to ofs + len - 1.

val blit : 'a t -> int -> 'a t -> int -> int -> unit

Weak.blit ar1 off1 ar2 off2 len copies len weak pointers from ar1 (starting at off1) to ar2 (starting at off2). It works correctly even if ar1 and ar2 are the same.

Weak hash sets

A weak hash set is a hashed set of values. Each value may magically disappear from the set when it is not used by the rest of the program any more. This is normally used to share data structures without inducing memory leaks. Weak hash sets are defined on values from a Hashtbl.HashedType module; the equal relation and hash function are taken from that module. We will say that v is an instance of x if equal x v is true.

The equal relation must be able to work on a shallow copy of the values and give the same result as with the values themselves.

Unsynchronized accesses

Unsynchronized accesses to weak hash sets are a programming error. Unsynchronized accesses to a weak hash set may lead to an invalid weak hash set state. Thus, concurrent accesses to weak hash sets must be synchronized (for instance with a Mutex.t).

module type S = sig .. end

The output signature of the functor Weak.Make.

module Make: 
functor (H : Hashtbl.HashedType-> S with type data = H.t

Functor building an implementation of the weak hash set structure.