19.32 
Module Weak: arrays of weak pointers 

type 'a t;;
The type of arrays of weak pointers (weak arrays). A weak
pointer is a value that the garbage collector may erase at
any time.
A weak pointer is said to be full if it points to a value,
empty if the value was erased by the GC.
val create : int > 'a t;;
Weak.create n
returns a new weak array of length n
.
All the pointers are initially empty. Raise Invalid_argument
if n
is negative or greater than Sys.max_array_length1
.
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 n
th cell of ar
to be a
(full) pointer to el
; Weak.set ar n None
sets the n
th
cell of ar
to empty.
Raise Invalid_argument "Weak.set"
if n
is not in the range
0 to Weak.length a  1
.
val get : 'a t > int > 'a option;;
Weak.get ar n
returns None if the n
th cell of ar
is
empty, Some x
(where x
is the value) if it is full.
Raise Invalid_argument "Weak.get"
if n
is not in the range
0 to Weak.length a  1
.
val get_copy : 'a t > int > 'a option;;
Weak.get_copy ar n
returns None if the n
th 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).
Raise Invalid_argument "Weak.get"
if n
is not in the range
0 to Weak.length a  1
.
val check: 'a t > int > bool;;
Weak.check ar n
returns true
if the n
th 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 > pos:int > len:int > 'a option > unit;;
Weak.fill ar ofs len el
sets to el
all pointers of ar
from
ofs
to ofs + len  1
. Raise Invalid_argument "Weak.fill"
if ofs
and len
do not designate a valid subarray of a
.
val blit : src:'a t > src_pos:int >
dst:'a t > dst_pos:int > len: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.
Raise Invalid_argument "Weak.blit"
if off1
and len
do
not designate a valid subarray of ar1
, or if off2
and len
do not designate a valid subarray of ar2
.