`module Weak: sig .. end`

Arrays of weak pointers and hash tables of weak pointers.

```
type t('a);
```

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 after the
weak pointers are erased.

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

Notes:

- Integers are not allocated and cannot be stored in weak arrays.
- Weak arrays cannot be marshaled using
`Pervasives.output_value`

nor the functions of the`Marshal`

module.

```
let create: int => t('a);
```

`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_length`

`-1`

.```
let length: t('a) => int;
```

`Weak.length ar`

returns the length (number of elements) of
`ar`

.```
let set: (t('a), int, option('a)) => 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`

.```
let get: (t('a), int) => option('a);
```

`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`

.```
let get_copy: (t('a), int) => option('a);
```

`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`

.```
let check: (t('a), 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`

.```
let fill: (t('a), int, int, option('a)) => 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`

.```
let blit: (t('a), int, t('a), 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.
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`

.A weak hash table 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 tables 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.

`module type S = sig .. end`

The output signature of the functor

`Weak.Make`

.
```
module Make: (H: Hashtbl.HashedType) => S with type data = H.t;
```

Functor building an implementation of the weak hash table structure.