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## 6.17 Weak pointers

A weak pointer is a reference to an object that doesn’t prevent the object from being garbage-collected. Gauche provides weak pointers as a weak vector object. A weak vector is like a vector of objects, except each object can be garbage collected if it is not referenced from objects other than weak vectors. If the object is collected, the entry of the weak vector is replaced for `#f.`

 ```gosh> (define v (make-weak-vector 1)) v gosh> (weak-vector-ref v 0) #f gosh> (weak-vector-set! v 0 (cons 1 1)) # gosh> (weak-vector-ref v 0) (1 . 1) gosh> (gc) # gosh> (gc) # gosh> (weak-vector-ref v 0) #f ```
Builtin Class: <weak-vector>

The weak vector class. Inherits `<sequence>` and `<collection>`, so you can use `gauche.collection` (See section `gauche.collection` - Collection framework) and `gauche.sequence` (See section `gauche.sequence` - Sequence framework).

 ```(coerce-to '(1 2 3 4)) ⇒ a weak vector with four elements ```
Function: make-weak-vector size

Creates and returns a weak vector of size size.

Function: weak-vector-length wvec

Returns the length of a weak vector wvec.

Function: weak-vector-ref wvec k &optioal fallback

Returns k-th element of a weak vector wvec.

By default, `weak-vector-ref` signals an error if k is negative, or greater than or equal to the size of wvec. However, if an optional argument fallback is given, it is returned for such case.

If the element has been garbage collected, this procedure returns fallback if it is provided, `#f` otherwise.

With `gauche.sequence` module, you can also use a method `ref`.

Function: weak-vector-set! wvec k obj

Sets k-th element of the weak vector wvec to obj. It is an error if k is negative or greater than or equal to the size of wec.

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