► |  | The Racket Reference

1 | Language Model
2 | Syntactic Forms
3 | Datatypes
4 | Structures
5 | Classes and Objects
6 | Units
7 | Contracts
8 | Pattern Matching
9 | Control Flow
10 | Concurrency and Parallelism
11 | Macros
12 | Input and Output
13 | Reflection and Security
14 | Operating System
15 | Memory Management
16 | Unsafe Operations
17 | Running Racket
| Bibliography
| Index

▼ | 15 | Memory Management

15.1 | Weak Boxes
15.2 | Ephemerons
15.3 | Wills and Executors
15.4 | Garbage Collection
15.5 | Phantom Byte Strings

On this page:

make-weak-box
weak-box-value
weak-box?

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#### 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).

> > procedure

> > (make-weak-box v) → weak-box?

> v : any/c

Returns a new weak box that initially contains v.

> > procedure

> > (weak-box-value weak-box [gced-v]) → any/c

> weak-box : weak-box?
> gced-v : any/c = #f

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 gced-v (which defaults to #f) is returned.

> > procedure

> > (weak-box? v) → boolean?

> v : any/c

Returns #t if v is a weak box, #f otherwise.
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