In programming, a reference which does not protect its object from garbage collection
In
computer programming
, a
weak reference
is a
reference
that does not protect the referenced
object
from collection by a
garbage collector
, unlike a strong reference. An object referenced
only
by weak references ? meaning "every chain of references that reaches the object includes at least one weak reference as a link" ? is considered
weakly reachable
,
and can be treated as
unreachable
and so may be collected at any time. Some garbage-collected languages feature or support various levels of weak references, such as
C#
,
Lua
,
Java
,
Lisp
,
OCaml
,
Perl
,
Python
[1]
and
PHP
since the version 7.4.
[2]
Uses
[
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]
Weak references have a number of common uses. When using
reference counting
garbage collection, weak references can break
reference cycles
, by using a weak reference for a link in the cycle. When one has an
associative array
(mapping, hash map) whose keys are (references to) objects, for example to hold auxiliary data about objects, using weak references for the keys avoids keeping the objects alive just because of their use as keys. When one has an object where other objects are registered, such as in the
observer pattern
(particularly in
event handling
), if a strong reference is kept, objects must be explicitly unregistered, otherwise a memory leak occurs (the
lapsed listener problem
), while a weak reference removes the need to unregister. When holding cached data that can be recreated if necessary, weak references allow the cache to be reclaimed, effectively producing discardable memory. This last case (a cache) is distinct from others, as it is preferable that the objects only be garbage collected if necessary, and there is thus a need for finer distinctions within weak references, here a stronger form of a weak reference. In many cases weak references do not need to be directly used, instead simply using a weak array or other
container
whose keys or values are weak references.
Garbage collection
[
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]
Garbage collection is used to clean up unused objects and so reduce the potential for
memory leaks
and data corruption. There are two main types of garbage collection: tracing and
reference counting
. Reference counting schemes record the number of references to a given object and collect the object when the reference count becomes zero. Reference-counting cannot collect cyclic (or circular) references because only one object may be collected at a time. Groups of mutually referencing objects which are not directly referenced by other objects and are unreachable can thus become permanently resident; if an application continually generates such unreachable groups of unreachable objects this will have the effect of a
memory leak
. Weak references (references which are not counted in reference counting) may be used to solve the problem of circular references if the reference cycles are avoided by using weak references for some of the references within the group.
A very common case of such strong vs. weak reference distinctions is in tree structures, such as the
Document Object Model
(DOM), where parent-to-child references are strong, but child-to-parent references are weak. For example, Apple's
Cocoa
framework recommends this approach.
[3]
Indeed, even when the object graph is not a tree, a tree structure can often be imposed by the notion of object ownership, where ownership relationships are strong and form a tree, and non-ownership relationships are weak and not needed to form the tree ? this approach is common in
C++
(pre-C++11), using raw pointers as weak references. This approach, however, has the downside of not allowing the ability to detect when a parent branch has been removed and deleted. Since the
C++11
standard, a solution was added by using
shared_ptr and weak_ptr
, inherited from the
Boost
library.
Weak references are also used to minimize the number of unnecessary objects in memory by allowing the program to indicate which objects are of minor importance by only weakly referencing them.
[
citation needed
]
Variations
[
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]
Some languages have multiple levels of weak reference strength. For example,
Java
has, in order of decreasing strength,
soft
, weak, and
phantom
references, defined in the
package
java.lang.ref
.
[4]
Each reference type has an associated notion of reachability. The garbage collector (GC) uses an object's type of reachability to determine when to free the object. It is safe for the GC to free an object that is softly reachable, but the GC may decide not to do so if it believes the JVM can spare the memory (e.g. the JVM has much unused heap space). The GC will free a weakly reachable object as soon as the GC notices the object. Unlike the other reference types, a phantom reference cannot be followed. On the other hand, phantom references provide a mechanism to notify the program when an object has been freed (notification is implemented using ReferenceQueues).
In C#, weak references are distinguished by whether they track
object resurrection
or not. This distinction does not occur for strong references, as objects are not
finalized
if they have any strong references to them. By default, in C# weak reference do not track resurrection, meaning a weak reference is not updated if an object is resurrected; these are called
short weak references
, and weak references that track resurrection are called
long weak references
.
Some non-garbage-collected languages, such as
C++
, provide weak/strong reference functionality as part of supporting garbage collection libraries. The Boost C++ library provides strong and weak references. It is a mistake to use regular C++ pointers as the
weak
counterparts of
smart pointers
because such usage removes the ability to detect when the
strong
reference count has gone to 0 and the object has been deleted. Worse yet, it does not allow for detection of whether another strong reference is already tracking a given plain pointer. This introduces the possibility of having two (or more) smart pointers tracking the same plain pointer (which causes corruption as soon as one of these smart pointers' reference count reaches 0 and the object gets deleted).
Examples
[
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]
Weak references can be useful when keeping a list of the current variables being referenced in the application. This list must have weak links to the objects. Otherwise, once objects are added to the list, they will be referenced by it and will persist for the duration of the program.
Java
[
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]
Java 1.2 in 1998 introduced
[6]
two kinds of weak references, one known as a "soft reference" (intended to be used for maintaining GC-managed in-memory caches, but which doesn't work very well in practice on some platforms with dynamic heap like Android
[7]
) and the other simply as a "weak reference". It also added a related experimental mechanism dubbed "phantom references" as an alternative to the dangerous and inefficient finalize() mechanism.
[8]
If a weak reference is created, and then elsewhere in the code
get()
is used to get the actual object, the weak reference is not strong enough to prevent garbage collection, so it may be (if there are no strong references to the object) that
get()
suddenly starts returning null.
[9]
import
java.lang.ref.WeakReference
;
public
class
ReferenceTest
{
public
static
void
main
(
String
[]
args
)
throws
InterruptedException
{
WeakReference
r
=
new
WeakReference
(
"I'm here"
);
StrongReference
sr
=
new
StrongReference
(
"I'm here"
);
System
.
out
.
println
(
"Before gc: r="
+
r
.
get
()
+
", static="
+
sr
.
get
());
System
.
gc
();
Thread
.
sleep
(
100
);
// Only r.get() becomes null.
System
.
out
.
println
(
"After gc: r="
+
r
.
get
()
+
", static="
+
sr
.
get
());
}
}
Another use of weak references is in writing a
cache
. Using, for example, a weak
hash map
, one can store in the cache the various referred objects via a weak reference. When the garbage collector runs — when for example the application's memory usage gets sufficiently high — those cached objects which are no longer directly referenced by other objects are removed from the cache.
Smalltalk
[
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]
|
a s1 s2
|
s1
:=
'hello'
copy
.
"that's a strong reference"
s2
:=
'world'
copy
.
"that's a strong reference"
a
:=
WeakArray
with:
s1
with:
s2
.
a
printOn:
Transcript
.
ObjectMemory
collectGarbage
.
a
printOn:
Transcript
.
"both elements still there"
s1
:=
nil
.
"strong reference goes away"
ObjectMemory
collectGarbage
.
a
printOn:
Transcript
.
"first element gone"
s2
:=
nil
.
"strong reference goes away"
ObjectMemory
collectGarbage
.
a
printOn:
Transcript
.
"second element gone"
weak_table
=
setmetatable
({},
{
__mode
=
"v"
})
weak_table
.
item
=
{}
print
(
weak_table
.
item
)
collectgarbage
()
print
(
weak_table
.
item
)
Objective-C 2.0
[
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]
In
Objective-C
2.0, not only garbage collection, but also
automatic reference counting
will be affected by weak references. All variables and properties in the following example are weak.
@interface
WeakRef
:
NSObject
{
__weak
NSString
*
str1
;
__unsafe_unretained
NSString
*
str2
;
}
@property
(
nonatomic
,
weak
)
NSString
*
str3
;
@property
(
nonatomic
,
unsafe_unretained
)
NSString
*
str4
;
@end
The difference between
weak
(
__weak
) and
unsafe_unretained
(
__unsafe_unretained
) is that when the object the variable pointed to is being deallocated, whether the value of the variable is going to be changed or not.
weak
ones will be updated to
nil
and the
unsafe_unretained
one will be left unchanged, as a
dangling pointer
. The
weak
references is added to Objective-C since
Mac OS X 10.7 "Lion"
and
iOS 5
, together with
Xcode
4.1 (4.2 for iOS), and only when using ARC. Older versions of Mac OS X, iOS, and GNUstep support only
unsafe_unretained
references as weak ones.
class
Node
{
public
weak
Node
prev
;
// a weak reference is used to avoid circular references between nodes of a doubly-linked list
public
Node
next
;
}
Python
[
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]
>>>
import
weakref
>>>
import
gc
>>>
class
Egg
:
...
def
spam
(
self
):
...
print
(
"I'm alive!"
)
...
>>>
obj
=
Egg
()
>>>
weak_obj
=
weakref
.
ref
(
obj
)
>>>
weak_obj
()
.
spam
()
I'm alive!
>>>
obj
=
"Something else"
>>>
gc
.
collect
()
35
>>>
weak_obj
()
.
spam
()
Traceback (most recent call last):
File
"<stdin>"
, line
1
, in
<module>
AttributeError
:
'NoneType' object has no attribute 'spam'
See also
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]
References
[
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]
External links
[
edit
]
Java
[
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]
Python
[
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]