<s>
In	O
computer	B-General_Concept
programming	I-General_Concept
,	O
a	O
weak	B-General_Concept
reference	I-General_Concept
is	O
a	O
reference	O
that	O
does	O
not	O
protect	O
the	O
referenced	O
object	O
from	O
collection	O
by	O
a	O
garbage	B-General_Concept
collector	I-General_Concept
,	O
unlike	O
a	O
strong	B-General_Concept
reference	I-General_Concept
.	O
</s>
<s>
An	O
object	O
referenced	O
only	O
by	O
weak	B-General_Concept
references	I-General_Concept
–	O
meaning	O
"	O
every	O
chain	O
of	O
references	O
that	O
reaches	O
the	O
object	O
includes	O
at	O
least	O
one	O
weak	B-General_Concept
reference	I-General_Concept
as	O
a	O
link	O
"	O
–	O
is	O
considered	O
weakly	B-General_Concept
reachable	I-General_Concept
,	O
and	O
can	O
be	O
treated	O
as	O
unreachable	B-General_Concept
and	O
so	O
may	O
be	O
collected	O
at	O
any	O
time	O
.	O
</s>
<s>
Some	O
garbage-collected	O
languages	O
feature	O
or	O
support	O
various	O
levels	O
of	O
weak	B-General_Concept
references	I-General_Concept
,	O
such	O
as	O
C#	B-Application
,	O
Java	B-Language
,	O
Lisp	B-Language
,	O
OCaml	B-Language
,	O
Perl	B-Language
,	O
Python	B-Language
and	O
PHP	B-Application
since	O
the	O
version	O
7.4	O
.	O
</s>
<s>
Weak	B-General_Concept
references	I-General_Concept
have	O
a	O
number	O
of	O
common	O
uses	O
.	O
</s>
<s>
When	O
using	O
reference	B-General_Concept
counting	I-General_Concept
garbage	B-General_Concept
collection	I-General_Concept
,	O
weak	B-General_Concept
references	I-General_Concept
can	O
break	O
reference	O
cycles	O
,	O
by	O
using	O
a	O
weak	B-General_Concept
reference	I-General_Concept
for	O
a	O
link	O
in	O
the	O
cycle	O
.	O
</s>
<s>
When	O
one	O
has	O
an	O
associative	B-Application
array	I-Application
(	O
mapping	O
,	O
hash	B-Algorithm
map	I-Algorithm
)	O
whose	O
keys	O
are	O
(	O
references	O
to	O
)	O
objects	O
,	O
for	O
example	O
to	O
hold	O
auxiliary	O
data	O
about	O
objects	O
,	O
using	O
weak	B-General_Concept
references	I-General_Concept
for	O
the	O
keys	O
avoids	O
keeping	O
the	O
objects	O
alive	O
just	O
because	O
of	O
their	O
use	O
as	O
keys	O
.	O
</s>
<s>
When	O
one	O
has	O
an	O
object	O
where	O
other	O
objects	O
are	O
registered	O
,	O
such	O
as	O
in	O
the	O
observer	B-Language
pattern	I-Language
(	O
particularly	O
in	O
event	O
handling	O
)	O
,	O
if	O
a	O
strong	B-General_Concept
reference	I-General_Concept
is	O
kept	O
,	O
objects	O
must	O
be	O
explicitly	O
unregistered	O
,	O
otherwise	O
a	O
memory	B-Error_Name
leak	I-Error_Name
occurs	O
(	O
the	O
lapsed	B-Error_Name
listener	I-Error_Name
problem	I-Error_Name
)	O
,	O
while	O
a	O
weak	B-General_Concept
reference	I-General_Concept
removes	O
the	O
need	O
to	O
unregister	O
.	O
</s>
<s>
When	O
holding	O
cached	O
data	O
that	O
can	O
be	O
recreated	O
if	O
necessary	O
,	O
weak	B-General_Concept
references	I-General_Concept
allow	O
the	O
cache	B-General_Concept
to	O
be	O
reclaimed	O
,	O
effectively	O
producing	O
discardable	O
memory	O
.	O
</s>
<s>
This	O
last	O
case	O
(	O
a	O
cache	B-General_Concept
)	O
is	O
distinct	O
from	O
others	O
,	O
as	O
it	O
is	O
preferable	O
that	O
the	O
objects	O
only	O
be	O
garbage	B-General_Concept
collected	I-General_Concept
if	O
necessary	O
,	O
and	O
there	O
is	O
thus	O
a	O
need	O
for	O
finer	O
distinctions	O
within	O
weak	B-General_Concept
references	I-General_Concept
,	O
here	O
a	O
stronger	O
form	O
of	O
a	O
weak	B-General_Concept
reference	I-General_Concept
.	O
</s>
<s>
In	O
many	O
cases	O
weak	B-General_Concept
references	I-General_Concept
do	O
not	O
need	O
to	O
be	O
directly	O
used	O
,	O
instead	O
simply	O
using	O
a	O
weak	B-General_Concept
array	I-General_Concept
or	O
other	O
container	B-Application
whose	O
keys	O
or	O
values	O
are	O
weak	B-General_Concept
references	I-General_Concept
.	O
</s>
<s>
Garbage	B-General_Concept
collection	I-General_Concept
is	O
used	O
to	O
clean	O
up	O
unused	O
objects	O
and	O
so	O
reduce	O
the	O
potential	O
for	O
memory	B-Error_Name
leaks	I-Error_Name
and	O
data	O
corruption	O
.	O
</s>
<s>
There	O
are	O
two	O
main	O
types	O
of	O
garbage	B-General_Concept
collection	I-General_Concept
:	O
tracing	O
and	O
reference	B-General_Concept
counting	I-General_Concept
.	O
</s>
<s>
Reference	B-General_Concept
counting	I-General_Concept
schemes	O
record	O
the	O
number	O
of	O
references	O
to	O
a	O
given	O
object	O
and	O
collect	O
the	O
object	O
when	O
the	O
reference	B-General_Concept
count	I-General_Concept
becomes	O
zero	O
.	O
</s>
<s>
Groups	O
of	O
mutually	O
referencing	O
objects	O
which	O
are	O
not	O
directly	O
referenced	O
by	O
other	O
objects	O
and	O
are	O
unreachable	B-General_Concept
can	O
thus	O
become	O
permanently	O
resident	O
;	O
if	O
an	O
application	O
continually	O
generates	O
such	O
unreachable	B-General_Concept
groups	O
of	O
unreachable	B-General_Concept
objects	I-General_Concept
this	O
will	O
have	O
the	O
effect	O
of	O
a	O
memory	B-Error_Name
leak	I-Error_Name
.	O
</s>
<s>
Weak	B-General_Concept
references	I-General_Concept
(	O
references	O
which	O
are	O
not	O
counted	O
in	O
reference	B-General_Concept
counting	I-General_Concept
)	O
may	O
be	O
used	O
to	O
solve	O
the	O
problem	O
of	O
circular	O
references	O
if	O
the	O
reference	O
cycles	O
are	O
avoided	O
by	O
using	O
weak	B-General_Concept
references	I-General_Concept
for	O
some	O
of	O
the	O
references	O
within	O
the	O
group	O
.	O
</s>
<s>
A	O
very	O
common	O
case	O
of	O
such	O
strong	O
vs.	O
weak	B-General_Concept
reference	I-General_Concept
distinctions	O
is	O
in	O
tree	O
structures	O
,	O
such	O
as	O
the	O
Document	B-General_Concept
Object	I-General_Concept
Model	I-General_Concept
(	O
DOM	B-General_Concept
)	O
,	O
where	O
parent-to-child	O
references	O
are	O
strong	O
,	O
but	O
child-to-parent	O
references	O
are	O
weak	O
.	O
</s>
<s>
For	O
example	O
,	O
Apple	O
's	O
Cocoa	B-Operating_System
framework	O
recommends	O
this	O
approach	O
.	O
</s>
<s>
Indeed	O
,	O
even	O
when	O
the	O
object	O
graph	O
is	O
not	O
a	O
tree	O
,	O
a	O
tree	O
structure	O
can	O
often	O
be	O
imposed	O
by	O
the	O
notion	O
of	O
object	O
ownership	O
,	O
where	O
ownership	O
relationships	O
are	O
strong	O
and	O
form	O
a	O
tree	O
,	O
and	O
non-ownership	O
relationships	O
are	O
weak	O
and	O
not	O
needed	O
to	O
form	O
the	O
tree	O
–	O
this	O
approach	O
is	O
common	O
in	O
C++	B-Language
(	O
pre-C	O
++11	O
)	O
,	O
using	O
raw	O
pointers	O
as	O
weak	B-General_Concept
references	I-General_Concept
.	O
</s>
<s>
Since	O
the	O
C++11	B-Language
standard	O
,	O
a	O
solution	O
was	O
added	O
by	O
using	O
shared_ptr	O
and	O
weak_ptr	O
,	O
inherited	O
from	O
the	O
Boost	B-Language
library	I-Language
.	O
</s>
<s>
Weak	B-General_Concept
references	I-General_Concept
are	O
also	O
used	O
to	O
minimize	O
the	O
number	O
of	O
unnecessary	O
objects	O
in	O
memory	O
by	O
allowing	O
the	O
program	O
to	O
indicate	O
which	O
objects	O
are	O
of	O
minor	O
importance	O
by	O
only	O
weakly	O
referencing	O
them	O
.	O
</s>
<s>
Some	O
languages	O
have	O
multiple	O
levels	O
of	O
weak	B-General_Concept
reference	I-General_Concept
strength	O
.	O
</s>
<s>
For	O
example	O
,	O
Java	B-Language
has	O
,	O
in	O
order	O
of	O
decreasing	O
strength	O
,	O
soft	B-General_Concept
,	O
weak	O
,	O
and	O
phantom	B-General_Concept
references	I-General_Concept
,	O
defined	O
in	O
the	O
package	B-Language
java.lang.ref	O
.	O
</s>
<s>
The	O
garbage	B-General_Concept
collector	I-General_Concept
(	O
GC	O
)	O
uses	O
an	O
object	O
's	O
type	O
of	O
reachability	O
to	O
determine	O
when	O
to	O
free	O
the	O
object	O
.	O
</s>
<s>
It	O
is	O
safe	O
for	O
the	O
GC	O
to	O
free	O
an	O
object	O
that	O
is	O
softly	B-General_Concept
reachable	I-General_Concept
,	O
but	O
the	O
GC	O
may	O
decide	O
not	O
to	O
do	O
so	O
if	O
it	O
believes	O
the	O
JVM	O
can	O
spare	O
the	O
memory	O
(	O
e.g.	O
</s>
<s>
The	O
GC	O
will	O
free	O
a	O
weakly	B-General_Concept
reachable	I-General_Concept
object	O
as	O
soon	O
as	O
the	O
GC	O
notices	O
the	O
object	O
.	O
</s>
<s>
Unlike	O
the	O
other	O
reference	O
types	O
,	O
a	O
phantom	B-General_Concept
reference	I-General_Concept
cannot	O
be	O
followed	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
phantom	B-General_Concept
references	I-General_Concept
provide	O
a	O
mechanism	O
to	O
notify	O
the	O
program	O
when	O
an	O
object	O
has	O
been	O
freed	O
(	O
notification	O
is	O
implemented	O
using	O
ReferenceQueues	O
)	O
.	O
</s>
<s>
In	O
C#	B-Application
,	O
weak	B-General_Concept
references	I-General_Concept
are	O
distinguished	O
by	O
whether	O
they	O
track	O
object	B-General_Concept
resurrection	I-General_Concept
or	O
not	O
.	O
</s>
<s>
This	O
distinction	O
does	O
not	O
occur	O
for	O
strong	B-General_Concept
references	I-General_Concept
,	O
as	O
objects	O
are	O
not	O
finalized	B-Application
if	O
they	O
have	O
any	O
strong	B-General_Concept
references	I-General_Concept
to	O
them	O
.	O
</s>
<s>
By	O
default	O
,	O
in	O
C#	B-Application
weak	B-General_Concept
reference	I-General_Concept
do	O
not	O
track	O
resurrection	O
,	O
meaning	O
a	O
weak	B-General_Concept
reference	I-General_Concept
is	O
not	O
updated	O
if	O
an	O
object	O
is	O
resurrected	O
;	O
these	O
are	O
called	O
short	O
weak	B-General_Concept
references	I-General_Concept
,	O
and	O
weak	B-General_Concept
references	I-General_Concept
that	O
track	O
resurrection	O
are	O
called	O
long	O
weak	B-General_Concept
references	I-General_Concept
.	O
</s>
<s>
Some	O
non-garbage-collected	O
languages	O
,	O
such	O
as	O
C++	B-Language
,	O
provide	O
weak/strong	O
reference	O
functionality	O
as	O
part	O
of	O
supporting	O
garbage	B-General_Concept
collection	I-General_Concept
libraries	O
.	O
</s>
<s>
The	O
Boost	B-Language
C++	I-Language
library	O
provides	O
strong	O
and	O
weak	B-General_Concept
references	I-General_Concept
.	O
</s>
<s>
It	O
is	O
a	O
mistake	O
to	O
use	O
regular	O
C++	B-Language
pointers	O
as	O
the	O
weak	O
counterparts	O
of	O
smart	B-Language
pointers	I-Language
because	O
such	O
usage	O
removes	O
the	O
ability	O
to	O
detect	O
when	O
the	O
strong	B-General_Concept
reference	I-General_Concept
count	O
has	O
gone	O
to	O
0	O
and	O
the	O
object	O
has	O
been	O
deleted	O
.	O
</s>
<s>
Worse	O
yet	O
,	O
it	O
does	O
not	O
allow	O
for	O
detection	O
of	O
whether	O
another	O
strong	B-General_Concept
reference	I-General_Concept
is	O
already	O
tracking	O
a	O
given	O
plain	O
pointer	O
.	O
</s>
<s>
This	O
introduces	O
the	O
possibility	O
of	O
having	O
two	O
(	O
or	O
more	O
)	O
smart	B-Language
pointers	I-Language
tracking	O
the	O
same	O
plain	O
pointer	O
(	O
which	O
causes	O
corruption	O
as	O
soon	O
as	O
one	O
of	O
these	O
smart	B-Language
pointers	I-Language
 '	O
reference	B-General_Concept
count	I-General_Concept
reaches	O
0	O
and	O
the	O
object	O
gets	O
deleted	O
)	O
.	O
</s>
<s>
Weak	B-General_Concept
references	I-General_Concept
can	O
be	O
useful	O
when	O
keeping	O
a	O
list	O
of	O
the	O
current	O
variables	O
being	O
referenced	O
in	O
the	O
application	O
.	O
</s>
<s>
Java	B-Language
1.2	O
in	O
1998	O
introduced	O
two	O
kinds	O
of	O
weak	B-General_Concept
references	I-General_Concept
,	O
one	O
known	O
as	O
a	O
"	O
soft	B-General_Concept
reference	I-General_Concept
"	O
(	O
intended	O
to	O
be	O
used	O
for	O
maintaining	O
GC-managed	O
in-memory	O
caches	O
,	O
but	O
which	O
does	O
n't	O
work	O
very	O
well	O
in	O
practice	O
on	O
some	O
platforms	O
with	O
dynamic	O
heap	O
like	O
Android	O
)	O
and	O
the	O
other	O
simply	O
as	O
a	O
"	O
weak	B-General_Concept
reference	I-General_Concept
"	O
.	O
</s>
<s>
It	O
also	O
added	O
a	O
related	O
experimental	O
mechanism	O
dubbed	O
"	O
phantom	B-General_Concept
references	I-General_Concept
"	O
as	O
an	O
alternative	O
to	O
the	O
dangerous	O
and	O
inefficient	O
finalize( )	O
mechanism	O
.	O
</s>
<s>
If	O
a	O
weak	B-General_Concept
reference	I-General_Concept
is	O
created	O
,	O
and	O
then	O
elsewhere	O
in	O
the	O
code	O
get( )	O
is	O
used	O
to	O
get	O
the	O
actual	O
object	O
,	O
the	O
weak	B-General_Concept
reference	I-General_Concept
is	O
not	O
strong	O
enough	O
to	O
prevent	O
garbage	B-General_Concept
collection	I-General_Concept
,	O
so	O
it	O
may	O
be	O
(	O
if	O
there	O
are	O
no	O
strong	B-General_Concept
references	I-General_Concept
to	O
the	O
object	O
)	O
that	O
get( )	O
suddenly	O
starts	O
returning	O
null	O
.	O
</s>
<s>
Another	O
use	O
of	O
weak	B-General_Concept
references	I-General_Concept
is	O
in	O
writing	O
a	O
cache	B-General_Concept
.	O
</s>
<s>
Using	O
,	O
for	O
example	O
,	O
a	O
weak	O
hash	B-Algorithm
map	I-Algorithm
,	O
one	O
can	O
store	O
in	O
the	O
cache	B-General_Concept
the	O
various	O
referred	O
objects	O
via	O
a	O
weak	B-General_Concept
reference	I-General_Concept
.	O
</s>
<s>
When	O
the	O
garbage	B-General_Concept
collector	I-General_Concept
runs	O
when	O
for	O
example	O
the	O
application	O
's	O
memory	O
usage	O
gets	O
sufficiently	O
high	O
those	O
cached	O
objects	O
which	O
are	O
no	O
longer	O
directly	O
referenced	O
by	O
other	O
objects	O
are	O
removed	O
from	O
the	O
cache	B-General_Concept
.	O
</s>
<s>
In	O
Objective-C	B-Language
2.0	O
,	O
not	O
only	O
garbage	B-General_Concept
collection	I-General_Concept
,	O
but	O
also	O
automatic	B-General_Concept
reference	I-General_Concept
counting	I-General_Concept
will	O
be	O
affected	O
by	O
weak	B-General_Concept
references	I-General_Concept
.	O
</s>
<s>
weak	O
ones	O
will	O
be	O
updated	O
to	O
nil	O
and	O
the	O
unsafe_unretained	O
one	O
will	O
be	O
left	O
unchanged	O
,	O
as	O
a	O
dangling	B-Error_Name
pointer	I-Error_Name
.	O
</s>
<s>
The	O
weak	B-General_Concept
references	I-General_Concept
is	O
added	O
to	O
Objective-C	B-Language
since	O
Mac	O
OS	O
X	O
10.7	B-Application
"	O
Lion	B-Application
"	O
and	O
iOS	B-Operating_System
5	I-Operating_System
,	O
together	O
with	O
Xcode	B-Operating_System
4.1	O
(	O
4.2	O
for	O
iOS	O
)	O
,	O
and	O
only	O
when	O
using	O
ARC	O
.	O
</s>
