A weak reference is a reference that does not protect the referent object from collection by a garbage collector.
- An object referenced only by weak references is considered
unreachable (or "weakly reachable") and so may be collected at any
time.
- Weak references are used to avoid keeping memory referenced by
unneeded objects. Some garbage-collected languages feature or support
various levels of weak references, such as Java, C#, Python, Perl or
Lisp.
- Garbage collection is used to 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 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.
- Weak references are also used to minimize the number of unnecessary
objects in memory by allowing the program to indicate which objects
are not critical by only weakly referencing them.
