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In languages without primitive types (where everything is an object), all fields of the copy B are references to the same objects as the fields of original A. The referenced objects are thus shared , so if one of these objects is modified (from A or B), the change is visible in the other.
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 ...
In some programming language environments (at least one proprietary Lisp implementation, for example), [citation needed] the value used as the null pointer (called nil in Lisp) may actually be a pointer to a block of internal data useful to the implementation (but not explicitly reachable from user programs), thus allowing the same register to be used as a useful constant and a quick way of ...
For example, strings and arrays are passed by reference, but when modified, they are duplicated if they have non-zero reference counts. This allows them to act as value types without the performance problems of copying on assignment or making them immutable. [8] In the Qt framework, many types are copy-on-write ("implicitly shared" in Qt's terms).
In this Java example, the Printer class has a print method. This print method, rather than performing the print itself, forwards to an object of class RealPrinter . To the outside world it appears that the Printer object is doing the print, but the RealPrinter object is the one actually doing the work.
Python sets are very much like mathematical sets, and support operations like set intersection and union. Python also features a frozenset class for immutable sets, see Collection types. Dictionaries (class dict) are mutable mappings tying keys and corresponding values. Python has special syntax to create dictionaries ({key: value})
If such a reference is destroyed quickly, then there is no need to update the counter. This eliminates a large number of updates associated with short-lived references (such as the above list-length-counting example). However, if such a reference is copied into a data structure, then the deferred increment must be performed at that time.
Circular references can appear in computer programming when one piece of code requires the result from another, but that code needs the result from the first. For example, the two functions, posn and plus1 in the following Python program comprise a circular reference: [further explanation needed]