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Concept
Indirection
Résumé
In computer programming, indirection (also called dereferencing) is the ability to reference something using a name, reference, or container instead of the value itself. The most common form of indirection is the act of manipulating a value through its memory address. For example, accessing a variable through the use of a pointer. A stored pointer that exists to provide a reference to an object by double indirection is called an indirection node. In some older computer architectures, indirect words supported a variety of more-or-less complicated addressing modes.
Another important example is the domain name system which enables names such as en.wikipedia.org to be used in place
of network addresses such as 208.80.154.224. The indirection from human-readable names to network addresses means that the references to a web page become more memorable, and links do not need to change when a web site is relocated to a different server.
A famous aphorism of Butler Lampson goes: "All problems in computer science can be solved by another level of indirection" (the "fundamental theorem of software engineering").
This is often deliberately mis-quoted with "abstraction layer" substituted for "level of indirection". An often cited corollary to this is, "...except for the problem of too many layers of indirection."
A humorous Internet memorandum, , insists that:
(6) It is easier to move a problem around (for example, by moving the problem to a different part of the overall network architecture) than it is to solve it.
(6a) (corollary). It is always possible to add another level of indirection.
Object-oriented programming makes use of indirection extensively, a simple example being dynamic dispatch. Higher-level examples of indirection are the design patterns of the proxy and the proxy server. Delegation is another classic example of an indirection pattern. In strongly typed interpreted languages with dynamic datatypes, most variable references require a level of indirection: first the type of the variable is checked for safety, and then the pointer to the actual value is dereferenced and acted on.
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