Indirection
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In
Another important example is the
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.
Overview
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").[1] 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
(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.
Recursive data types are usually implemented using indirection, because otherwise if a value of a data type can contain the entirety of another value of the same data type, there is no limit to the size a value of this data type could need.
When doing symbolic programming from a formal mathematical specification the use of indirection can be quite helpful. To start with a simple example the variables x, y and z in an equation such as can refer to any number. One could imagine objects for various numbers and then x, y and z could point to the specific numbers being used for a particular problem. The simple example has its limitation as there are infinitely many real numbers. In various other parts of symbolic programming there are only so many symbols. So to move on to a more significant example, in logic the formula α can refer to any formula, so it could be β, γ, δ, ... or η→π, ς ∨ σ, ... When set-builder notation is employed the statement Δ={α} means the set of all formulae — so although the reference is to α there are two levels of indirection here, the first to the set of all α and then the second to a specific formula for each occurrence of α in the set Δ.
See also
- Handle
- Delegation pattern
- Pointer
- Reference
- Dereference operator
- Law of Demeter
References
- ^ Spinellis, Diomidis (2007). "Another level of indirection". In Oram, Andy; Wilson, Greg (eds.). Beautiful Code: Leading Programmers Explain How They Think. Sebastopol, California: O'Reilly and Associates. pp. 279–291.