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class="noprint">From Wikipedia, the free encyclopedia</div> </div> <div id="contentSub"><div id="mw-content-subtitle"></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Computer programming construct</div> <style data-mw-deduplicate="TemplateStyles:r1251242444">.mw-parser-output .ambox{border:1px solid #a2a9b1;border-left:10px solid #36c;background-color:#fbfbfb;box-sizing:border-box}.mw-parser-output .ambox+link+.ambox,.mw-parser-output .ambox+link+style+.ambox,.mw-parser-output .ambox+link+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+style+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+link+.ambox{margin-top:-1px}html body.mediawiki .mw-parser-output .ambox.mbox-small-left{margin:4px 1em 4px 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.ambox{display:none!important}}</style><table class="box-More_citations_needed plainlinks metadata ambox ambox-content ambox-Refimprove" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><a href="/wiki/File:Question_book-new.svg" class="mw-file-description"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/50px-Question_book-new.svg.png" decoding="async" width="50" height="39" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/75px-Question_book-new.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/100px-Question_book-new.svg.png 2x" data-file-width="512" data-file-height="399" /></a></span></div></td><td class="mbox-text"><div class="mbox-text-span">This article <b>needs additional citations for <a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability">verification</a></b>.<span class="hide-when-compact"> Please help <a href="/wiki/Special:EditPage/Indirection" title="Special:EditPage/Indirection">improve this article</a> by <a href="/wiki/Help:Referencing_for_beginners" title="Help:Referencing for beginners">adding citations to reliable sources</a>. Unsourced material may be challenged and removed.<br /><small><span class="plainlinks"><i>Find sources:</i> <a rel="nofollow" class="external text" href="https://www.google.com/search?as_eq=wikipedia&q=%22Indirection%22">"Indirection"</a> – <a rel="nofollow" class="external text" href="https://www.google.com/search?tbm=nws&q=%22Indirection%22+-wikipedia&tbs=ar:1">news</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.google.com/search?&q=%22Indirection%22&tbs=bkt:s&tbm=bks">newspapers</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.google.com/search?tbs=bks:1&q=%22Indirection%22+-wikipedia">books</a> <b>·</b> <a rel="nofollow" class="external text" href="https://scholar.google.com/scholar?q=%22Indirection%22">scholar</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.jstor.org/action/doBasicSearch?Query=%22Indirection%22&acc=on&wc=on">JSTOR</a></span></small></span> <span class="date-container"><i>(<span class="date">February 2022</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <p>In <a href="/wiki/Computer_programming" title="Computer programming">computer programming</a>, an <b>indirection</b> (also called a <b>reference</b>) is a way of referring to 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 <a href="/wiki/Memory_address" title="Memory address">memory address</a>. For example, accessing a <a href="/wiki/Variable_(programming)" class="mw-redirect" title="Variable (programming)">variable</a> through the use of a <a href="/wiki/Pointer_(computer_programming)" title="Pointer (computer programming)">pointer</a>. A stored pointer that exists to provide a reference to an object by double indirection is called an <i>indirection node</i>. In some older computer architectures, indirect words supported a variety of more-or-less complicated <a href="/wiki/Addressing_mode" title="Addressing mode">addressing modes</a>. </p><p>Another important example is the <a href="/wiki/Domain_name_system" class="mw-redirect" title="Domain name system">domain name system</a> which enables names such as <code>en.wikipedia.org</code> to be used in place of network addresses such as <code>208.80.154.224</code>. 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. </p> <div class="mw-heading mw-heading2"><h2 id="Overview">Overview</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Indirection&action=edit&section=1" title="Edit section: Overview"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A famous <a href="/wiki/Aphorism" title="Aphorism">aphorism</a> of <a href="/wiki/Butler_Lampson" title="Butler Lampson">Butler Lampson</a> that is attributed to <a href="/wiki/David_Wheeler_(computer_scientist)" title="David Wheeler (computer scientist)">David Wheeler</a> goes: "All problems in computer science can be solved by another level of indirection" (the "<a href="/wiki/Fundamental_theorem_of_software_engineering" title="Fundamental theorem of software engineering">fundamental theorem of software engineering</a>").<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> This is often deliberately mis-quoted<sup class="noprint Inline-Template" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Manual_of_Style/Words_to_watch#Unsupported_attributions" title="Wikipedia:Manual of Style/Words to watch"><span title="The material near this tag may use weasel words or too-vague attribution. (July 2024)">by whom?</span></a></i>]</sup> with "<a href="/wiki/Abstraction_layer" title="Abstraction layer">abstraction layer</a>" substituted for "level of indirection". A <a href="/wiki/Corollary" title="Corollary">corollary</a> to this aphorism, and the original conclusion from Wheeler, is "...except for the problem of too many layers of indirection." </p><p>A <a href="/wiki/April_Fools%27_Day_Request_for_Comments" title="April Fools' Day Request for Comments">humorous Internet memorandum</a>, <style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><a href="/wiki/RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc1925">1925</a>, insists that: </p> <style data-mw-deduplicate="TemplateStyles:r1244412712">.mw-parser-output .templatequote{overflow:hidden;margin:1em 0;padding:0 32px}.mw-parser-output .templatequotecite{line-height:1.5em;text-align:left;margin-top:0}@media(min-width:500px){.mw-parser-output .templatequotecite{padding-left:1.6em}}</style><blockquote class="templatequote"><p>(6) It is easier to move a problem around (for example, by moving the problem to a different part of the overall <a href="/wiki/Network_architecture" title="Network architecture">network architecture</a>) than it is to solve it. </p><dl><dd>(6a) (corollary). It is always possible to add another level of indirection.</dd></dl></blockquote> <p><a href="/wiki/Object-oriented_programming" title="Object-oriented programming">Object-oriented programming</a> makes use of indirection extensively, a simple example being <a href="/wiki/Dynamic_dispatch" title="Dynamic dispatch">dynamic dispatch</a>. Higher-level examples of indirection are the <a href="/wiki/Design_pattern_(computer_science)" class="mw-redirect" title="Design pattern (computer science)">design patterns</a> of the <a href="/wiki/Proxy_pattern" title="Proxy pattern">proxy</a> and the <a href="/wiki/Proxy_server" title="Proxy server">proxy server</a>. <a href="/wiki/Delegation_(programming)" class="mw-redirect" title="Delegation (programming)">Delegation</a> is another classic example of an indirection pattern. In <a href="/wiki/Strongly_typed_programming_language" class="mw-redirect" title="Strongly typed programming language">strongly typed</a> <a href="/wiki/Interpreted_language" class="mw-redirect" title="Interpreted language">interpreted languages</a> with dynamic <a href="/wiki/Data_type" title="Data type">data types</a>, 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. </p><p><a href="/wiki/Recursive_data_type" title="Recursive data type">Recursive data types</a> 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. </p><p>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 <i>x</i>, <i>y</i> and <i>z</i> in an equation such as <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\textstyle z={\sqrt {x^{2}+y^{2}}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="false" scriptlevel="0"> <mi>z</mi> <mo>=</mo> <mrow class="MJX-TeXAtom-ORD"> <msqrt> <msup> <mi>x</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msup> <mo>+</mo> <msup> <mi>y</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msup> </msqrt> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\textstyle z={\sqrt {x^{2}+y^{2}}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/52e9b960c2983658066e13b9fd2c81cafba5edf6" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; width:13.949ex; height:3.509ex;" alt="{\textstyle z={\sqrt {x^{2}+y^{2}}}}"></span> can refer to any number. One could imagine objects for various numbers and then <i>x</i>, <i>y</i> and <i>z</i> 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 <i>α</i> can refer to any formula, so it could be <i>β</i>, <i>γ</i>, <i>δ</i>, ... or <i>η</i>→<i>π</i>, <i>ς</i> ∨ <i>σ</i>, ... When <a href="/wiki/Set-builder_notation" title="Set-builder notation">set-builder notation</a> is employed the statement Δ={<i>α</i>} means the set of all formulae — so although the reference is to <i>α</i> there are two levels of indirection here, the first to the set of all <i>α</i> and then the second to a specific formula for each occurrence of <i>α</i> in the set Δ. </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Indirection&action=edit&section=2" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Handle_(computing)" title="Handle (computing)">Handle</a></li> <li><a href="/wiki/Delegation_pattern" title="Delegation pattern">Delegation pattern</a></li> <li><a href="/wiki/Pointer_(computer_programming)" title="Pointer (computer programming)">Pointer</a></li> <li><a href="/wiki/Reference_(computer_science)" title="Reference (computer science)">Reference</a></li> <li><a href="/wiki/Dereference_operator" class="mw-redirect" title="Dereference operator">Dereference operator</a></li> <li><a href="/wiki/Law_of_Demeter" title="Law of Demeter">Law of Demeter</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Indirection&action=edit&section=3" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFSpinellis2007" class="citation book cs1">Spinellis, Diomidis (2007). "Another level of indirection". In Oram, Andy; Wilson, Greg (eds.). <a rel="nofollow" class="external text" href="http://www.dmst.aueb.gr/dds/pubs/inbook/beautiful_code/html/Spi07g.html"><i>Beautiful Code: Leading Programmers Explain How They Think</i></a>. Sebastopol, California: O'Reilly and Associates. pp. 279–291.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=Another+level+of+indirection&rft.btitle=Beautiful+Code%3A+Leading+Programmers+Explain+How+They+Think&rft.place=Sebastopol%2C+California&rft.pages=279-291&rft.pub=O%27Reilly+and+Associates&rft.date=2007&rft.aulast=Spinellis&rft.aufirst=Diomidis&rft_id=http%3A%2F%2Fwww.dmst.aueb.gr%2Fdds%2Fpubs%2Finbook%2Fbeautiful_code%2Fhtml%2FSpi07g.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AIndirection" class="Z3988"></span></span> </li> </ol></div></div>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?type=1x1" alt="" width="1" height="1" style="border: none; position: absolute;"></noscript> <div class="printfooter" data-nosnippet="">Retrieved from "<a dir="ltr" href="https://en.wikipedia.org/w/index.php?title=Indirection&oldid=1232446174">https://en.wikipedia.org/w/index.php?title=Indirection&oldid=1232446174</a>"</div></div> <div id="catlinks" class="catlinks" data-mw="interface"><div id="mw-normal-catlinks" class="mw-normal-catlinks"><a href="/wiki/Help:Category" title="Help:Category">Categories</a>: <ul><li><a href="/wiki/Category:Data_types" title="Category:Data types">Data types</a></li><li><a href="/wiki/Category:Programming_constructs" title="Category:Programming constructs">Programming constructs</a></li><li><a href="/wiki/Category:Computing_terminology" title="Category:Computing terminology">Computing terminology</a></li><li><a href="/wiki/Category:Unary_operations" title="Category:Unary operations">Unary operations</a></li></ul></div><div id="mw-hidden-catlinks" class="mw-hidden-catlinks mw-hidden-cats-hidden">Hidden categories: <ul><li><a href="/wiki/Category:Articles_with_short_description" title="Category:Articles with short description">Articles with short description</a></li><li><a href="/wiki/Category:Short_description_is_different_from_Wikidata" title="Category:Short description is different from Wikidata">Short description is different from Wikidata</a></li><li><a href="/wiki/Category:Articles_needing_additional_references_from_February_2022" title="Category:Articles needing additional references from February 2022">Articles needing additional references from February 2022</a></li><li><a href="/wiki/Category:All_articles_needing_additional_references" title="Category:All articles needing additional references">All articles needing additional references</a></li><li><a href="/wiki/Category:Articles_with_specifically_marked_weasel-worded_phrases_from_July_2024" title="Category:Articles with specifically marked weasel-worded phrases from July 2024">Articles with specifically marked weasel-worded phrases from July 2024</a></li></ul></div></div> </div> </main> </div> <div class="mw-footer-container"> <footer id="footer" class="mw-footer" > <ul id="footer-info"> <li id="footer-info-lastmod"> This page was last edited on 3 July 2024, at 19:41<span class="anonymous-show"> (UTC)</span>.</li> <li id="footer-info-copyright">Text is available under the <a href="/wiki/Wikipedia:Text_of_the_Creative_Commons_Attribution-ShareAlike_4.0_International_License" title="Wikipedia:Text of the Creative Commons Attribution-ShareAlike 4.0 International License">Creative Commons Attribution-ShareAlike 4.0 License</a>; additional terms may apply. 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