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vector-toc-level-2"> <a class="vector-toc-link" href="#Polymorphism"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.3</span> <span>Polymorphism</span> </div> </a> <ul id="toc-Polymorphism-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Conditional_action" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Conditional_action"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.4</span> <span>Conditional action</span> </div> </a> <ul id="toc-Conditional_action-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Implementation" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Implementation"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>Implementation</span> </div> </a> <ul id="toc-Implementation-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Example_code" class="vector-toc-list-item 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id="toc-C++-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-C#" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#C#"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.3</span> <span>C#</span> </div> </a> <ul id="toc-C#-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Kotlin" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Kotlin"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.4</span> <span>Kotlin</span> </div> </a> <ul id="toc-Kotlin-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-JavaScript" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#JavaScript"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.5</span> <span>JavaScript</span> </div> </a> <ul id="toc-JavaScript-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Red_and_REBOL" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Red_and_REBOL"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.6</span> <span>Red and REBOL</span> </div> </a> <ul id="toc-Red_and_REBOL-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Lua" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Lua"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.7</span> <span>Lua</span> </div> </a> <ul id="toc-Lua-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Python" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Python"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.8</span> <span>Python</span> </div> </a> <ul id="toc-Python-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Julia" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Julia"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.9</span> <span>Julia</span> </div> </a> <ul id="toc-Julia-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-See_also" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#See_also"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>See also</span> </div> </a> <ul id="toc-See_also-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>References</span> </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-External_links" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#External_links"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>External links</span> </div> </a> <ul id="toc-External_links-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet mw-ui-icon-wikimedia-listBullet"></span> <span class="vector-dropdown-label-text">Toggle the table of contents</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading"><span class="mw-page-title-main">Callback (computer programming)</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="Go to an article in another language. Available in 20 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-20" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">20 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-zh-min-nan mw-list-item"><a href="https://zh-min-nan.wikipedia.org/wiki/Callback" title="Callback – Minnan" lang="nan" hreflang="nan" data-title="Callback" data-language-autonym="閩南語 / Bân-lâm-gú" data-language-local-name="Minnan" class="interlanguage-link-target"><span>閩南語 / Bân-lâm-gú</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/Callback_(programaci%C3%B3)" title="Callback (programació) – Catalan" lang="ca" hreflang="ca" data-title="Callback (programació)" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/R%C3%BCckruffunktion" title="Rückruffunktion – German" lang="de" hreflang="de" data-title="Rückruffunktion" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-el mw-list-item"><a href="https://el.wikipedia.org/wiki/Callback_(%CF%80%CF%81%CE%BF%CE%B3%CF%81%CE%B1%CE%BC%CE%BC%CE%B1%CF%84%CE%B9%CF%83%CE%BC%CF%8C%CF%82)" title="Callback (προγραμματισμός) – Greek" lang="el" hreflang="el" data-title="Callback (προγραμματισμός)" data-language-autonym="Ελληνικά" data-language-local-name="Greek" class="interlanguage-link-target"><span>Ελληνικά</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Retrollamada_(inform%C3%A1tica)" title="Retrollamada (informática) – Spanish" lang="es" hreflang="es" data-title="Retrollamada (informática)" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%D8%A8%D8%A7%D8%B2%D8%AE%D9%88%D8%A7%D9%86%DB%8C_(%D8%A8%D8%B1%D9%86%D8%A7%D9%85%D9%87%E2%80%8C%D9%86%D9%88%DB%8C%D8%B3%DB%8C_%D8%B1%D8%A7%DB%8C%D8%A7%D9%86%D9%87)" title="بازخوانی (برنامه‌نویسی رایانه) – Persian" lang="fa" hreflang="fa" data-title="بازخوانی (برنامه‌نویسی رایانه)" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Fonction_de_rappel" title="Fonction de rappel – French" lang="fr" hreflang="fr" data-title="Fonction de rappel" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/%EC%BD%9C%EB%B0%B1" title="콜백 – Korean" lang="ko" hreflang="ko" data-title="콜백" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/Callback" title="Callback – Italian" lang="it" hreflang="it" data-title="Callback" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E3%82%B3%E3%83%BC%E3%83%AB%E3%83%90%E3%83%83%E3%82%AF_(%E6%83%85%E5%A0%B1%E5%B7%A5%E5%AD%A6)" title="コールバック (情報工学) – Japanese" lang="ja" hreflang="ja" data-title="コールバック (情報工学)" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-nn mw-list-item"><a href="https://nn.wikipedia.org/wiki/Attendekall" title="Attendekall – Norwegian Nynorsk" lang="nn" hreflang="nn" data-title="Attendekall" data-language-autonym="Norsk nynorsk" data-language-local-name="Norwegian Nynorsk" class="interlanguage-link-target"><span>Norsk nynorsk</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/Wywo%C5%82anie_zwrotne" title="Wywołanie zwrotne – Polish" lang="pl" hreflang="pl" data-title="Wywołanie zwrotne" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/Callback" title="Callback – Portuguese" lang="pt" hreflang="pt" data-title="Callback" data-language-autonym="Português" data-language-local-name="Portuguese" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/Callback_(%D0%BF%D1%80%D0%BE%D0%B3%D1%80%D0%B0%D0%BC%D0%BC%D0%B8%D1%80%D0%BE%D0%B2%D0%B0%D0%BD%D0%B8%D0%B5)" title="Callback (программирование) – Russian" lang="ru" hreflang="ru" data-title="Callback (программирование)" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-ta mw-list-item"><a href="https://ta.wikipedia.org/wiki/%E0%AE%A4%E0%AE%BF%E0%AE%B0%E0%AF%81%E0%AE%AA%E0%AF%8D%E0%AE%AA%E0%AE%BF_%E0%AE%85%E0%AE%B4%E0%AF%88_(%E0%AE%95%E0%AE%A3%E0%AE%BF%E0%AE%A9%E0%AE%BF%E0%AE%AF%E0%AE%BF%E0%AE%AF%E0%AE%B2%E0%AF%8D)" title="திருப்பி அழை (கணினியியல்) – Tamil" lang="ta" hreflang="ta" data-title="திருப்பி அழை (கணினியியல்)" data-language-autonym="தமிழ்" data-language-local-name="Tamil" class="interlanguage-link-target"><span>தமிழ்</span></a></li><li class="interlanguage-link interwiki-th mw-list-item"><a href="https://th.wikipedia.org/wiki/%E0%B8%81%E0%B8%B2%E0%B8%A3%E0%B9%80%E0%B8%A3%E0%B8%B5%E0%B8%A2%E0%B8%81%E0%B8%81%E0%B8%A5%E0%B8%B1%E0%B8%9A_(%E0%B8%81%E0%B8%B2%E0%B8%A3%E0%B9%80%E0%B8%82%E0%B8%B5%E0%B8%A2%E0%B8%99%E0%B9%82%E0%B8%9B%E0%B8%A3%E0%B9%81%E0%B8%81%E0%B8%A3%E0%B8%A1)" title="การเรียกกลับ (การเขียนโปรแกรม) – Thai" lang="th" hreflang="th" data-title="การเรียกกลับ (การเขียนโปรแกรม)" data-language-autonym="ไทย" data-language-local-name="Thai" class="interlanguage-link-target"><span>ไทย</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/Callback_(%D0%BF%D1%80%D0%BE%D0%B3%D1%80%D0%B0%D0%BC%D1%83%D0%B2%D0%B0%D0%BD%D0%BD%D1%8F)" title="Callback (програмування) – Ukrainian" lang="uk" hreflang="uk" data-title="Callback (програмування)" data-language-autonym="Українська" data-language-local-name="Ukrainian" class="interlanguage-link-target"><span>Українська</span></a></li><li class="interlanguage-link interwiki-vi mw-list-item"><a href="https://vi.wikipedia.org/wiki/Callback" title="Callback – Vietnamese" lang="vi" hreflang="vi" data-title="Callback" data-language-autonym="Tiếng Việt" data-language-local-name="Vietnamese" class="interlanguage-link-target"><span>Tiếng Việt</span></a></li><li class="interlanguage-link interwiki-zh-yue mw-list-item"><a href="https://zh-yue.wikipedia.org/wiki/%E5%9B%9E%E5%91%BC%E5%87%BD%E6%95%B8" title="回呼函數 – Cantonese" lang="yue" hreflang="yue" data-title="回呼函數" data-language-autonym="粵語" data-language-local-name="Cantonese" class="interlanguage-link-target"><span>粵語</span></a></li><li class="interlanguage-link interwiki-zh mw-list-item"><a href="https://zh.wikipedia.org/wiki/%E5%9B%9E%E8%B0%83%E5%87%BD%E6%95%B0" title="回调函数 – Chinese" lang="zh" hreflang="zh" data-title="回调函数" data-language-autonym="中文" data-language-local-name="Chinese" class="interlanguage-link-target"><span>中文</span></a></li> </ul> <div class="after-portlet after-portlet-lang"><span class="wb-langlinks-edit wb-langlinks-link"><a href="https://www.wikidata.org/wiki/Special:EntityPage/Q1205349#sitelinks-wikipedia" title="Edit interlanguage links" class="wbc-editpage">Edit links</a></span></div> </div> </div> </div> </header> <div class="vector-page-toolbar"> <div class="vector-page-toolbar-container"> <div id="left-navigation"> <nav aria-label="Namespaces"> <div id="p-associated-pages" class="vector-menu vector-menu-tabs mw-portlet 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.hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">For a discussion of callback with computer <a href="/wiki/Modem" title="Modem">modems</a>, see <a href="/wiki/Callback_(telecommunications)" title="Callback (telecommunications)">Callback (telecommunications)</a>.</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 0;overflow:hidden;width:238px;border-collapse:collapse;font-size:88%;line-height:1.25em}.mw-parser-output .ambox-speedy{border-left:10px solid #b32424;background-color:#fee7e6}.mw-parser-output .ambox-delete{border-left:10px solid #b32424}.mw-parser-output .ambox-content{border-left:10px solid #f28500}.mw-parser-output .ambox-style{border-left:10px solid #fc3}.mw-parser-output .ambox-move{border-left:10px solid #9932cc}.mw-parser-output .ambox-protection{border-left:10px solid #a2a9b1}.mw-parser-output .ambox .mbox-text{border:none;padding:0.25em 0.5em;width:100%}.mw-parser-output .ambox .mbox-image{border:none;padding:2px 0 2px 0.5em;text-align:center}.mw-parser-output .ambox .mbox-imageright{border:none;padding:2px 0.5em 2px 0;text-align:center}.mw-parser-output .ambox .mbox-empty-cell{border:none;padding:0;width:1px}.mw-parser-output .ambox .mbox-image-div{width:52px}@media(min-width:720px){.mw-parser-output .ambox{margin:0 10%}}@media print{body.ns-0 .mw-parser-output .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/Callback_(computer_programming)" title="Special:EditPage/Callback (computer programming)">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=%22Callback%22+computer+programming">"Callback" computer programming</a> – <a rel="nofollow" class="external text" href="https://www.google.com/search?tbm=nws&q=%22Callback%22+computer+programming+-wikipedia&tbs=ar:1">news</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.google.com/search?&q=%22Callback%22+computer+programming&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=%22Callback%22+computer+programming+-wikipedia">books</a> <b>·</b> <a rel="nofollow" class="external text" href="https://scholar.google.com/scholar?q=%22Callback%22+computer+programming">scholar</a> <b>·</b> <a rel="nofollow" class="external text" href="https://www.jstor.org/action/doBasicSearch?Query=%22Callback%22+computer+programming&acc=on&wc=on">JSTOR</a></span></small></span> <span class="date-container"><i>(<span class="date">September 2015</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> <figure typeof="mw:File/Thumb"><a href="/wiki/File:Callback-notitle.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/d/d4/Callback-notitle.svg/370px-Callback-notitle.svg.png" decoding="async" width="370" height="104" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/d/d4/Callback-notitle.svg/555px-Callback-notitle.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/d/d4/Callback-notitle.svg/740px-Callback-notitle.svg.png 2x" data-file-width="512" data-file-height="144" /></a><figcaption>A callback is often back on the level of the original caller.</figcaption></figure> <p>In <a href="/wiki/Computer_programming" title="Computer programming">computer programming</a>, a <b>callback</b> is a <a href="/wiki/Function_(computer_programming)" title="Function (computer programming)">function</a> that is stored as data (a <a href="/wiki/Reference_(computer_science)" title="Reference (computer science)">reference</a>) and designed to be called by another function – often <i>back</i> to the original <a href="/wiki/Abstraction_(computer_science)" title="Abstraction (computer science)">abstraction layer</a>. </p><p>A function that accepts a callback <a href="/wiki/Parameter_(computer_programming)" title="Parameter (computer programming)">parameter</a> may be designed to call back before <a href="/wiki/Return_statement" title="Return statement">returning</a> to its caller which is known as <i><a href="/wiki/Synchronization_(computer_science)" title="Synchronization (computer science)">synchronous</a></i> or <i>blocking</i>. The function that accepts a callback may be designed to store the callback so that it can be called back after returning which is known as <i>asynchronous</i>, <i><a href="/wiki/Non-blocking_algorithm" title="Non-blocking algorithm">non-blocking</a></i> or <i>deferred</i>. </p><p><a href="/wiki/Programming_languages" class="mw-redirect" title="Programming languages">Programming languages</a> support callbacks in different ways such as <a href="/wiki/Function_pointers" class="mw-redirect" title="Function pointers">function pointers</a>, <a href="/wiki/Lambda_(programming)" class="mw-redirect" title="Lambda (programming)">lambda expressions</a> and <a href="/wiki/Block_(programming)" title="Block (programming)">blocks</a>. </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Analogy">Analogy</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=1" title="Edit section: Analogy"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>To aid understanding the concept, the following is an analogy from real life. </p><p>A customer visits a store to place an order. This is like the first call. </p><p>The customer gives to a clerk a list of items, a check to cover their cost and delivery instructions. These are the parameters of the first call including the callback which is the delivery instructions. It is understood that the check will be cashed and that the instructions will be followed. </p><p>When the staff are able, they deliver the items as instructed which is like calling the callback. </p><p>Notably, the delivery need not be made by the clerk who took the order. A callback need not be called by the function that accepted the callback as a parameter. </p><p>Also, the delivery need not be made directly to the customer. A callback need not be to the calling function. In fact, a function would generally not pass itself as a callback. Some find the use of <i>back</i> to be misleading since the call is (generally) not back to the original caller as it is for a <a href="/wiki/Callback_(telecommunications)" title="Callback (telecommunications)">telephone call</a>. </p> <div class="mw-heading mw-heading2"><h2 id="Use"><span class="anchor" id="TYPES"></span>Use</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=2" title="Edit section: Use"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A blocking callback runs in the <a href="/wiki/Execution_(computing)" title="Execution (computing)">execution</a> context of the function that passes the callback. A deferred callback can run in a different context such as during <a href="/wiki/Interrupt" title="Interrupt">interrupt</a> or from a <a href="/wiki/Thread_(computing)" title="Thread (computing)">thread</a>. As such, a deferred callback can be used for synchronization and delegating work to another thread. </p> <div class="mw-heading mw-heading3"><h3 id="Event_handling">Event handling</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=3" title="Edit section: Event handling"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A callback can be used for event handling. Often, consuming code registers a callback for a particular type of event. When that event occurs, the callback is called. </p><p>Callbacks are often used to program the <a href="/wiki/Graphical_user_interface" title="Graphical user interface">graphical user interface</a> (GUI) of a program that runs in a <a href="/wiki/Windowing_system" title="Windowing system">windowing system</a>. The application supplies a reference to a custom callback function for the windowing system to call. The windowing system calls this function to notify the application of events like <a href="/wiki/Computer_mouse" title="Computer mouse">mouse</a> clicks and <a href="/wiki/Computer_keyboard" title="Computer keyboard">key</a> presses. </p> <div class="mw-heading mw-heading3"><h3 id="Asynchronous_action">Asynchronous action</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=4" title="Edit section: Asynchronous action"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A callback can be used to implement asynchronous processing. A caller requests an action and provides a callback to be called when the action completes which might be long after the request is made. </p> <div class="mw-heading mw-heading3"><h3 id="Polymorphism">Polymorphism</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=5" title="Edit section: Polymorphism"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A callback can be used to implement <a href="/wiki/Polymorphism_(computer_science)" title="Polymorphism (computer science)">polymorphism</a>. In the following pseudocode, <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">SayHi</code> can take either <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">WriteStatus</code> or <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">WriteError</code>. </p> <div class="mw-highlight mw-highlight-lang-python mw-content-ltr" dir="ltr"><pre><span></span><span class="k">def</span> <span class="nf">WriteStatus</span><span class="p">(</span><span class="n">string</span> <span class="n">message</span><span class="p">):</span> <span class="n">Write</span><span class="p">(</span><span class="n">stdout</span><span class="p">,</span> <span class="n">message</span><span class="p">)</span> <span class="k">def</span> <span class="nf">WriteError</span><span class="p">(</span><span class="n">string</span> <span class="n">message</span><span class="p">):</span> <span class="n">Write</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span> <span class="n">message</span><span class="p">)</span> <span class="k">def</span> <span class="nf">SayHi</span><span class="p">(</span><span class="n">write</span><span class="p">):</span> <span class="n">write</span><span class="p">(</span><span class="s2">"Hello world"</span><span class="p">)</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="Conditional_action">Conditional action</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=6" title="Edit section: Conditional action"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A callback can be used to implement conditional behavior. In the following pseudocode, if logging is enabled, <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">Log</code> calls the callback, <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">getMessage</code>, and writes the result. But, if logging is not enabled, then <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">getMessage</code> is not called; saving the runtime cost. </p> <div class="mw-highlight mw-highlight-lang-python mw-content-ltr" dir="ltr"><pre><span></span><span class="k">def</span> <span class="nf">Log</span><span class="p">(</span><span class="n">getMessage</span><span class="p">):</span> <span class="k">if</span> <span class="n">isLoggingEnabled</span><span class="p">:</span> <span class="n">message</span> <span class="o">=</span> <span class="n">getMessage</span><span class="p">();</span> <span class="n">WriteLine</span><span class="p">(</span><span class="n">message</span><span class="p">);</span> </pre></div> <div class="mw-heading mw-heading2"><h2 id="Implementation">Implementation</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=7" title="Edit section: Implementation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The callback technology is implemented differently by <a href="/wiki/Programming_language" title="Programming language">programming language</a>. </p><p>In <a href="/wiki/Assembly_language" title="Assembly language">assembly</a>, <a href="/wiki/C_(programming_language)" title="C (programming language)">C</a>, <a href="/wiki/C%2B%2B" title="C++">C++</a>, <a href="/wiki/Pascal_(programming_language)" title="Pascal (programming language)">Pascal</a>, <a href="/wiki/Modula2" class="mw-redirect" title="Modula2">Modula2</a> and other languages, a callback function is stored internally as a <a href="/wiki/Function_pointer" title="Function pointer">function pointer</a>. Using the same storage allows different languages to directly share callbacks without a <a href="/wiki/Program_lifecycle_phase" title="Program lifecycle phase">design-time or runtime</a> <a href="/wiki/Interoperability" title="Interoperability">interoperability</a> <a href="/wiki/Abstraction_layer" title="Abstraction layer">layer</a>. For example, the <a href="/wiki/Windows_API" title="Windows API">Windows API</a> is accessible via multiple languages, compilers and assemblers. </p><p>C++ also allows objects to provide an implementation of the function call operation. The <a href="/wiki/Standard_Template_Library" title="Standard Template Library">Standard Template Library</a> accepts these objects (called <i><a href="/wiki/Function_object" title="Function object">functors</a></i>) as parameters. </p><p>Many <a href="/wiki/Dynamic_programming_language" title="Dynamic programming language">dynamic languages</a>, such as <a href="/wiki/JavaScript" title="JavaScript">JavaScript</a>, <a href="/wiki/Lua_(programming_language)" title="Lua (programming language)">Lua</a>, <a href="/wiki/Python_(programming_language)" title="Python (programming language)">Python</a>, <a href="/wiki/Perl" title="Perl">Perl</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><sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> and <a href="/wiki/PHP" title="PHP">PHP</a>, allow a function object to be passed. </p><p><a href="/wiki/List_of_CLI_languages" title="List of CLI languages">CLI languages</a> such as <a href="/wiki/C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a> and <a href="/wiki/VB.NET" class="mw-redirect" title="VB.NET">VB.NET</a> provide a <a href="/wiki/Type_safety" title="Type safety">type-safe</a> encapsulating function reference known as <a href="/wiki/Delegate_(CLI)" title="Delegate (CLI)">delegate</a>. </p><p>Events and <a href="/wiki/Event_handlers" class="mw-redirect" title="Event handlers">event handlers</a>, as used in .NET languages, provide for callbacks. </p><p>Functional languages generally support <a href="/wiki/First-class_functions" class="mw-redirect" title="First-class functions">first-class functions</a>, which can be passed as callbacks to other functions, stored as data or returned from functions. </p><p>Many languages, including Perl, Python, <a href="/wiki/Ruby_(programming_language)" title="Ruby (programming language)">Ruby</a>, <a href="/wiki/Smalltalk" title="Smalltalk">Smalltalk</a>, <a href="/wiki/C%2B%2B" title="C++">C++</a> (11+), C# and VB.NET (new versions) and most functional languages, support <a href="/wiki/Lambda_(programming)" class="mw-redirect" title="Lambda (programming)">lambda expressions</a>, unnamed functions with inline syntax, that generally acts as callbacks.. </p><p>In some languages, including <a href="/wiki/Scheme_(programming_language)" title="Scheme (programming language)">Scheme</a>, <a href="/wiki/ML_(programming_language)" title="ML (programming language)">ML</a>, JavaScript, Perl, Python, Smalltalk, PHP (since 5.3.0),<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> C++ (11+), Java (since 8),<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> and many others, a lambda can be a <a href="/wiki/Closure_(computer_science)" class="mw-redirect" title="Closure (computer science)">closure</a>, i.e. can access variables locally defined in the context in which the lambda is defined. </p><p>In an <a href="/wiki/Object-oriented_programming" title="Object-oriented programming">object-oriented programming</a> language such as <a href="/wiki/Java_(programming_language)" title="Java (programming language)">Java</a> versions before function-valued arguments, the behavior of a callback can be achieved by passing an object that implements an interface. The methods of this object are callbacks. </p><p>In <a href="/wiki/PL/I" title="PL/I">PL/I</a> and <a href="/wiki/ALGOL_60" title="ALGOL 60">ALGOL 60</a> a callback procedure may need to be able to access local variables in containing blocks, so it is called through an <i>entry variable</i> containing both the entry point and context information. <sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Example_code">Example code</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=8" title="Edit section: Example code"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading3"><h3 id="C">C</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=9" title="Edit section: C"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Callbacks have a wide variety of uses, for example in error signaling: a <a href="/wiki/Unix" title="Unix">Unix</a> program might not want to terminate immediately when it receives <a href="/wiki/SIGTERM" class="mw-redirect" title="SIGTERM">SIGTERM</a>, so to make sure that its termination is handled properly, it would register the cleanup function as a callback. Callbacks may also be used to control whether a function acts or not: <a href="/wiki/Xlib" title="Xlib">Xlib</a> allows custom predicates to be specified to determine whether a program wishes to handle an event. </p><p>In the following <a href="/wiki/C_(programming_language)" title="C (programming language)">C</a> code, function <code>PrintNumber</code> uses parameter <code>getNumber</code> as a blocking callback. <code>PrintNumber</code> is called with <code>GetAnswerToMostImportantQuestion</code> which acts as a callback function. When run the output is: "Value: 42". </p> <div class="mw-highlight mw-highlight-lang-c mw-content-ltr" dir="ltr"><pre><span></span><span class="cp">#include</span><span class="w"> </span><span class="cpf"><stdio.h></span> <span class="cp">#include</span><span class="w"> </span><span class="cpf"><stdlib.h></span> <span class="kt">void</span><span class="w"> </span><span class="nf">PrintNumber</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="p">(</span><span class="o">*</span><span class="n">getNumber</span><span class="p">)(</span><span class="kt">void</span><span class="p">))</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">val</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">getNumber</span><span class="p">();</span> <span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Value: %d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="w"> </span><span class="n">val</span><span class="p">);</span> <span class="p">}</span> <span class="kt">int</span><span class="w"> </span><span class="nf">GetAnswerToMostImportantQuestion</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">42</span><span class="p">;</span> <span class="p">}</span> <span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="n">PrintNumber</span><span class="p">(</span><span class="n">GetAnswerToMostImportantQuestion</span><span class="p">);</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span> <span class="p">}</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="C++"><span id="C.2B.2B"></span>C++</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=10" title="Edit section: C++"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In C++, <a href="/wiki/Function_object" title="Function object">functor</a> can be used in addition to function pointer. </p> <div class="mw-heading mw-heading3"><h3 id="C#"><span id="C.23"></span>C#</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=11" title="Edit section: C#"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In the following <a href="/wiki/C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a> code, method <code>Helper.Method</code> uses parameter <code>callback</code> as a blocking callback. <code>Helper.Method</code> is called with <code>Log</code> which acts as a callback function. When run, the following is written to the console: "Callback was: Hello world". </p> <div class="mw-highlight mw-highlight-lang-c# mw-content-ltr" dir="ltr"><pre><span></span><span class="k">public</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">MainClass</span> <span class="p">{</span> <span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="k">void</span><span class="w"> </span><span class="nf">Main</span><span class="p">(</span><span class="kt">string</span><span class="p">[]</span><span class="w"> </span><span class="n">args</span><span class="p">)</span> <span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="n">Helper</span><span class="w"> </span><span class="n">helper</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">Helper</span><span class="p">();</span> <span class="w"> </span><span class="n">helper</span><span class="p">.</span><span class="n">Method</span><span class="p">(</span><span class="n">Log</span><span class="p">);</span> <span class="w"> </span><span class="p">}</span> <span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="k">void</span><span class="w"> </span><span class="nf">Log</span><span class="p">(</span><span class="kt">string</span><span class="w"> </span><span class="n">str</span><span class="p">)</span> <span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="n">Console</span><span class="p">.</span><span class="n">WriteLine</span><span class="p">(</span><span class="s">$"Callback was: {str}"</span><span class="p">);</span> <span class="w"> </span><span class="p">}</span> <span class="p">}</span> <span class="k">public</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Helper</span> <span class="p">{</span> <span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="k">void</span><span class="w"> </span><span class="nf">Method</span><span class="p">(</span><span class="n">Action</span><span class="o"><</span><span class="kt">string</span><span class="o">></span><span class="w"> </span><span class="n">callback</span><span class="p">)</span> <span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="n">callback</span><span class="p">(</span><span class="s">"Hello world"</span><span class="p">);</span> <span class="w"> </span><span class="p">}</span> <span class="p">}</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="Kotlin">Kotlin</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=12" title="Edit section: Kotlin"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In the following <a href="/wiki/Kotlin_(programming_language)" title="Kotlin (programming language)">Kotlin</a> code, function <code>askAndAnswer</code> uses parameter <code>getAnswer</code> as a blocking callback. <code>askAndAnswer</code> is called with <code>getAnswerToMostImportantQuestion</code> which acts as a callback function. Running this will tell the user that the answer to their question is "42". </p> <div class="mw-highlight mw-highlight-lang-kotlin mw-content-ltr" dir="ltr"><pre><span></span><span class="kd">fun</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="n">print</span><span class="p">(</span><span class="s">"Enter the most important question: "</span><span class="p">)</span> <span class="w"> </span><span class="kd">val</span><span class="w"> </span><span class="nv">question</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">readLine</span><span class="p">()</span> <span class="w"> </span><span class="n">askAndAnswer</span><span class="p">(</span><span class="n">question</span><span class="p">,</span><span class="w"> </span><span class="o">::</span><span class="n">getAnswerToMostImportantQuestion</span><span class="p">)</span> <span class="p">}</span> <span class="kd">fun</span><span class="w"> </span><span class="nf">getAnswerToMostImportantQuestion</span><span class="p">():</span><span class="w"> </span><span class="kt">Int</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="m">42</span> <span class="p">}</span> <span class="kd">fun</span><span class="w"> </span><span class="nf">askAndAnswer</span><span class="p">(</span><span class="n">question</span><span class="p">:</span><span class="w"> </span><span class="kt">String?</span><span class="p">,</span><span class="w"> </span><span class="n">getAnswer</span><span class="p">:</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="kt">Int</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="n">println</span><span class="p">(</span><span class="s">"Question: </span><span class="si">$</span><span class="n">question</span><span class="s">"</span><span class="p">)</span> <span class="w"> </span><span class="n">println</span><span class="p">(</span><span class="s">"Answer: </span><span class="si">${</span><span class="n">getAnswer</span><span class="p">()</span><span class="si">}</span><span class="s">"</span><span class="p">)</span> <span class="p">}</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="JavaScript">JavaScript</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=13" title="Edit section: JavaScript"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In the following <a href="/wiki/JavaScript" title="JavaScript">JavaScript</a> code, function <code>calculate</code> uses parameter <code>operate</code> as a blocking callback. <code>calculate</code> is called with <code>multiply</code> and then with <code>sum</code> which act as callback functions. </p> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre><span></span><span class="kd">function</span><span class="w"> </span><span class="nx">calculate</span><span class="p">(</span><span class="nx">a</span><span class="p">,</span><span class="w"> </span><span class="nx">b</span><span class="p">,</span><span class="w"> </span><span class="nx">operate</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="nx">operate</span><span class="p">(</span><span class="nx">a</span><span class="p">,</span><span class="w"> </span><span class="nx">b</span><span class="p">);</span> <span class="p">}</span> <span class="kd">function</span><span class="w"> </span><span class="nx">multiply</span><span class="p">(</span><span class="nx">a</span><span class="p">,</span><span class="w"> </span><span class="nx">b</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="nx">a</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="nx">b</span><span class="p">;</span> <span class="p">}</span> <span class="kd">function</span><span class="w"> </span><span class="nx">sum</span><span class="p">(</span><span class="nx">a</span><span class="p">,</span><span class="w"> </span><span class="nx">b</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="nx">a</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="nx">b</span><span class="p">;</span> <span class="p">}</span> <span class="c1">// outputs 20</span> <span class="nx">alert</span><span class="p">(</span><span class="nx">calculate</span><span class="p">(</span><span class="mf">10</span><span class="p">,</span><span class="w"> </span><span class="mf">2</span><span class="p">,</span><span class="w"> </span><span class="nx">multiply</span><span class="p">));</span> <span class="c1">// outputs 12</span> <span class="nx">alert</span><span class="p">(</span><span class="nx">calculate</span><span class="p">(</span><span class="mf">10</span><span class="p">,</span><span class="w"> </span><span class="mf">2</span><span class="p">,</span><span class="w"> </span><span class="nx">sum</span><span class="p">));</span> </pre></div> <p>The collection method <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">.each()</code> of the <a href="/wiki/JQuery" title="JQuery">jQuery</a> <a href="/wiki/JavaScript_libraries" class="mw-redirect" title="JavaScript libraries">library</a> uses the function passed to it as a blocking callback. It calls the callback for each item of the collection. For example: </p> <div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre><span></span><span class="nx">$</span><span class="p">(</span><span class="s2">"li"</span><span class="p">).</span><span class="nx">each</span><span class="p">(</span><span class="kd">function</span><span class="p">(</span><span class="nx">index</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="nx">index</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="s2">": "</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="nx">$</span><span class="p">(</span><span class="k">this</span><span class="p">).</span><span class="nx">text</span><span class="p">());</span> <span class="p">});</span> </pre></div> <p>Deferred callbacks are commonly used for handling events from the user, the client and timers. Examples can be found in <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">addEventListener</code>, <a href="/wiki/Ajax_(programming)" title="Ajax (programming)">Ajax</a> and <code><a href="/wiki/XMLHttpRequest" title="XMLHttpRequest">XMLHttpRequest</a></code>. <sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> </p><p>In addition to using callbacks in JavaScript source code, C functions that take a function are supported via js-ctypes.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Red_and_REBOL">Red and REBOL</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=14" title="Edit section: Red and REBOL"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The following <a href="/wiki/REBOL" class="mw-redirect" title="REBOL">REBOL</a>/<a href="/wiki/Red_(programming_language)" title="Red (programming language)">Red</a> code demonstrates callback use. </p> <ul><li>As alert requires a string, form produces a string from the result of calculate</li> <li>The get-word! values (i.e., :calc-product and :calc-sum) trigger the interpreter to return the code of the function rather than evaluate with the function.</li> <li>The datatype! references in a block! [float! integer!] restrict the type of values passed as arguments.</li></ul> <div class="mw-highlight mw-highlight-lang-red mw-content-ltr" dir="ltr"><pre><span></span><span class="nv">Red</span> <span class="gs">[</span><span class="gu">Title:</span> <span class="s">"Callback example"</span><span class="gs">]</span> <span class="gu">calculate:</span> <span class="nb">func</span> <span class="gs">[</span> <span class="nv">num1</span> <span class="gs">[</span><span class="kt">number!</span><span class="gs">]</span> <span class="nv">num2</span> <span class="gs">[</span><span class="kt">number!</span><span class="gs">]</span> <span class="nv">callback-function</span> <span class="gs">[</span><span class="kt">function!</span><span class="gs">]</span> <span class="gs">][</span> <span class="nv">callback-function</span> <span class="nv">num1</span> <span class="nv">num2</span> <span class="gs">]</span> <span class="gu">calc-product:</span> <span class="nb">func</span> <span class="gs">[</span> <span class="nv">num1</span> <span class="gs">[</span><span class="kt">number!</span><span class="gs">]</span> <span class="nv">num2</span> <span class="gs">[</span><span class="kt">number!</span><span class="gs">]</span> <span class="gs">][</span> <span class="nv">num1</span> <span class="o">*</span> <span class="nv">num2</span> <span class="gs">]</span> <span class="gu">calc-sum:</span> <span class="nb">func</span> <span class="gs">[</span> <span class="nv">num1</span> <span class="gs">[</span><span class="kt">number!</span><span class="gs">]</span> <span class="nv">num2</span> <span class="gs">[</span><span class="kt">number!</span><span class="gs">]</span> <span class="gs">][</span> <span class="nv">num1</span> <span class="o">+</span> <span class="nv">num2</span> <span class="gs">]</span> <span class="c">; alerts 75, the product of 5 and 15</span> <span class="nv">alert</span> <span class="nf">form</span> <span class="nv">calculate</span> <span class="m">5</span> <span class="m">15</span> <span class="gu">:calc-product</span> <span class="c">; alerts 20, the sum of 5 and 15</span> <span class="nv">alert</span> <span class="nf">form</span> <span class="nv">calculate</span> <span class="m">5</span> <span class="m">15</span> <span class="gu">:calc-sum</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="Lua">Lua</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=15" title="Edit section: Lua"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A color tweening example using the <a href="/wiki/Roblox" title="Roblox">Roblox</a> engine that takes an optional .done callback: </p> <div class="mw-highlight mw-highlight-lang-lua mw-content-ltr" dir="ltr"><pre><span></span><span class="n">wait</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="kd">local</span> <span class="n">DT</span> <span class="o">=</span> <span class="n">wait</span><span class="p">()</span> <span class="kr">function</span> <span class="nf">tween_color</span><span class="p">(</span><span class="n">object</span><span class="p">,</span> <span class="n">finish_color</span><span class="p">,</span> <span class="n">fade_time</span><span class="p">)</span> <span class="kd">local</span> <span class="n">step_r</span> <span class="o">=</span> <span class="n">finish_color</span><span class="p">.</span><span class="n">r</span> <span class="o">-</span> <span class="n">object</span><span class="p">.</span><span class="n">BackgroundColor3</span><span class="p">.</span><span class="n">r</span> <span class="kd">local</span> <span class="n">step_g</span> <span class="o">=</span> <span class="n">finish_color</span><span class="p">.</span><span class="n">g</span> <span class="o">-</span> <span class="n">object</span><span class="p">.</span><span class="n">BackgroundColor3</span><span class="p">.</span><span class="n">g</span> <span class="kd">local</span> <span class="n">step_b</span> <span class="o">=</span> <span class="n">finish_color</span><span class="p">.</span><span class="n">b</span> <span class="o">-</span> <span class="n">object</span><span class="p">.</span><span class="n">BackgroundColor3</span><span class="p">.</span><span class="n">b</span> <span class="kd">local</span> <span class="n">total_steps</span> <span class="o">=</span> <span class="mi">1</span><span class="o">/</span><span class="p">(</span><span class="n">DT</span><span class="o">*</span><span class="p">(</span><span class="mi">1</span><span class="o">/</span><span class="n">fade_time</span><span class="p">))</span> <span class="kd">local</span> <span class="n">completed</span><span class="p">;</span> <span class="nb">coroutine.wrap</span><span class="p">(</span><span class="kr">function</span><span class="p">()</span> <span class="kr">for</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="n">DT</span><span class="o">*</span><span class="p">(</span><span class="mi">1</span> <span class="o">/</span> <span class="n">fade_time</span><span class="p">)</span> <span class="kr">do</span> <span class="n">object</span><span class="p">.</span><span class="n">BackgroundColor3</span> <span class="o">=</span> <span class="n">Color3</span><span class="p">.</span><span class="n">new</span> <span class="p">(</span> <span class="n">object</span><span class="p">.</span><span class="n">BackgroundColor3</span><span class="p">.</span><span class="n">r</span> <span class="o">+</span> <span class="p">(</span><span class="n">step_r</span><span class="o">/</span><span class="n">total_steps</span><span class="p">),</span> <span class="n">object</span><span class="p">.</span><span class="n">BackgroundColor3</span><span class="p">.</span><span class="n">g</span> <span class="o">+</span> <span class="p">(</span><span class="n">step_g</span><span class="o">/</span><span class="n">total_steps</span><span class="p">),</span> <span class="n">object</span><span class="p">.</span><span class="n">BackgroundColor3</span><span class="p">.</span><span class="n">b</span> <span class="o">+</span> <span class="p">(</span><span class="n">step_b</span><span class="o">/</span><span class="n">total_steps</span><span class="p">)</span> <span class="p">)</span> <span class="n">wait</span><span class="p">()</span> <span class="kr">end</span> <span class="kr">if</span> <span class="n">completed</span> <span class="kr">then</span> <span class="n">completed</span><span class="p">()</span> <span class="kr">end</span> <span class="kr">end</span><span class="p">)()</span> <span class="kr">return</span> <span class="p">{</span> <span class="n">done</span> <span class="o">=</span> <span class="kr">function</span><span class="p">(</span><span class="n">callback</span><span class="p">)</span> <span class="n">completed</span> <span class="o">=</span> <span class="n">callback</span> <span class="kr">end</span> <span class="p">}</span> <span class="kr">end</span> <span class="n">tween_color</span><span class="p">(</span><span class="n">some_object</span><span class="p">,</span> <span class="n">Color3</span><span class="p">.</span><span class="n">new</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">),</span> <span class="mi">1</span><span class="p">).</span><span class="n">done</span><span class="p">(</span><span class="kr">function</span><span class="p">()</span> <span class="nb">print</span> <span class="s2">"Color tweening finished!"</span> <span class="kr">end</span><span class="p">)</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="Python">Python</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=16" title="Edit section: Python"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In the following <a href="/wiki/Python_(programming_language)" title="Python (programming language)">Python</a> code, function <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">calculate</code> accepts a parameter <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">operate</code> that is used as a blocking callback. <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">calculate</code> is called with <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">square</code> which acts as a callback function. </p> <div class="mw-highlight mw-highlight-lang-python mw-content-ltr" dir="ltr"><pre><span></span><span class="k">def</span> <span class="nf">square</span><span class="p">(</span><span class="n">val</span><span class="p">):</span> <span class="k">return</span> <span class="n">val</span> <span class="o">**</span> <span class="mi">2</span> <span class="k">def</span> <span class="nf">calculate</span><span class="p">(</span><span class="n">operate</span><span class="p">,</span> <span class="n">val</span><span class="p">):</span> <span class="k">return</span> <span class="n">operate</span><span class="p">(</span><span class="n">val</span><span class="p">)</span> <span class="c1"># outputs: 25</span> <span class="n">calculate</span><span class="p">(</span><span class="n">square</span><span class="p">,</span> <span class="mi">5</span><span class="p">)</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="Julia">Julia</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Callback_(computer_programming)&action=edit&section=17" title="Edit section: Julia"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In the following <a href="/wiki/Julia_(programming_language)" title="Julia (programming language)">Julia</a> code, function <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">calculate</code> accepts a parameter <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">operate</code> that is used as a blocking callback. <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">calculate</code> is called with <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">square</code> which acts as a callback function. </p> <div class="mw-highlight mw-highlight-lang-jlcon mw-content-ltr" dir="ltr"><pre><span></span><span class="gp">julia></span><span class="w"> </span><span class="n">square</span><span class="p">(</span><span class="n">val</span><span class="p">)</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">val</span><span class="o">^</span><span class="mi">2</span> <span class="go">square (generic function with 1 method)</span> <span class="gp">julia></span><span class="w"> </span><span class="n">calculate</span><span class="p">(</span><span class="n">operate</span><span class="p">,</span><span class="n">val</span><span class="p">)</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">operate</span><span class="p">(</span><span class="n">val</span><span class="p">)</span> <span class="go">calculate (generic function with 1 method)</span> <span class="gp">julia></span><span class="w"> </span><span class="n">calculate</span><span class="p">(</span><span class="n">square</span><span class="p">,</span><span class="mi">5</span><span class="p">)</span> <span class="go">25</span> </pre></div> <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=Callback_(computer_programming)&action=edit&section=18" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239009302">.mw-parser-output .portalbox{padding:0;margin:0.5em 0;display:table;box-sizing:border-box;max-width:175px;list-style:none}.mw-parser-output .portalborder{border:1px solid var(--border-color-base,#a2a9b1);padding:0.1em;background:var(--background-color-neutral-subtle,#f8f9fa)}.mw-parser-output .portalbox-entry{display:table-row;font-size:85%;line-height:110%;height:1.9em;font-style:italic;font-weight:bold}.mw-parser-output .portalbox-image{display:table-cell;padding:0.2em;vertical-align:middle;text-align:center}.mw-parser-output .portalbox-link{display:table-cell;padding:0.2em 0.2em 0.2em 0.3em;vertical-align:middle}@media(min-width:720px){.mw-parser-output .portalleft{clear:left;float:left;margin:0.5em 1em 0.5em 0}.mw-parser-output .portalright{clear:right;float:right;margin:0.5em 0 0.5em 1em}}</style><ul role="navigation" aria-label="Portals" class="noprint portalbox portalborder portalright"> <li class="portalbox-entry"><span class="portalbox-image"><span class="noviewer" typeof="mw:File"><a href="/wiki/File:Octicons-terminal.svg" class="mw-file-description"><img alt="icon" src="//upload.wikimedia.org/wikipedia/commons/thumb/6/6f/Octicons-terminal.svg/24px-Octicons-terminal.svg.png" decoding="async" width="24" height="28" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/6/6f/Octicons-terminal.svg/37px-Octicons-terminal.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/6/6f/Octicons-terminal.svg/49px-Octicons-terminal.svg.png 2x" data-file-width="896" data-file-height="1024" /></a></span></span><span class="portalbox-link"><a href="/wiki/Portal:Computer_programming" title="Portal:Computer programming">Computer programming portal</a></span></li></ul> <style data-mw-deduplicate="TemplateStyles:r1184024115">.mw-parser-output .div-col{margin-top:0.3em;column-width:30em}.mw-parser-output .div-col-small{font-size:90%}.mw-parser-output .div-col-rules{column-rule:1px solid #aaa}.mw-parser-output .div-col dl,.mw-parser-output .div-col ol,.mw-parser-output .div-col ul{margin-top:0}.mw-parser-output .div-col li,.mw-parser-output .div-col dd{page-break-inside:avoid;break-inside:avoid-column}</style><div class="div-col" style="column-width: 25em;"> <ul><li><a href="/wiki/Command_pattern" title="Command pattern">Command pattern</a></li> <li><a href="/wiki/Continuation-passing_style" title="Continuation-passing style">Continuation-passing style</a></li> <li><a href="/wiki/Event_loop" title="Event loop">Event loop</a></li> <li><a href="/wiki/Event-driven_programming" title="Event-driven programming">Event-driven programming</a></li> <li><a href="/wiki/Implicit_invocation" title="Implicit invocation">Implicit invocation</a></li> <li><a href="/wiki/Inversion_of_control" title="Inversion of control">Inversion of control</a></li> <li><a href="/wiki/Libsigc%2B%2B" title="Libsigc++">libsigc++</a>, a callback library for C++</li> <li><a href="/wiki/Signals_and_slots" title="Signals and slots">Signals and slots</a></li> <li><a href="/wiki/User_exit" title="User exit">User exit</a></li></ul> </div> <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=Callback_(computer_programming)&action=edit&section=19" 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 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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 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(prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.unix.org.ua/orelly/perl/cookbook/ch11_05.htm">"Perl Cookbook - 11.4. 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(1979). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=IFmaqTI9-KsC&pg=PA164"><i>Encyclopedia of Computer Science and Technology: Volume 12</i></a>. Marcel Dekker, inc. p. 164. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-8247-2262-0" title="Special:BookSources/0-8247-2262-0"><bdi>0-8247-2262-0</bdi></a><span class="reference-accessdate">. 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