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docSidebarContainer_YfHR"><div class="sidebarViewport_aRkj"><div class="sidebar_njMd"><nav aria-label="Docs sidebar" class="menu thin-scrollbar menu_SIkG"><ul class="theme-doc-sidebar-menu menu__list"><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-1 menu__list-item"><a class="menu__link" href="../index.html">Introduction</a></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-0/intro.html">Contents and Figures</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-1/b-b-scope-purpose-and-history.html">1. Introduction</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-2/c-b-character-syntax.html">2. Syntax</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-3/d-b-evaluation.html">3. Evaluation and Compilation</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-4/e-b-introduction.html">4. Types and Classes</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-5/f-b-generalized-reference.html">5. Data and Control Flow</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-6/g-b-the-loop-facility.html">6. Iteration</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret menu__link--active" aria-expanded="true" href="h-b-object-creation-and-initialization.html">7. Objects</a></div><ul style="display:block;overflow:visible;height:auto" class="menu__list"><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link" tabindex="0" href="h-b-object-creation-and-initialization.html">7.1 Object Creation and Initialization</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link" tabindex="0" href="h-c-changing-the-class-of-an-instance.html">7.2 Changing the Class of an Instance</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link" tabindex="0" href="h-d-reinitializing-an-instance.html">7.3 Reinitializing an Instance</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link" tabindex="0" href="h-e-metaobjects.html">7.4 Meta</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link menu__link--active" aria-current="page" tabindex="0" href="h-f-slots.html">7.5 Slots</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link" tabindex="0" href="h-g-generic-functions-and-methods.html">7.6 Generic Functions and Methods</a></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-2 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist" aria-expanded="false" tabindex="0" href="../category/77-objects-dictionary.html">7.7 Objects Dictionary</a><button aria-label="Expand sidebar category '7.7 Objects Dictionary'" type="button" class="clean-btn menu__caret"></button></div></li></ul></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../category/81-structures-dictionary.html">8. Structures</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-9/j-b-condition-system-concepts.html">9. Conditions</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-10/ba-b-symbol-concepts.html">10. Symbols</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-11/bb-b-package-concepts.html">11. Packages</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-12/bc-b-number-concepts.html">12. Numbers</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-13/bd-b-character-concepts.html">13. Characters</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-14/be-b-cons-concepts.html">14. Conses</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-15/bf-b-array-concepts.html">15. Arrays</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-16/bg-b-string-concepts.html">16. Strings</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-17/bh-b-sequence-concepts.html">17. Sequences</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-18/bi-b-hash-table-concepts.html">18. Hash Tables</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-19/bj-b-overview-of-filenames.html">19. Filenames</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-20/ca-b-file-system-concepts.html">20. Files</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-21/cb-b-stream-concepts.html">21. Streams</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-22/cc-b-the-lisp-printer.html">22. Printer</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-23/cd-b-reader-concepts.html">23. Reader</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-24/ce-b-system-construction-concepts.html">24. System Construction</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-25/cf-b-the-external-environment.html">25. External Environment</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../environments.html">26. Environments</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../meta-object-protocol.html">27. Meta Object Protocol</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../data-structures.html">28. Data Structures</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../portability/trivial-packages.html">29. Portability</a></div></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-1 menu__list-item"><a class="menu__link" href="../dictionary-entries.html">dictionary-entries</a></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-26/intro.html">Glossary</a></div></li></ul></nav></div></div></aside><main class="docMainContainer_TBSr"><div class="container padding-top--md padding-bottom--lg"><div class="row"><div class="col docItemCol_VOVn"><div class="docItemContainer_Djhp"><article><nav class="theme-doc-breadcrumbs breadcrumbsContainer_Z_bl" aria-label="Breadcrumbs"><ul class="breadcrumbs" itemscope="" itemtype="https://schema.org/BreadcrumbList"><li class="breadcrumbs__item"><a aria-label="Home page" class="breadcrumbs__link" href="../index.html"><svg viewBox="0 0 24 24" class="breadcrumbHomeIcon_YNFT"><path d="M10 19v-5h4v5c0 .55.45 1 1 1h3c.55 0 1-.45 1-1v-7h1.7c.46 0 .68-.57.33-.87L12.67 3.6c-.38-.34-.96-.34-1.34 0l-8.36 7.53c-.34.3-.13.87.33.87H5v7c0 .55.45 1 1 1h3c.55 0 1-.45 1-1z" fill="currentColor"></path></svg></a></li><li class="breadcrumbs__item"><span class="breadcrumbs__link">7. Objects</span><meta itemprop="position" content="1"></li><li itemscope="" itemprop="itemListElement" itemtype="https://schema.org/ListItem" class="breadcrumbs__item breadcrumbs__item--active"><span class="breadcrumbs__link" itemprop="name">7.5 Slots</span><meta itemprop="position" content="2"></li></ul></nav><div class="tocCollapsible_ETCw theme-doc-toc-mobile tocMobile_ITEo"><button type="button" class="clean-btn tocCollapsibleButton_TO0P">On this page</button></div><div class="theme-doc-markdown markdown"><h1>7.5 Slots</h1>
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<h2 class="anchor anchorWithStickyNavbar_LWe7" id="751-introduction-to-slots">7.5.1 Introduction to Slots<a href="h-f-slots.html#751-introduction-to-slots" class="hash-link" aria-label="Direct link to 7.5.1 Introduction to Slots" title="Direct link to 7.5.1 Introduction to Slots"></a></h2>
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<p>An <span><i>object</i></span> of <span><i>metaclass</i></span> <span><b>standard-class</b></span> has zero or more named <span><i>slots</i></span>. The <span><i>slots</i></span> of an <span><i>object</i></span> are determined by the <span><i>class</i></span> of the <span><i>object</i></span>. Each <span><i>slot</i></span> can hold one value. The <span><i>name</i></span> of a <span><i>slot</i></span> is a <span><i>symbol</i></span> that is syntactically valid for use as a variable name.</p>
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<p>When a <span><i>slot</i></span> does not have a value, the <span><i>slot</i></span> is said to be <span><i>unbound</i></span>. When an unbound <span><i>slot</i></span> is read, the <span><i>generic function</i></span> <span><b>slot-unbound</b></span> is invoked. The system-supplied primary <span><i>method</i></span> for <span><b>slot-unbound</b></span> on <span><i>class</i></span> <span><b>t</b></span> signals an error. If <span><b>slot-unbound</b></span> returns, its <span><i>primary value</i></span> is used that time as the <span><i>value</i></span> of the <span><i>slot</i></span>.</p>
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<p>The default initial value form for a <span><i>slot</i></span> is defined by the <!-- -->:initform<!-- --> slot option. When the <!-- -->:initform<!-- --> form is used to supply a value, it is evaluated in the lexical environment in which the <span><b>defclass</b></span> form was evaluated. The <!-- -->:initform<!-- --> along with the lexical environment in which the <span><b>defclass</b></span> form was evaluated is called a <span><i>captured initialization form</i></span>. For more details, see Section 7.1 (Object Creation and Initialization).</p>
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<p>A <span><i>local slot</i></span> is defined to be a <span><i>slot</i></span> that is <span><i>accessible</i></span> to exactly one <span><i>instance</i></span>, namely the one in which the <span><i>slot</i></span> is allocated. A <span><i>shared slot</i></span> is defined to be a <span><i>slot</i></span> that is visible to more than one <span><i>instance</i></span> of a given <span><i>class</i></span> and its <em>subclasses</em>.</p>
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<p>A <span><i>class</i></span> is said to define a <span><i>slot</i></span> with a given <span><i>name</i></span> when the <span><b>defclass</b></span> form for that <span><i>class</i></span> contains a <span><i>slot specifier</i></span> with that <span><i>name</i></span>. Defining a <span><i>local slot</i></span> does not immediately create a <span><i>slot</i></span>; it causes a <span><i>slot</i></span> to be created each time an <span><i>instance</i></span> of the <span><i>class</i></span> is created. Defining a <span><i>shared slot</i></span> immediately creates a <span><i>slot</i></span>.</p>
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<p>The <!-- -->:allocation<!-- --> slot option to <span><b>defclass</b></span> controls the kind of <span><i>slot</i></span> that is defined. If the value of the <!-- -->:allocation<!-- --> slot option is <!-- -->:instance<!-- -->, a <span><i>local slot</i></span> is created. If the value of <!-- -->:allocation<!-- --> is <!-- -->:class<!-- -->, a <span><i>shared slot</i></span> is created.</p>
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<p>A <span><i>slot</i></span> is said to be <span><i>accessible</i></span> in an <span><i>instance</i></span> of a <span><i>class</i></span> if the <span><i>slot</i></span> is defined by the <span><i>class</i></span> of the <span><i>instance</i></span> or is inherited from a <span><i>superclass</i></span> of that <span><i>class</i></span>. At most one <span><i>slot</i></span> of a given <span><i>name</i></span> can be <span><i>accessible</i></span> in an <span><i>instance</i></span>. A <span><i>shared slot</i></span> defined by a <span><i>class</i></span> is <span><i>accessible</i></span> in all <span><i>instances</i></span> of that <span><i>class</i></span>. A detailed explanation of the inheritance of <span><i>slots</i></span> is given in Section 7.5.3 (Inheritance of Slots and Slot Options).</p>
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<h2 class="anchor anchorWithStickyNavbar_LWe7" id="752-accessing-slots">7.5.2 Accessing Slots<a href="h-f-slots.html#752-accessing-slots" class="hash-link" aria-label="Direct link to 7.5.2 Accessing Slots" title="Direct link to 7.5.2 Accessing Slots"></a></h2>
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<p><span><i>Slots</i></span> can be <em>accessed</em> in two ways: by use of the primitive function <span><b>slot-value</b></span> and by use of <em>generic functions</em> generated by the <span><b>defclass</b></span> form.</p>
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<p>The <span><i>function</i></span> <span><b>slot-value</b></span> can be used with any of the <span><i>slot</i></span> names specified in the <span><b>defclass</b></span> form to <span><i>access</i></span> a specific <em>slot accessible</em> in an <span><i>instance</i></span> of the given <span><i>class</i></span>.</p>
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<p>The macro <span><b>defclass</b></span> provides syntax for generating <span><i>methods</i></span> to read and write <span><i>slots</i></span>. If a reader <span><i>method</i></span> is requested, a <span><i>method</i></span> is automatically generated for reading the value of the <span><i>slot</i></span>, but no <span><i>method</i></span> for storing a value into it is generated. If a writer <span><i>method</i></span> is requested, a <span><i>method</i></span> is automatically generated for storing a value into the <span><i>slot</i></span>, but no <span><i>method</i></span> for reading its value is generated. If an accessor <span><i>method</i></span> is requested, a <span><i>method</i></span> for reading the value of the <span><i>slot</i></span> and a <span><i>method</i></span> for storing a value into the <span><i>slot</i></span> are automatically generated. Reader and writer <span><i>methods</i></span></p>
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<p>are implemented using <span><b>slot-value</b></span>.</p>
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<p>When a reader or writer <span><i>method</i></span> is specified for a <span><i>slot</i></span>, the name of the <em>generic function</em> to which the generated <span><i>method</i></span> belongs is directly specified. If the <span><i>name</i></span> specified for the writer <span><i>method</i></span> is the symbol name, the <span><i>name</i></span> of the <em>generic function</em> for writing the <span><i>slot</i></span> is the symbol name, and the <em>generic function</em> takes two arguments: the new value and the <span><i>instance</i></span>, in that order. If the <span><i>name</i></span> specified for the accessor <span><i>method</i></span> is the symbol name, the <span><i>name</i></span> of the <em>generic function</em> for reading</p>
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<p>the <span><i>slot</i></span> is the symbol name, and the <span><i>name</i></span> of the <em>generic function</em> for writing the <span><i>slot</i></span> is the list (setf name).</p>
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<p>A <span><i>generic function</i></span> created or modified by supplying <!-- -->:reader<!-- -->, <!-- -->:writer<!-- -->, or <!-- -->:accessor<!-- --> <span><i>slot</i></span> options can be treated exactly as an ordinary <span><i>generic function</i></span>.</p>
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<p>Note that <span><b>slot-value</b></span> can be used to read or write the value of a <span><i>slot</i></span> whether or not reader or writer <span><i>methods</i></span> exist for that <span><i>slot</i></span>. When <span><b>slot-value</b></span> is used, no reader or writer <span><i>methods</i></span> are invoked.</p>
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<p>The macro <span><b>with-slots</b></span> can be used to establish a <span><i>lexical environment</i></span> in which specified <span><i>slots</i></span> are lexically available as if they were variables. The macro <span><b>with-slots</b></span> invokes the <span><i>function</i></span> <span><b>slot-value</b></span> to <span><i>access</i></span> the specified <span><i>slots</i></span>.</p>
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<p>The macro <span><b>with-accessors</b></span> can be used to establish a lexical environment in which specified <span><i>slots</i></span> are lexically available through their accessors as if they were variables. The macro <span><b>with-accessors</b></span> invokes the appropriate accessors to <span><i>access</i></span> the specified <span><i>slots</i></span>.</p>
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<h2 class="anchor anchorWithStickyNavbar_LWe7" id="753-inheritance-of-slots-and-slot-options">7.5.3 Inheritance of Slots and Slot Options<a href="h-f-slots.html#753-inheritance-of-slots-and-slot-options" class="hash-link" aria-label="Direct link to 7.5.3 Inheritance of Slots and Slot Options" title="Direct link to 7.5.3 Inheritance of Slots and Slot Options"></a></h2>
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<p>The set of the <span><i>names</i></span> of all <em>slots accessible</em> in an <span><i>instance</i></span> of a <em>class C</em> is the union of the sets of <span><i>names</i></span> of <span><i>slots</i></span> defined by <em>C</em> and its <em>superclasses</em>. The structure of an <span><i>instance</i></span> is the set of <span><i>names</i></span> of <span><i>local slots</i></span> in that <span><i>instance</i></span>.</p>
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<p>In the simplest case, only one <span><i>class</i></span> among <em>C</em> and its <em>superclasses</em> defines a <span><i>slot</i></span> with a given <span><i>slot</i></span> name. If a <span><i>slot</i></span> is defined by a <span><i>superclass</i></span> of <em>C</em>, the <span><i>slot</i></span> is said to be inherited. The characteristics of the <span><i>slot</i></span> are determined by the <span><i>slot specifier</i></span> of the defining <span><i>class</i></span>. Consider the defining <span><i>class</i></span> for</p>
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<p>a slot <em>S</em>. If the value of the <!-- -->:allocation<!-- --> slot option is <!-- -->:instance<!-- -->, then <em>S</em> is a <span><i>local slot</i></span> and each <span><i>instance</i></span> of <em>C</em> has its own <span><i>slot</i></span> named <em>S</em> that stores its own value. If the value of the <!-- -->:allocation<!-- --> slot option is <!-- -->:class<!-- -->, then <em>S</em> is a <span><i>shared slot</i></span>, the <span><i>class</i></span> that defined <em>S</em> stores the value, and all <span><i>instances</i></span> of <em>C</em> can <span><i>access</i></span> that single <span><i>slot</i></span>. If the <!-- -->:allocation<!-- --> slot option is omitted, <!-- -->:instance<!-- --> is used.</p>
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<p>In general, more than one <span><i>class</i></span> among <em>C</em> and its <em>superclasses</em> can define a <span><i>slot</i></span> with a given <span><i>name</i></span>. In such cases, only one <span><i>slot</i></span> with the given name is <span><i>accessible</i></span> in an <span><i>instance</i></span> of <em>C</em>, and the characteristics of that <span><i>slot</i></span> are a combination of the several <span><i>slot</i></span> specifiers, computed as follows:</p>
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<p><em>•</em> All the <span><i>slot specifiers</i></span> for a given <span><i>slot</i></span> name are ordered from most specific to least specific, according to the order in <em>C</em>’s <span><i>class precedence list</i></span> of the <em>classes</em> that define them. All references to the specificity of <span><i>slot specifiers</i></span> immediately below refers to this ordering.</p>
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<p><em>•</em> The allocation of a <span><i>slot</i></span> is controlled by the most specific <span><i>slot specifier</i></span> . If the most specific <span><i>slot specifier</i></span> does not contain an <!-- -->:allocation<!-- --> slot option, <!-- -->:instance<!-- --> is used. Less specific <span><i>slot specifiers</i></span> do not affect the allocation.</p>
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<p><em>•</em> The default initial value form for a <span><i>slot</i></span> is the value of the <!-- -->:initform<!-- --> slot option in the most specific <span><i>slot specifier</i></span> that contains one. If no <span><i>slot specifier</i></span> contains an <!-- -->:initform<!-- --> slot option, the <span><i>slot</i></span> has no default initial value form.</p>
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<p><i>•</i> The contents of a <i>slot</i> will always be of type (and <i>T</i><sub>1</sub> <i>. . . T<sub>n</sub></i>) where <i>T</i><sub>1</sub> <i>. . . T<sub>n</sub></i> are the values of the <!-- -->:type<!-- --> slot options contained in all of the <i>slot specifiers</i>. If no <i>slot specifier</i> contains the <!-- -->:type<!-- --> slot option, the contents of the <i>slot</i> will always be of <i>type</i> <b>t</b>. The consequences of attempting to store in a <i>slot</i> a value that does not satisfy the <i>type</i> of the <i>slot</i> are undefined.</p>
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<p><em>•</em> The set of initialization arguments that initialize a given <span><i>slot</i></span> is the union of the initialization arguments declared in the <!-- -->:initarg<!-- --> slot options in all the <span><i>slot specifiers</i></span>.</p>
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<p><em>•</em> The <span><i>documentation string</i></span> for a <span><i>slot</i></span> is the value of the <!-- -->:documentation<!-- --> slot option in the most specific <span><i>slot</i></span> specifier that contains one. If no <span><i>slot specifier</i></span> contains a <!-- -->:documentation<!-- --> slot option, the <span><i>slot</i></span> has no <span><i>documentation string</i></span>.</p>
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<p>A consequence of the allocation rule is that a <span><i>shared slot</i></span> can be <em>shadowed</em>. For example, if a class <em>C</em><sub>1</sub> defines a <span><i>slot</i></span> named <em>S</em> whose value for the <!-- -->:allocation<!-- --> slot option is <!-- -->:class<!-- -->, that <span><i>slot</i></span> is <span><i>accessible</i></span> in <span><i>instances</i></span> of <em>C</em><sub>1</sub> and all of its <em>subclasses</em>. However, if <em>C</em><sub>2</sub> is a <span><i>subclass</i></span> of <em>C</em><sub>1</sub> and also defines a <span><i>slot</i></span> named <em>S</em>, <em>C</em><sub>1</sub>’s <span><i>slot</i></span> is not shared by <span><i>instances</i></span> of <em>C</em><sub>2</sub> and its <em>subclasses</em>. When a class <em>C</em><sub>1</sub> defines a <span><i>shared slot</i></span>, any subclass <em>C</em><sub>2</sub> of <em>C</em><sub>1</sub> will share this single <span><i>slot</i></span> unless the <span><b>defclass</b></span> form for <em>C</em><sub>2</sub> specifies a <span><i>slot</i></span> of the same <span><i>name</i></span> or there is a <span><i>superclass</i></span> of <em>C</em><sub>2</sub> that precedes <em>C</em><sub>1</sub> in the <span><i>class precedence list</i></span> of <em>C</em><sub>2</sub> that defines a <span><i>slot</i></span> of the same name.</p>
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<p>A consequence of the type rule is that the value of a <span><i>slot</i></span> satisfies the type constraint of each <span><i>slot specifier</i></span> that contributes to that <span><i>slot</i></span>. Because the result of attempting to store in a <span><i>slot</i></span> a value that does not satisfy the type constraint for the <span><i>slot</i></span> is undefined, the value in a <span><i>slot</i></span> might fail to satisfy its type constraint.</p>
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<p>The <!-- -->:reader<!-- -->, <!-- -->:writer<!-- -->, and <!-- -->:accessor<!-- --> slot options create <span><i>methods</i></span> rather than define the characteristics of a <span><i>slot</i></span>. Reader and writer <span><i>methods</i></span> are inherited in the sense described in Section 7.6.7 (Inheritance of Methods).</p>
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<p><span><i>Methods</i></span> that <em>access slots</em> use only the name of the <span><i>slot</i></span> and the <span><i>type</i></span> of the <span><i>slot</i></span>’s value. Suppose a <span><i>superclass</i></span> provides a <span><i>method</i></span> that expects to <span><i>access</i></span> a <span><i>shared slot</i></span> of a given <span><i>name</i></span>, and a <span><i>subclass</i></span> defines a <span><i>local slot</i></span> with the same <span><i>name</i></span>. If the <span><i>method</i></span> provided by the <span><i>superclass</i></span> is used on an <span><i>instance</i></span> of the <span><i>subclass</i></span>, the <em>method accesses</em> the <span><i>local slot</i></span>.</p></div><footer class="theme-doc-footer docusaurus-mt-lg"><div class="theme-doc-footer-edit-meta-row row"><div class="col"><a href="https://github.com/lisp-docs/cl-language-reference/tree/main/docs/chap-7/h-f-slots.md" target="_blank" rel="noopener noreferrer" class="theme-edit-this-page"><svg fill="currentColor" height="20" width="20" viewBox="0 0 40 40" class="iconEdit_Z9Sw" aria-hidden="true"><g><path d="m34.5 11.7l-3 3.1-6.3-6.3 3.1-3q0.5-0.5 1.2-0.5t1.1 0.5l3.9 3.9q0.5 0.4 0.5 1.1t-0.5 1.2z m-29.5 17.1l18.4-18.5 6.3 6.3-18.4 18.4h-6.3v-6.2z"></path></g></svg>Edit this page</a></div><div class="col lastUpdated_vwxv"><span class="theme-last-updated">Last updated<!-- --> by <b>daninus14</b></span></div></div></footer></article><nav class="pagination-nav 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