216 lines
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216 lines
15 KiB
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Accessing Slots (Guile Reference Manual)</title>
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<meta name="description" content="Accessing Slots (Guile Reference Manual)">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Introspection.html" rel="up" title="Introspection">
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<body lang="en">
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<div class="subsection-level-extent" id="Accessing-Slots">
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<div class="nav-panel">
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<p>
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Previous: <a href="Generic-Functions.html" accesskey="p" rel="prev">Generic Functions</a>, Up: <a href="Introspection.html" accesskey="u" rel="up">Introspection</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<h4 class="subsection" id="Accessing-Slots-1"><span>8.8.5 Accessing Slots<a class="copiable-link" href="#Accessing-Slots-1"> ¶</a></span></h4>
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<p>Any slot, regardless of its allocation, can be queried, referenced and
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set using the following four primitive procedures.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-slot_002dexists_003f"><span class="category-def">primitive procedure: </span><span><strong class="def-name">slot-exists?</strong> <var class="def-var-arguments">obj slot-name</var><a class="copiable-link" href="#index-slot_002dexists_003f"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">obj</var> has a slot with name <var class="var">slot-name</var>,
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otherwise <code class="code">#f</code>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-slot_002dbound_003f"><span class="category-def">primitive procedure: </span><span><strong class="def-name">slot-bound?</strong> <var class="def-var-arguments">obj slot-name</var><a class="copiable-link" href="#index-slot_002dbound_003f"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if the slot named <var class="var">slot-name</var> in <var class="var">obj</var> has a
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value, otherwise <code class="code">#f</code>.
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</p>
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<p><code class="code">slot-bound?</code> calls the generic function <code class="code">slot-missing</code> if
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<var class="var">obj</var> does not have a slot called <var class="var">slot-name</var> (see <a class="pxref" href="#Accessing-Slots">slot-missing</a>).
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-slot_002dref-1"><span class="category-def">primitive procedure: </span><span><strong class="def-name">slot-ref</strong> <var class="def-var-arguments">obj slot-name</var><a class="copiable-link" href="#index-slot_002dref-1"> ¶</a></span></dt>
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<dd><p>Return the value of the slot named <var class="var">slot-name</var> in <var class="var">obj</var>.
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</p>
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<p><code class="code">slot-ref</code> calls the generic function <code class="code">slot-missing</code> if
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<var class="var">obj</var> does not have a slot called <var class="var">slot-name</var> (see <a class="pxref" href="#Accessing-Slots">slot-missing</a>).
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</p>
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<p><code class="code">slot-ref</code> calls the generic function <code class="code">slot-unbound</code> if the
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named slot in <var class="var">obj</var> does not have a value (see <a class="pxref" href="#Accessing-Slots">slot-unbound</a>).
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-slot_002dset_0021-1"><span class="category-def">primitive procedure: </span><span><strong class="def-name">slot-set!</strong> <var class="def-var-arguments">obj slot-name value</var><a class="copiable-link" href="#index-slot_002dset_0021-1"> ¶</a></span></dt>
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<dd><p>Set the value of the slot named <var class="var">slot-name</var> in <var class="var">obj</var> to <var class="var">value</var>.
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</p>
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<p><code class="code">slot-set!</code> calls the generic function <code class="code">slot-missing</code> if
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<var class="var">obj</var> does not have a slot called <var class="var">slot-name</var> (see <a class="pxref" href="#Accessing-Slots">slot-missing</a>).
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</p></dd></dl>
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<p>GOOPS stores information about slots in classes. Internally,
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all of these procedures work by looking up the slot definition for the
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slot named <var class="var">slot-name</var> in the class <code class="code">(class-of
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<var class="var">obj</var>)</code>, and then using the slot definition’s “getter” and
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“setter” closures to get and set the slot value.
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</p>
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<p>The next four procedures differ from the previous ones in that they take
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the class as an explicit argument, rather than assuming
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<code class="code">(class-of <var class="var">obj</var>)</code>. Therefore they allow you to apply the
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“getter” and “setter” closures of a slot definition in one class to
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an instance of a different class.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-slot_002dexists_002dusing_002dclass_003f"><span class="category-def">primitive procedure: </span><span><strong class="def-name">slot-exists-using-class?</strong> <var class="def-var-arguments">class obj slot-name</var><a class="copiable-link" href="#index-slot_002dexists_002dusing_002dclass_003f"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">class</var> has a slot definition for a slot with
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name <var class="var">slot-name</var>, otherwise <code class="code">#f</code>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-slot_002dbound_002dusing_002dclass_003f"><span class="category-def">primitive procedure: </span><span><strong class="def-name">slot-bound-using-class?</strong> <var class="def-var-arguments">class obj slot-name</var><a class="copiable-link" href="#index-slot_002dbound_002dusing_002dclass_003f"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if applying <code class="code">slot-ref-using-class</code> to the same
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arguments would call the generic function <code class="code">slot-unbound</code>, otherwise
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<code class="code">#f</code>.
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</p>
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<p><code class="code">slot-bound-using-class?</code> calls the generic function
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<code class="code">slot-missing</code> if <var class="var">class</var> does not have a slot definition for a
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slot called <var class="var">slot-name</var> (see <a class="pxref" href="#Accessing-Slots">slot-missing</a>).
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-slot_002dref_002dusing_002dclass"><span class="category-def">primitive procedure: </span><span><strong class="def-name">slot-ref-using-class</strong> <var class="def-var-arguments">class obj slot-name</var><a class="copiable-link" href="#index-slot_002dref_002dusing_002dclass"> ¶</a></span></dt>
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<dd><p>Apply the “getter” closure for the slot named <var class="var">slot-name</var> in
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<var class="var">class</var> to <var class="var">obj</var>, and return its result.
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</p>
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<p><code class="code">slot-ref-using-class</code> calls the generic function
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<code class="code">slot-missing</code> if <var class="var">class</var> does not have a slot definition for a
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slot called <var class="var">slot-name</var> (see <a class="pxref" href="#Accessing-Slots">slot-missing</a>).
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</p>
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<p><code class="code">slot-ref-using-class</code> calls the generic function
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<code class="code">slot-unbound</code> if the application of the “getter” closure to
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<var class="var">obj</var> returns an unbound value (see <a class="pxref" href="#Accessing-Slots">slot-unbound</a>).
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-slot_002dset_002dusing_002dclass_0021"><span class="category-def">primitive procedure: </span><span><strong class="def-name">slot-set-using-class!</strong> <var class="def-var-arguments">class obj slot-name value</var><a class="copiable-link" href="#index-slot_002dset_002dusing_002dclass_0021"> ¶</a></span></dt>
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<dd><p>Apply the “setter” closure for the slot named <var class="var">slot-name</var> in
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<var class="var">class</var> to <var class="var">obj</var> and <var class="var">value</var>.
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</p>
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<p><code class="code">slot-set-using-class!</code> calls the generic function
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<code class="code">slot-missing</code> if <var class="var">class</var> does not have a slot definition for a
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slot called <var class="var">slot-name</var> (see <a class="pxref" href="#Accessing-Slots">slot-missing</a>).
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</p></dd></dl>
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<p>Slots whose allocation is per-class rather than per-instance can be
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referenced and set without needing to specify any particular instance.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-class_002dslot_002dref"><span class="category-def">procedure: </span><span><strong class="def-name">class-slot-ref</strong> <var class="def-var-arguments">class slot-name</var><a class="copiable-link" href="#index-class_002dslot_002dref"> ¶</a></span></dt>
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<dd><p>Return the value of the slot named <var class="var">slot-name</var> in class <var class="var">class</var>.
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The named slot must have <code class="code">#:class</code> or <code class="code">#:each-subclass</code>
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allocation (see <a class="pxref" href="Slot-Options.html">allocation</a>).
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</p>
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<p>If there is no such slot with <code class="code">#:class</code> or <code class="code">#:each-subclass</code>
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allocation, <code class="code">class-slot-ref</code> calls the <code class="code">slot-missing</code> generic
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function with arguments <var class="var">class</var> and <var class="var">slot-name</var>. Otherwise, if
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the slot value is unbound, <code class="code">class-slot-ref</code> calls the
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<code class="code">slot-unbound</code> generic function, with the same arguments.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-class_002dslot_002dset_0021"><span class="category-def">procedure: </span><span><strong class="def-name">class-slot-set!</strong> <var class="def-var-arguments">class slot-name value</var><a class="copiable-link" href="#index-class_002dslot_002dset_0021"> ¶</a></span></dt>
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<dd><p>Set the value of the slot named <var class="var">slot-name</var> in class <var class="var">class</var> to
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<var class="var">value</var>. The named slot must have <code class="code">#:class</code> or
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<code class="code">#:each-subclass</code> allocation (see <a class="pxref" href="Slot-Options.html">allocation</a>).
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</p>
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<p>If there is no such slot with <code class="code">#:class</code> or <code class="code">#:each-subclass</code>
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allocation, <code class="code">class-slot-ref</code> calls the <code class="code">slot-missing</code> generic
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function with arguments <var class="var">class</var> and <var class="var">slot-name</var>.
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</p></dd></dl>
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<p>When a <code class="code">slot-ref</code> or <code class="code">slot-set!</code> call specifies a non-existent
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slot name, or tries to reference a slot whose value is unbound, GOOPS
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calls one of the following generic functions.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-slot_002dmissing"><span class="category-def">generic: </span><span><strong class="def-name">slot-missing</strong><a class="copiable-link" href="#index-slot_002dmissing"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-slot_002dmissing-1"><span class="category-def">method: </span><span><strong class="def-name">slot-missing</strong> <var class="def-var-arguments">(class <class>) slot-name</var><a class="copiable-link" href="#index-slot_002dmissing-1"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-slot_002dmissing-2"><span class="category-def">method: </span><span><strong class="def-name">slot-missing</strong> <var class="def-var-arguments">(class <class>) (object <object>) slot-name</var><a class="copiable-link" href="#index-slot_002dmissing-2"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-slot_002dmissing-3"><span class="category-def">method: </span><span><strong class="def-name">slot-missing</strong> <var class="def-var-arguments">(class <class>) (object <object>) slot-name value</var><a class="copiable-link" href="#index-slot_002dmissing-3"> ¶</a></span></dt>
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<dd><p>When an application attempts to reference or set a class or instance
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slot by name, and the slot name is invalid for the specified <var class="var">class</var>
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or <var class="var">object</var>, GOOPS calls the <code class="code">slot-missing</code> generic function.
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</p>
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<p>The default methods all call <code class="code">goops-error</code> with an appropriate
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message.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-slot_002dunbound"><span class="category-def">generic: </span><span><strong class="def-name">slot-unbound</strong><a class="copiable-link" href="#index-slot_002dunbound"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-slot_002dunbound-1"><span class="category-def">method: </span><span><strong class="def-name">slot-unbound</strong> <var class="def-var-arguments">(object <object>)</var><a class="copiable-link" href="#index-slot_002dunbound-1"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-slot_002dunbound-2"><span class="category-def">method: </span><span><strong class="def-name">slot-unbound</strong> <var class="def-var-arguments">(class <class>) slot-name</var><a class="copiable-link" href="#index-slot_002dunbound-2"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-slot_002dunbound-3"><span class="category-def">method: </span><span><strong class="def-name">slot-unbound</strong> <var class="def-var-arguments">(class <class>) (object <object>) slot-name</var><a class="copiable-link" href="#index-slot_002dunbound-3"> ¶</a></span></dt>
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<dd><p>When an application attempts to reference a class or instance slot, and
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the slot’s value is unbound, GOOPS calls the <code class="code">slot-unbound</code> generic
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function.
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</p>
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<p>The default methods all call <code class="code">goops-error</code> with an appropriate
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message.
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</p></dd></dl>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Previous: <a href="Generic-Functions.html">Generic Functions</a>, Up: <a href="Introspection.html">Introspection</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</html>
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