169 lines
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169 lines
9 KiB
HTML
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Class Definition (Guile Reference Manual)</title>
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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="GOOPS.html" rel="up" title="GOOPS">
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<div class="section-level-extent" id="Class-Definition">
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<p>
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Next: <a href="Instance-Creation.html" accesskey="n" rel="next">Instance Creation and Slot Access</a>, Previous: <a href="Copyright-Notice.html" accesskey="p" rel="prev">Copyright Notice</a>, Up: <a href="GOOPS.html" accesskey="u" rel="up">GOOPS</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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<hr>
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<h3 class="section" id="Class-Definition-1"><span>8.2 Class Definition<a class="copiable-link" href="#Class-Definition-1"> ¶</a></span></h3>
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<p>A new class is defined with the <code class="code">define-class</code> syntax:
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</p>
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<a class="index-entry-id" id="index-define_002dclass"></a>
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<a class="index-entry-id" id="index-class"></a>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define-class <var class="var">class</var> (<var class="var">superclass</var> ...)
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<var class="var">slot-description</var> ...
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<var class="var">class-option</var> ...)
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</pre></div>
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<p><var class="var">class</var> is the class being defined. The list of <var class="var">superclass</var>es
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specifies which existing classes, if any, to inherit slots and
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properties from. <em class="dfn">Slots</em> hold per-instance<a class="footnote" id="DOCF29" href="#FOOT29"><sup>29</sup></a> data, for instances of
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that class — like “fields” or “member variables” in other object
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oriented systems. Each <var class="var">slot-description</var> gives the name of a slot
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and optionally some “properties” of this slot; for example its initial
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value, the name of a function which will access its value, and so on.
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Class options, slot descriptions and inheritance are discussed more
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below.
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<a class="index-entry-id" id="index-slot"></a>
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-define_002dclass-1"><span class="category-def">syntax: </span><span><strong class="def-name">define-class</strong> <var class="def-var-arguments">name (super …) slot-definition … class-option …</var><a class="copiable-link" href="#index-define_002dclass-1"> ¶</a></span></dt>
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<dd><p>Define a class called <var class="var">name</var> that inherits from <var class="var">super</var>s, with
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direct slots defined by <var class="var">slot-definition</var>s and <var class="var">class-option</var>s.
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The newly created class is bound to the variable name <var class="var">name</var> in the
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current environment.
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</p>
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<p>Each <var class="var">slot-definition</var> is either a symbol that names the slot or a
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list,
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</p>
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<div class="example">
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<pre class="example-preformatted">(<var class="var">slot-name-symbol</var> . <var class="var">slot-options</var>)
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</pre></div>
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<p>where <var class="var">slot-name-symbol</var> is a symbol and <var class="var">slot-options</var> is a
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list with an even number of elements. The even-numbered elements of
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<var class="var">slot-options</var> (counting from zero) are slot option keywords; the
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odd-numbered elements are the corresponding values for those keywords.
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</p>
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<p>Each <var class="var">class-option</var> is an option keyword and corresponding value.
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</p></dd></dl>
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<p>As an example, let us define a type for representing a complex number
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in terms of two real numbers.<a class="footnote" id="DOCF30" href="#FOOT30"><sup>30</sup></a> This can be done with the following class definition:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define-class <my-complex> (<number>)
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r i)
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</pre></div>
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<p>This binds the variable <code class="code"><my-complex></code> to a new class whose
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instances will contain two slots. These slots are called <code class="code">r</code> and
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<code class="code">i</code> and will hold the real and imaginary parts of a complex
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number. Note that this class inherits from <code class="code"><number></code>, which is a
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predefined class.<a class="footnote" id="DOCF31" href="#FOOT31"><sup>31</sup></a>
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</p>
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<p>Slot options are described in the next section. The possible class
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options are as follows.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-_0023_003ametaclass"><span class="category-def">class option: </span><span><strong class="def-name">#:metaclass</strong> <var class="def-var-arguments">metaclass</var><a class="copiable-link" href="#index-_0023_003ametaclass"> ¶</a></span></dt>
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<dd><p>The <code class="code">#:metaclass</code> class option specifies the metaclass of the class
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being defined. <var class="var">metaclass</var> must be a class that inherits from
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<code class="code"><class></code>. For the use of metaclasses, see <a class="ref" href="Metaobjects-and-the-Metaobject-Protocol.html">Metaobjects and the Metaobject Protocol</a> and <a class="ref" href="Metaclasses.html">Metaclasses</a>.
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</p>
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<p>If the <code class="code">#:metaclass</code> option is absent, GOOPS reuses or constructs a
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metaclass for the new class by calling <code class="code">ensure-metaclass</code>
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(see <a class="pxref" href="Class-Definition-Protocol.html">ensure-metaclass</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-_0023_003aname"><span class="category-def">class option: </span><span><strong class="def-name">#:name</strong> <var class="def-var-arguments">name</var><a class="copiable-link" href="#index-_0023_003aname"> ¶</a></span></dt>
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<dd><p>The <code class="code">#:name</code> class option specifies the new class’s name. This
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name is used to identify the class whenever related objects - the class
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itself, its instances and its subclasses - are printed.
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</p>
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<p>If the <code class="code">#:name</code> option is absent, GOOPS uses the first argument to
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<code class="code">define-class</code> as the class name.
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</p></dd></dl>
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</div>
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<div class="footnotes-segment">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h5 class="footnote-body-heading"><a id="FOOT29" href="#DOCF29">(29)</a></h5>
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<p>Usually — but
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see also the <code class="code">#:allocation</code> slot option.</p>
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<h5 class="footnote-body-heading"><a id="FOOT30" href="#DOCF30">(30)</a></h5>
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<p>Of course Guile already
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provides complex numbers, and <code class="code"><complex></code> is in fact a predefined
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class in GOOPS; but the definition here is still useful as an
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example.</p>
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<h5 class="footnote-body-heading"><a id="FOOT31" href="#DOCF31">(31)</a></h5>
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<p><code class="code"><number></code> is the direct superclass of
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the predefined class <code class="code"><complex></code>; <code class="code"><complex></code> is the
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superclass of <code class="code"><real></code>, and <code class="code"><real></code> is the superclass of
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<code class="code"><integer></code>.</p>
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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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Next: <a href="Instance-Creation.html">Instance Creation and Slot Access</a>, Previous: <a href="Copyright-Notice.html">Copyright Notice</a>, Up: <a href="GOOPS.html">GOOPS</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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