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<h3 class="section" id="Inheritance-1"><span>8.7 Inheritance<a class="copiable-link" href="#Inheritance-1"> &para;</a></span></h3>
<p>Here are some class definitions to help illustrate inheritance:
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(define-class A () a)
(define-class B () b)
(define-class C () c)
(define-class D (A B) d a)
(define-class E (A C) e c)
(define-class F (D E) f)
</pre></div>
<p><code class="code">A</code>, <code class="code">B</code>, <code class="code">C</code> have a null list of superclasses. In this
case, the system will replace the null list by a list which only
contains <code class="code">&lt;object&gt;</code>, the root of all the classes defined by
<code class="code">define-class</code>. <code class="code">D</code>, <code class="code">E</code>, <code class="code">F</code> use multiple
inheritance: each class inherits from two previously defined classes.
Those class definitions define a hierarchy which is shown in
<a class="ref" href="#fig_003ahier">Figure 8.2</a>. In this figure, the class <code class="code">&lt;top&gt;</code> is also shown;
this class is the superclass of all Scheme objects. In particular,
<code class="code">&lt;top&gt;</code> is the superclass of all standard Scheme
types.
</p>
<div class="float" id="fig_003ahier">
<pre class="verbatim"> &lt;top&gt;
/ \\\_____________________
/ \\___________ \
/ \ \ \
&lt;object&gt; &lt;pair&gt; &lt;procedure&gt; &lt;number&gt;
/ | \ |
/ | \ |
A B C &lt;complex&gt;
|\__/__ | |
\ / \ / |
D E &lt;real&gt;
\ / |
F |
&lt;integer&gt;
</pre>
<div class="caption"><p><strong class="strong">Figure 8.2: </strong>A class hierarchy.</p></div></div>
<p>When a class has superclasses, its set of slots is calculated by taking
the union of its own slots and those of all its superclasses. Thus each
instance of D will have three slots, <code class="code">a</code>, <code class="code">b</code> and
<code class="code">d</code>). The slots of a class can be discovered using the
<code class="code">class-slots</code> primitive. For instance,
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(class-slots A) &rArr; ((a))
(class-slots E) &rArr; ((a) (e) (c))
(class-slots F) &rArr; ((e) (c) (b) (d) (a) (f))
</pre></div>
<p>The ordering of the returned slots is not significant.
</p>
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