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154 lines
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<title>Class Precedence List (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Class-Precedence-List">
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<p>
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Next: <a href="Sorting-Methods.html" accesskey="n" rel="next">Sorting Methods</a>, Up: <a href="Inheritance.html" accesskey="u" rel="up">Inheritance</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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<h4 class="subsection" id="Class-Precedence-List-1"><span>8.7.1 Class Precedence List<a class="copiable-link" href="#Class-Precedence-List-1"> ¶</a></span></h4>
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<p>What happens when a class inherits from two or more superclasses that
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have a slot with the same name but incompatible definitions — for
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example, different init values or slot allocations? We need a rule for
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deciding which slot definition the derived class ends up with, and this
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rule is provided by the class’s <em class="dfn">Class Precedence
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List</em>.<a class="footnote" id="DOCF34" href="#FOOT34"><sup>34</sup></a>
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</p>
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<p>Another problem arises when invoking a generic function, and there is
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more than one method that could apply to the call arguments. Here we
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need a way of ordering the applicable methods, so that Guile knows which
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method to use first, which to use next if that method calls
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<code class="code">next-method</code>, and so on. One of the ingredients for this ordering
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is determining, for each given call argument, which of the specializing
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classes, from each applicable method’s definition, is the most specific
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for that argument; and here again the class precedence list helps.
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</p>
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<p>If inheritance was restricted such that each class could only have one
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superclass — which is known as <em class="dfn">single</em> inheritance — class
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ordering would be easy. The rule would be simply that a subclass is
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considered more specific than its superclass.
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</p>
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<p>With multiple inheritance, ordering is less obvious, and we have to
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impose an arbitrary rule to determine precedence. Suppose we have
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define-class X ()
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(x #:init-value 1))
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(define-class Y ()
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(x #:init-value 2))
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(define-class Z (X Y)
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(...))
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</pre></div>
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<p>Clearly the <code class="code">Z</code> class is more specific than <code class="code">X</code> or <code class="code">Y</code>,
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for instances of <code class="code">Z</code>. But which is more specific out of <code class="code">X</code>
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and <code class="code">Y</code> — and hence, for the definitions above, which
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<code class="code">#:init-value</code> will take effect when creating an instance of
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<code class="code">Z</code>? The rule in GOOPS is that the superclasses listed earlier
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are more specific than those listed later. Hence <code class="code">X</code> is more
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specific than <code class="code">Y</code>, and the <code class="code">#:init-value</code> for slot <code class="code">x</code> in
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instances of <code class="code">Z</code> will be 1.
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</p>
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<p>Hence there is a linear ordering for a class and all its
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superclasses, from most specific to least specific, and this ordering is
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called the Class Precedence List of the class.
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</p>
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<p>In fact the rules above are not quite enough to always determine a
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unique order, but they give an idea of how things work. For example,
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for the <code class="code">F</code> class shown in <a class="ref" href="Inheritance.html#fig_003ahier">Figure 8.2</a>, the class precedence
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list is
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</p>
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<div class="example">
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<pre class="example-preformatted">(f d e a c b <object> <top>)
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</pre></div>
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<p>In cases where there is any ambiguity (like this one), it is a bad idea
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for programmers to rely on exactly what the order is. If the order for
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some superclasses is important, it can be expressed directly in the
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class definition.
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</p>
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<p>The precedence list of a class can be obtained by calling
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<code class="code">class-precedence-list</code>. This function returns a ordered list
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whose first element is the most specific class. For instance:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(class-precedence-list B) ⇒ (#<<class> B 401b97c8>
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#<<class> <object> 401e4a10>
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#<<class> <top> 4026a9d8>)
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</pre></div>
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<p>Or for a more immediately readable result:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(map class-name (class-precedence-list B)) ⇒ (B <object> <top>)
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</pre></div>
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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="FOOT34" href="#DOCF34">(34)</a></h5>
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<p>This section is an adaptation of material from Jeff
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Dalton’s (J.Dalton@ed.ac.uk) <cite class="cite">Brief introduction to CLOS</cite></p>
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</div>
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<p>
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Next: <a href="Sorting-Methods.html">Sorting Methods</a>, Up: <a href="Inheritance.html">Inheritance</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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