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144 lines
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Instance Creation Protocol (Guile Reference Manual)</title>
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<link href="The-Metaobject-Protocol.html" rel="up" title="The Metaobject Protocol">
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<div class="subsection-level-extent" id="Instance-Creation-Protocol">
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<p>
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Next: <a href="Class-Definition-Protocol.html" accesskey="n" rel="next">Class Definition Protocol</a>, Previous: <a href="MOP-Specification.html" accesskey="p" rel="prev">MOP Specification</a>, Up: <a href="The-Metaobject-Protocol.html" accesskey="u" rel="up">The Metaobject Protocol</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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<h4 class="subsection" id="Instance-Creation-Protocol-1"><span>8.11.4 Instance Creation Protocol<a class="copiable-link" href="#Instance-Creation-Protocol-1"> ¶</a></span></h4>
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<p><code class="code">make <class> . <var class="var">initargs</var></code> (method)
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</p>
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<ul class="itemize mark-bullet">
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<li><code class="code">allocate-instance <var class="var">class</var> <var class="var">initargs</var></code> (generic)
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<p>The applied <code class="code">allocate-instance</code> method should allocate storage for
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a new instance of class <var class="var">class</var> and return the uninitialized instance.
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</p>
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</li><li><code class="code">initialize <var class="var">instance</var> <var class="var">initargs</var></code> (generic)
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<p><var class="var">instance</var> is the uninitialized instance returned by
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<code class="code">allocate-instance</code>. The applied method should initialize the new
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instance in whatever sense is appropriate for its class. The method’s
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return value is ignored.
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</p></li></ul>
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<p><code class="code">make</code> itself is a generic function. Hence the <code class="code">make</code>
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invocation itself can be customized in the case where the new instance’s
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metaclass is more specialized than the default <code class="code"><class></code>, by
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defining a <code class="code">make</code> method that is specialized to that metaclass.
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</p>
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<p>Normally, however, the method for classes with metaclass <code class="code"><class></code>
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will be applied. This method calls two generic functions:
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</p>
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<ul class="itemize mark-bullet">
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<li>(allocate-instance <var class="var">class</var> . <var class="var">initargs</var>)
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</li><li>(initialize <var class="var">instance</var> . <var class="var">initargs</var>)
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</li></ul>
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<p><code class="code">allocate-instance</code> allocates storage for and returns the new
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instance, uninitialized. You might customize <code class="code">allocate-instance</code>,
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for example, if you wanted to provide a GOOPS wrapper around some other
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object programming system.
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</p>
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<p>To do this, you would create a specialized metaclass, which would act as
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the metaclass for all classes and instances from the other system. Then
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define an <code class="code">allocate-instance</code> method, specialized to that
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metaclass, which calls a Guile primitive C function (or FFI code), which
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in turn allocates the new instance using the interface of the other
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object system.
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</p>
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<p>In this case, for a complete system, you would also need to customize a
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number of other generic functions like <code class="code">make</code> and
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<code class="code">initialize</code>, so that GOOPS knows how to make classes from the
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other system, access instance slots, and so on.
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</p>
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<p><code class="code">initialize</code> initializes the instance that is returned by
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<code class="code">allocate-instance</code>. The standard GOOPS methods perform
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initializations appropriate to the instance class.
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</p>
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<ul class="itemize mark-bullet">
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<li>At the least specialized level, the method for instances of type
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<code class="code"><object></code> performs internal GOOPS instance initialization, and
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initializes the instance’s slots according to the slot definitions and
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any slot initialization keywords that appear in <var class="var">initargs</var>.
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</li><li>The method for instances of type <code class="code"><class></code> calls
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<code class="code">(next-method)</code>, then performs the class initializations described
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in <a class="ref" href="Class-Definition-Protocol.html">Class Definition Protocol</a>.
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</li><li>and so on for generic functions, methods, operator classes …
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</li></ul>
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<p>Similarly, you can customize the initialization of instances of any
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application-defined class by defining an <code class="code">initialize</code> method
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specialized to that class.
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</p>
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<p>Imagine a class whose instances’ slots need to be initialized at
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instance creation time by querying a database. Although it might be
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possible to achieve this a combination of <code class="code">#:init-thunk</code> keywords
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and closures in the slot definitions, it may be neater to write an
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<code class="code">initialize</code> method for the class that queries the database once
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and initializes all the dependent slot values according to the results.
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</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="Class-Definition-Protocol.html">Class Definition Protocol</a>, Previous: <a href="MOP-Specification.html">MOP Specification</a>, Up: <a href="The-Metaobject-Protocol.html">The Metaobject Protocol</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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