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<TITLE>CLHS: Issue ALLOCATE-INSTANCE Writeup</TITLE>
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<H2>Issue ALLOCATE-INSTANCE Writeup</H2>
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<PRE><B>Issue:</B> <A REL=DEFINITION HREF="../Body/f_alloca.htm#allocate-instance"><B>ALLOCATE-INSTANCE</B></A><P>
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<P>
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<B>References:</B> 88-002R p.1-41<P>
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89-003 p.3-35<P>
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<P>
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Related issues: <A HREF="iss215.htm">LOAD-OBJECTS</A><P>
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<P>
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<B>Category:</B> ADDITION<P>
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<P>
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<B>Edit history:</B> 30-Apr-90, Version 1 by Moon<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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The function page for the generic function <A REL=DEFINITION HREF="../Body/f_alloca.htm#allocate-instance"><B>ALLOCATE-INSTANCE</B></A> was<P>
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accidentally omitted from 88-002R.<P>
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<P>
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<B>Proposal (ALLOCATE-INSTANCE:ADD):<P>
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</B><P>
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Add a generic function <A REL=DEFINITION HREF="../Body/f_alloca.htm#allocate-instance"><B>ALLOCATE-INSTANCE</B></A> that takes one required<P>
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argument, a <A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A>, plus initialization arguments, and returns one value,<P>
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a freshly created instance of the given <A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A>. The <A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A> argument must<P>
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be an actual <A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A>, not a <A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A> name, and must not be a built-in <A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A>.<P>
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<P>
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The object returned by <A REL=DEFINITION HREF="../Body/f_alloca.htm#allocate-instance"><B>ALLOCATE-INSTANCE</B></A> is "uninitialized"; when the<P>
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<A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A> is a <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> <A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A>, this means the slots are unbound; when the<P>
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<A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A> is a <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> <A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A>, this means the slots' values are unspecified.<P>
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<P>
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Common Lisp does not specify how to add methods to ALLOCATE-INSTANCE;<P>
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this capability might be added by the Metaobject Protocol.<P>
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<P>
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This is Symbolics issue #32 and Loosemore's issue #6 of 27 Feb 90.<P>
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<P>
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<B>Examples:<P>
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</B><P>
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(<A REL=DEFINITION HREF="../Body/m_defcla.htm#defclass"><B>DEFCLASS</B></A> MYCLASS () (A B C))<P>
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(<A REL=DEFINITION HREF="../Body/f_alloca.htm#allocate-instance"><B>ALLOCATE-INSTANCE</B></A> (<A REL=DEFINITION HREF="../Body/f_find_c.htm#find-class"><B>FIND-CLASS</B></A> 'MYCLASS))<P>
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(<A REL=DEFINITION HREF="../Body/f_slt_bo.htm#slot-boundp"><B>SLOT-BOUNDP</B></A> * 'B) => <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A><P>
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<P>
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<B>Rationale:<P>
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</B><P>
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Some of the authors of 88-002R say the omission of <A REL=DEFINITION HREF="../Body/f_alloca.htm#allocate-instance"><B>ALLOCATE-INSTANCE</B></A> was<P>
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simply a mistake. Note that <A HREF="iss215.htm">LOAD-OBJECTS:MAKE-LOAD-FORM</A> assumes the<P>
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existence of <A REL=DEFINITION HREF="../Body/f_alloca.htm#allocate-instance"><B>ALLOCATE-INSTANCE</B></A> and that user-written <A REL=DEFINITION HREF="../Body/f_mk_ld_.htm#make-load-form"><B>MAKE-LOAD-FORM</B></A><P>
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methods that return two values will typically use <A REL=DEFINITION HREF="../Body/f_alloca.htm#allocate-instance"><B>ALLOCATE-INSTANCE</B></A> in<P>
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the first value.<P>
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<P>
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<B>Current practice:<P>
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</B><P>
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Symbolics Genera 8.0 does not <A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> <A REL=DEFINITION HREF="../Body/f_alloca.htm#allocate-instance"><B>ALLOCATE-INSTANCE</B></A> (although<P>
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it does exist in an internal package). Lucid 4.0.0 Beta-1 provides<P>
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<A REL=DEFINITION HREF="../Body/f_alloca.htm#allocate-instance"><B>ALLOCATE-INSTANCE</B></A>.<P>
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<P>
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<B>Cost to Implementors:<P>
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</B><P>
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Any CLOS implementation needs <A REL=DEFINITION HREF="../Body/f_alloca.htm#allocate-instance"><B>ALLOCATE-INSTANCE</B></A> or something like it<P>
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internally, so implementing this proposal is largely a matter of<P>
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exporting the symbol from the COMMON-LISP package and documenting it.<P>
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<P>
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<B>Cost to Users:<P>
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</B><P>
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None.<P>
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<P>
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<B>Cost of non-adoption:<P>
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</B><P>
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<A REL=DEFINITION HREF="../Body/f_mk_ld_.htm#make-load-form"><B>MAKE-LOAD-FORM</B></A> would be considerably less useful.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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None other than the impact of adding a symbol to the COMMON-LISP package.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Missing piece of CLOS provided.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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Neutral.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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None.<P>
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</PRE>
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