From ...
From: Erik Naggum <erik@naggum.no>
Subject: Re: aref vs svref
Date: 1999/01/25
Message-ID: <3126281048122906@naggum.no>#1/1
X-Deja-AN: 436785224
References: <wk7lubl3u1.fsf@mit.edu>
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Organization: Naggum Software; +47 8800 8879; http://www.naggum.no
Newsgroups: comp.lang.lisp

* David Bakhash <cadet@mit.edu>
| Let's say I define a class, `class-A', and I have an array of objects of
| type class-A.  If class-A is a complex class, with lots of slots, etc, is
| it better to define the (1-D) array of type T so I can use `svref', or is
| it better to tell the compiler the truth, which is that the :element-type
| is really class-A (in which case it may sometimes be (or null class-A)).
| Sometimes it's not so clear if telling the compiler things is really a
| good idea.

  I guess you're really asking whether a vector of class instances will be
  a simple vector unless you have fill pointers, displacement, or make it
  adjustable.  the answer is yes.  only a small set of types can be
  expected to specialize the array type and all of them are built-in types.
  the key is whether the object has to be referenced through a pointer or
  not, or can reasonably be allocated a fixed-width slot in a vector.  if
  it cannot be allocated a fixed-width slot, it has to use a pointer.

  you can probably find out by using UPGRADED-ARRAY-ELEMENT-TYPE on the
  type and see whether it is T.  if it is (and you do none of the other
  stuff that requires an array header), you must get a simple vector.

  of course, you know that you can find out whether a particular array is a
  simple vector with SIMPLE-VECTOR-P.

#:Erik
-- 
  SIGTHTBABW: a signal sent from Unix to its programmers at random
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