src/pcl/slot-name.lisp

   1 ;;;; This software is part of the SBCL system. See the README file for
   2 ;;;; more information.
   3
   4 ;;;; This software is derived from software originally released by Xerox
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   6 ;;;; to the software are in the public domain and are provided with
   7 ;;;; absolutely no warranty. See the COPYING and CREDITS files for more
   8 ;;;; information.
   9
  10 ;;;; copyright information from original PCL sources:
  11 ;;;;
  12 ;;;; Copyright (c) 1985, 1986, 1987, 1988, 1989, 1990 Xerox Corporation.
  13 ;;;; All rights reserved.
  14 ;;;;
  15 ;;;; Use and copying of this software and preparation of derivative works based
  16 ;;;; upon this software are permitted. Any distribution of this software or
  17 ;;;; derivative works must comply with all applicable United States export
  18 ;;;; control laws.
  19 ;;;;
  20 ;;;; This software is made available AS IS, and Xerox Corporation makes no
  21 ;;;; warranty about the software, its performance or its conformity to any
  22 ;;;; specification.
  23
  24 (in-package "SB!PCL")
  25 \f
  26 ;; This choice of naming structure is perhaps unfortunate, because were the
  27 ;; names 2-lists, the globaldb hack to support this would instead be
  28 ;; a natural use of the (SETF <x>) style naming that globaldb favors.
  29 ;; But this naming is documented, and changing it would be incompatible.
  30 ;; The 4-part name can be thought of as a 2-part name because
  31 ;; half of it is composed of constants:
  32 ;; (SB-PCL::SLOT-ACCESSOR :GLOBAL <foo> SB-PCL::{READER|WRITER|BOUNDP})
  33 ;; -> ({READER|WRITER|BOUNDP} <foo>)
  34 ;;
  35 (defun slot-reader-name (slot-name)
  36   (list 'slot-accessor :global slot-name 'reader))
  37
  38 (defun slot-writer-name (slot-name)
  39   (list 'slot-accessor :global slot-name 'writer))
  40
  41 (defun slot-boundp-name (slot-name)
  42   (list 'slot-accessor :global slot-name 'boundp))
  43
  44 ;;; This is the value that we stick into a slot to tell us that it is
  45 ;;; unbound. It may seem gross, but for performance reasons, we make
  46 ;;; this an interned symbol. That means that the fast check to see
  47 ;;; whether a slot is unbound is to say (EQ <val> '..SLOT-UNBOUND..).
  48 ;;; That is considerably faster than looking at the value of a special
  49 ;;; variable.
  50 ;;;
  51 ;;; It seems only reasonable to also export this for users, since
  52 ;;; otherwise dealing with STANDARD-INSTANCE-ACCESS becomes harder
  53 ;;; -- and slower -- than it needs to be.
  54 (defconstant +slot-unbound+ '..slot-unbound..
  55   #!+sb-doc
  56   "SBCL specific extensions to MOP: if this value is read from an
  57 instance using STANDARD-INSTANCE-ACCESS, the slot is unbound.
  58 Similarly, an :INSTANCE allocated slot can be made unbound by
  59 assigning this to it using (SETF STANDARD-INSTANCE-ACCESS).
  60
  61 Value of +SLOT-UNBOUND+ is unspecified, and should not be relied to be
  62 of any particular type, but it is guaranteed to be suitable for EQ
  63 comparison.")