with-positions

Arguments: ( &rest position-variables) &body body

This macro and the related with-boxes and with-positions-and-boxes stack-allocate position and/or box objects to provide temporary objects without consing, thereby possibly reducing the time spent garbage collecting.

The objects are valid only through the dynamic extent of the macro call, and unpredictable errors may occur if they are referenced afterward. Further, functions such as typep and positionp that query the type of an object should not be used on objects created by this macro. Type information for such objects is sacrificed to achieve the non-consing efficiency of stack allocation, and the return values from such type-checking calls are undefined.

Note that now that Common Graphics is multithreaded, the use of the alternative technique of placing position and box constants in source code via the #. reader macro, as in #.(make-position 0 0), is now strongly discouraged due to potential re-entrancy conflicts. Example:

(defun draw-diagonal-in-box (window box)
  ;; Avoids consing positions, as when calling box-top-left
  (with-positions (tpos1 tpos2)
    (draw-line window
      (nmake-position tpos1 (box-left box)(box-top box))
      (nmake-position tpos2 (box-right box)(box-bottom box)))))

See About using multiple threads in the IDE in cgide.htm.

Copyright (c) 1998-2002, Franz Inc. Oakland, CA., USA. All rights reserved.
Documentation for Allegro CL version 6.2. This page was not revised from the 6.1 page.
Created 2002.2.26.