5 - Forth Interest Group

I Listing Four. PCYerk source code. I
I
\ PCYerk
\ Object-oriented extensions to Forth
\ a la Yerk (once, NEON)
\ Written for Upper Deck Forth, version 2.0
\ R. E. Grehan
\ Sorry, I can't stand "then"
: endif [compile] then ; immediate
\ *************
\ ** STORAGE **
\ *************
34 $variable tstring \ Used in parsing names
\ Method stack
20 constant METH-STACK-SIZE
create mstack
METH-STACK-SIZE allot
\ Objects stack
20 constant OBJ-STACK-SIZE
create ostack
OBJ-STACK-SIZE allot
\ Instance variable names segment
variable ivar-seg \ Segment
20 constant IVAR-SEG-SIZE \ Segment size in paragraphs
variable ivar-next \ Offset to next free loc.
\ Methods names segment
variable methname-seg \ Segment
100 constant METHNAME-SEG-SIZE \ Segment size in paragraphs
variable methname-next \ Offset to next free slot
variable curr-meth# \ Current method #
variable my-meth# \ Method # about to be define1
\ Class definitions
variable curr-class-off \ Offset to current class
variable curr-meth-tail \ Current method tail
\ ..............................
\ ** METHOD AND OBJECT STACKS **
\ ..............................
\ NOTE: Neither stack do any bounds checking (for speed's
\ sake). If bounds checking is added, the stack manipulatior
1 \ words should be written in machine language.
\ Initialize the method stack ... stores selector ids
: mstack-init ( -- )
mstack dup !
\ Push top word onto method stack
: mpush (n-1
mstack @ 2+ ! \ Save item
2 mstack +! \ Increment
\ Copy top of mstack to dstack
: mstack->dstack ( -- n )
mstack @ @
I
Forth Dimensions 9
(Tisting continues
)olygon class. Then you
ould store the polygon's
ertex list into the one-di-
nensional array object.
It turns out you can do
his by defLning one of the
)olygon's instance variables
o be a pointer to an object of
lass Ida rray. Essentially,
leu build a headerless ob-
ect; that is, one that does not
lave a head in the Forth
iictionaly. The word in-
stantiate will build such
L headerless object. All in-
stantiate needs on the
;tack is a starting address in
he variable region (where
he new object will go) and
L pointer to the class defini-
ion. The code showing this
echnique is in Listing Three
:which presumes that you've
lefmed the ldarray class
dready).
Notice that I've defined
init : to multiply the num-
Der of vertex entries by two,
since the vertex list will carry
an x and y coordinate for
each vertex. The next line
stores the address of the next
Free location in the variable
segment into
^vertex-list, which be-
comes a pointer to our one-
dimensional array. I then use
[ ' 1 to retrieve the code ad-
dress of the ldar ray class,
then fetch the address of that
class defmition. The in-
stantiate word actually
creates the array. Keep in
mind that, when instan-
t i a t e executes, the
lda r ra y object's initial
method-which allocates the
memory space-will be ex-
ecuted.
Now we can store an x
and y coordinate into the
ldarray object using a
method called to:, which
we've presumably already
defined for the ldarray
dass. You can extend this idea
as far as you'd like to go,
aeatingpointelsto objwith
pointers to objects, and so on
Nuts and Bolts
Refer to Listing Four, the
complete source for PCYerk.
The heart of PCYerk is
January 1992 February