5 - Forth Interest Group

Figure Two. Object class words. I
\ oofclass.fth 910729 rwb
only forth also definitions
#vocs cells constant /context
struct :
; struct /class
#threads cells :field +threads
/context :field +context
cell :field +/body
: >context \ ( 'class -- ) MACROto select object's context.
+context context /context cmove ;
: class: \ ( -- '/body offset )
vocabulary
last @ name> >body >user \ ( -- 'body )
/context cell+ ualloc drop \ allocate space for
\ rest of body
( 'body ) dup context ! \ setup class to be
\ both context..
context over +context /context
\ ( -- 'body 'cxt 'c .cxt /cxt )
cmove definitions \ ( -- 'body ) ..and current vocabs.
+/body dup off 0 \ ( -- '/body offset )
does> ( pfa -- )
>user >context ;
: END \ ( - - I
1 only forth also definitions ;
: ;class \ ( '/body offset -- )
END swap ! ;
1 : METHODS definitions ;
cr . ( oofclass. fth loaded ) cr
that all messages are pref-
aced with a < (less-than)
character. This is done so
that the message and the
methodwill notbe confused.
The < is stripped off before
beiig compiled within the
message with the SWAP 1t
SWAP 1- code.
Figure Four contains
words which construct in-
stance OBJECTS and in-
stance VARiables. An in-
stance contains two fields: its
class pointer and its body.
An instance OBJECT'S body
contains its instance variables.
An instance vivtiable's body
contains its offset within its
parent OBJECT.
Figure Five (pg. 28) con-
tains the initial bootstrapped
object class, called PRIMARY.
I decided that PRIMARY
methods should just go in
the FORTH vocabulary, so that
PRIMARY would not have to
be INHERITed by each class;
thus, the phrase PRIMARY
METHODS has been com-
mented out. The only real
need for a PRIMARY class is
for indirect reference to the
object on top of the stack.
This data must be a declared
class. PRIMARY is declared
just for this situation. Other
than that, its "object-ness"
may be ignored and it can be
treated just like a Forth vari-
able or structure field.
Figure Six (page 28) is a
simple example of building
new classes. Note that the
methods' code is very Forth-
ish: the implicit stack opera-
my Forthmacs (by Mitch Bradley)
for the Atari ST is a 32bit
Forth, so my CELL is set
to four bytes. Adjust yours
accordingly. A simple method
for grouping data into
named records is provided
by the bee words STRUCT : ,
:FIELD, and ; STRUCT.
Common Forth contains
the seeds of an object-oriented
language. CREATE
DOES> provides the abiLity
to abstract data and create
new data types. Vocabularies
can provide privacy, as
well as inheritance. Thus,
only ten new words need be
used to program in OOF:
CLAS S : , ; CLASS, IN-
HERIT, METHODS, END,
MESSAGE, OBJECT, OB-
JECTS, :VAR, and :VARS.
Figure Two provides
definitions of object dur
words. I have implemented
a CLASS as a hybrid of
STRUCT : and VOCABU-
LARY-which reflects the
twofold nature of a class: to
provide the internal data
structure of its type, and to
house the data's routines.
See Figure Seven (page 29)
for a good example of how
these words are used. (In
Forthmacs, the parameter
Lld of a vocabulary is kept
in the user area. SO, while the
general idea is to build a
vocabulary with two extra
fields-he+contextfieldand
the +/body field-these must
be allocated in the user area
for Forthmacs.)
MESSAGE is defmed in
Figure Three. A message
merely records its name as a
suing within its body, and
then vies to Fid it in the
context vocabulary (CLASS)
at run time. CT~IS is an example
of late binding.) Note
tors (DUP, SWAP, TUCK, etc.)
srill needed,
are passed on the stack,
andRPNsynmissulluxL
thus, OOF blends with
common Forth. It is not necessary
for 2COMPLEX to expli.tly
INHERIT from the
COMPLEX in order for
the (like to pass
the