5 - Forth Interest Group

instance variables can re-
solve their addresses by cal-
culating offsets from the
object address on the
ostack.)
Late binding is also re-
ferred to as deferred bind-
ing. In simple terms, it means
the system doesn't know
what object you're going to
send a message to. Hence,
the system can't bind the
message to an execution
address at compile time.
Here's an example of
PCYerk's late binding:
get: { integerobj @ )
Here, I'm assuming that
the variable integerob j
holds an object's address. At
compile time, there's no way
the system can know what
objedwillbe in integerob j
when the program executes.
The system must, therefore,
determine the execution ad-
dress of get : at run time.
The words to handle late
binding are { and 1. (Yerk
used [ and I, but those words
were already taken in UD
Forth) The curly brackets
should immediately follow
the message, and may en-
close any Forth expression
that yields an object's ad-
dress.
Of course, late binding
yields code that runs more
slowly than code using early
binding. This is because late
binding defers until run time
processing that would have
been performed at compile
time.
Late binding lets you-
with only minor additional
programmin~ypass one
of PCYerk's deficiencies:
namely, that you define all
instance variables using the
ivar word. You cannot use
an object as an instance vari-
able. Suppose you've defined
a class called ldarray that
lets you build one-dimen-
sional array objects, and a
class called polygon that
builds polygon objects. It
would be nice to have a
ldarray object as one of
the instance variables of the
Figure One. Class header structure. I
Tokens Variable
Segment Segment
Listing Three. Building a headerless object. I
:class polygon