Detailed Description of Mini-OOF

Object oriented system with late binding typically use a "vtable"-approach: the first variable in each object is a pointer to a table, which contains the methods as function pointers. This vtable may contain some other informations, too.

instance var
instance var
instance var


So first, let's declare methods:

: method ( m v -- m' v ) Create  over , swap cell+ swap
  DOES> ( ... o -- ... ) @ over @ + @ execute ;

During method declaration, the number of methods and instance variables is on the stack (in address units). method creates one method and increments the method number. To execute a method, it takes the object, fetches the vtable pointer, adds the offset, and executes the xt stored there. Each method takes the object it is invoked from as top of stack parameter. The method itself should consume that object.

Now, we also have to declare instance variables

: var ( m v size -- m v' ) Create  over , +
  DOES> ( o -- addr ) @ + ;

Same as above, a word is created with the current offset. Instance variables can have different sizes (cells, floats, doubles, chars), so all we do is take the size and add it to the offset. If your machine has alignment restrictions, put the proper aligned or faligned before the variable, it will adjust the variable offset. That's why it is on the top of stack.

We need a starting point (the empty object) and some syntactic sugar:

Create object  1 cells , 2 cells ,
: class ( class -- class methods vars ) dup 2@ ;

Now, for inheritance, the vtable of the parent object has to be copied, when a new, derived class is declared. This gives all the methods of the parent class, which can be overridden, though.

: end-class  ( class methods vars -- )
  Create  here >r , dup , 2 cells ?DO ['] noop , 1 cells +LOOP
  cell+ dup cell+ r> rot @ 2 cells /string move ;

The first line creates the vtable, initialized with noops. The second line is the inheritance mechanism, it copies the xts from the parent vtable.

We still have no way to define new methods, let's do that now:

: defines ( xt class -- ) ' >body @ + ! ;

To allocate a new object, we need a word, too:

: new ( class -- o )  here over @ allot swap over ! ;

And sometimes derived classes want to access the method of the parent object. There are two ways to achieve this with this OOF: first, you could use named words, and second, you could look up the vtable of the parent object.

: :: ( class "name" -- ) ' >body @ + @ compile, ;

Note: this early binding means that the xt you have assigned to the method may not have non-default compilation semamtics applied with SET-COMPILER (e.g. in VFX Forth). Example: You want to perform the execution semantics of IF in a method

:noname ( -- if-sys ) postpone IF ; some-class defines if-method

is the only portable way to achive this. If you have a mix of interpretation and compilation semantics, like with S", achieving a deliberate selection portable may even be more difficult:

:noname state @ >r postpone [ ['] s" execute r> IF ] THEN ; class defines xxx

will most likely work on all systems.

An Example

Nothing can be more confusing than a good example, so here is one. First let's declare a text object (further called button), that stores text and position:

object class
  cell var text
  cell var len
  cell var x
  cell var y
  method init
  method draw
end-class button

Now, implement the two methods, draw and init:

:noname ( o -- ) >r
 r@ x @ r@ y @ at-xy  r@ text @ r> len @ type ;
 button defines draw
:noname ( addr u o -- ) >r
 0 r@ x ! 0 r@ y ! r@ len ! r> text ! ;
 button defines init

For inheritance, we define a class bold-button, with no new data and no new methods.

button class
end-class bold-button

: bold   27 emit ." [1m" ;
: normal 27 emit ." [0m" ;

:noname bold [ button :: draw ] normal ; bold-button defines draw

And finally, some code to demonstrate how to create objects and apply methods:

button new Constant foo
s" thin foo" foo init
foo draw
bold-button new Constant bar
s" fat bar" bar init
1 bar y !
bar draw

Larger Example

Gerry Jackson wrote a larger example, LexGen. This example extends Mini-OOF slightly, but it shows that with these small changes, Mini-OOF can be used for real-world applications.

Bernd Paysan, 24sep1998, 27jul2008