 
 
 
 Other Aspects of the Object Extension
In this section we describe the declaration of ``object'' types and
local declarations in classes. The latter can be used for class variables
by making constructors that reference the local environment.
 Interfaces
Class interfaces are generally infered by the type system, but they can also be
defined by a type declaration. Only public methods appear in this type.
 Syntax 
 
| class type name = | 
| object | 
| : | 
| val namei : typei | 
| : | 
| method namej : typej | 
| : | 
| end | 
Thus we can define the class point interface:
# class type interf_point  = 
 object 
   method get_x : int
   method get_y : int
   method moveto : (int * int ) -> unit
   method rmoveto : (int * int )  -> unit
   method to_string : unit -> string
   method distance : unit -> float
 end ;; 
This declaration is useful because the defined type can be
used as a type constraint.
# let seg_length (p1:interf_point) (p2:interf_point) =
  let x = float_of_int (p2#get_x - p1#get_x) 
  and y = float_of_int (p2#get_y - p1#get_y) in
   sqrt ((x*.x) +. (y*.y)) ;;
val seg_length : interf_point -> interf_point -> float = <fun>
Interfaces can only mask fields of instance variables and private methods.
They cannot mask abstract or public methods.
This is a restriction in their use, as shown by the following example:
# let p = ( new point_m1 (2,3) : interf_point);;
Characters 11-29:
This expression has type
  point_m1 =
    < distance : unit -> float; get_x : int; get_y : int;
      moveto : int * int -> unit; rmoveto : int * int -> unit;
      to_string : unit -> string; undo : unit -> unit >
but is here used with type
  interf_point =
    < distance : unit -> float; get_x : int; get_y : int;
      moveto : int * int -> unit; rmoveto : int * int -> unit;
      to_string : unit -> string >
Only the first object type has a method undo
Nevertheless, interfaces may use inheritance. 
Interfaces are especially useful in combination with modules:
it is possible to build the signature of a module using
object types, while only making available the description of class interfaces.
 Local Declarations in Classes
A class declaration produces a type and a constructor. In order to
make this chapter easier to read, we have been presenting constructors as functions
without an environment. In fact, it is possible to define constructors which
do not need initial values to create an instance: that means that they are no
longer functional. Furthermore one can use local declarations in the
class. Local variables captured by the constructor are shared and can
be treated as class variables.
 Constant Constructors
A class declaration does not need to use initial values passed to the
constructor. For example, in the following class: 
# class example1 = 
   object 
     method print () = ()
   end ;;
class example1 : object method print : unit -> unit end
# let p = new example1 ;;
val p : example1 = <obj>
The instance constructor is constant. The allocation does not require an
initial value for the instance variables. As a rule, it is better to use an
initial value such as (), in order to preserve the functional nature
of the constructor.
 Local Declarations for Constructors
A local declaration can be written directly with abstraction. 
# class example2 = 
   fun a -> 
     object 
       val mutable r = a
       method get_r = r
       method plus x = r <- r + x
    end;;
class example2 :
  int ->
  object val mutable r : int method get_r : int method plus : int -> unit end
Here it is easier to see the functional nature of the constructor. The
constructor is a closure which may have an environment that binds free variables
to an environment of declarations. The syntax for class declarations allows
local declarations in this functional expression.
 Class Variables
Class variables are declarations which are known at class level and therefore
shared by all instances of the class. Usually these class variables can be
used outside of any instance creation. 
In Objective CAML, thanks to the functional nature of a constructor with a non-empty 
environment, we can make these
values (particularly the modifiable ones) shared by all instances of a class.
We illustrate this facility with the following example, which allows us to
keep a register of the number of instances of a class. To do this we
define a parameterized abstract class 'a om.
# class virtual  ['a] om = 
   object 
     method  finalize ()  = () 
     method virtual destroy : unit -> unit
     method virtual to_string : unit -> string
     method virtual all : 'a list 
   end;;
Then we declare class 'a lo, whose constructor contains
local declarations for n, which associates a unique number with each
instance, and for l, which contains the list of pairs (number,
instance) of still active instances.
# class ['a] lo = 
   let l = ref [] 
   and n = ref 0 in
     fun s -> 
       object(self:'b )
         inherit ['a] om
         val mutable num = 0
         val name = s
         method to_string () = s
         method print () = print_string s
         method print_all () = 
           List.iter (function (a,b) -> 
                        Printf.printf "(%d,%s) " a (b#to_string())) !l
         method destroy () = self#finalize();
                             l:= List.filter (function (a,b) -> a <> num) !l; ()
         method all = List.map snd !l
         initializer incr n; num <- !n;  l:= (num, (self :> 'a  om) ) :: !l ; ()
       end;; 
class ['a] lo :
  string ->
  object
    constraint 'a = 'a om
    val name : string
    val mutable num : int
    method all : 'a list
    method destroy : unit -> unit
    method finalize : unit -> unit
    method print : unit -> unit
    method print_all : unit -> unit
    method to_string : unit -> string
  end
At each creation of an instance of class lo, the initializer
increments the reference n and adds the pair (number, self)
to the list l. Methods print and print_all display
respectively the receiving instance and all the instances containing in l. 
# let m1 = new lo "start";;
val m1 : ('a om as 'a) lo = <obj>
# let m2 = new lo "between";;
val m2 : ('a om as 'a) lo = <obj>
# let m3 = new lo "end";;
val m3 : ('a om as 'a) lo = <obj>
# m2#print_all();;
(3,end) (2,between) (1,start) - : unit = ()
# m2#all;;
- : ('a om as 'a) list = [<obj>; <obj>; <obj>]
Method destroy removes an instance from the list of instances, and
calls method finalize to perform a last action on this instance before
it disappears from the list. Method all returns all the instances of a
class created with new.
# m2#destroy();;
- : unit = ()
# m1#print_all();;
(3,end) (1,start) - : unit = ()
# m3#all;;
- : ('a om as 'a) list = [<obj>; <obj>]
We should note that instances of subclasses are also kept in this list. Nothing
prevents you from using the same technique by specializing some of these subclasses.
On the other hand, the instances obtained by a copy (Oo.copy or
{< >}) are not tracked.
 
 
