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                                       J3/03-113r2

Date:     02 April 2003 (J3 meeting 164)
To:       J3
From:     Aleksandar Donev
Subject:  Reinstating Deferred Bindings via Abstract Types
Reference: Based on J3/02-296r3, also see J3/02-271r1
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______________________________________
Summary
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See J3/03-113r2 for details.
We propose to:
1. Add the DEFERRED attribute for a specific binding.
2. Require the ABSTRACT attribute for a type with a deferred binding.
3. Allow (<interface-name>) to specify the deferred binding interface.
4. Ensure that no object has an abstract dynamic type.
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Edits to J3-007R3 + J3/03-166
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[42:3+] Add:
or ABSTRACT

[42:9+] Add:
C417a (R423) If the type definition contains or inherits (4.5.6.1)
 a deferred binding (4.5.4), ABSTRACT shall appear.
C417b (R423) If ABSTRACT appears, the type shall be extensible.

[52:12-13] Delete.
{Reworded version appears below at [53:22+]}

[52:14] Replace PROCEDURE with:
PROCEDURE [(<interface-name>)]

[52:16+] Add constraint:
C446a (R444) If =><binding> appears, <interface-name> shall not appear.

[52:17-18] Replace "If...appeared" by:
If neither =><binding> nor <interface-name> appears, it is as though
=><binding> had appeared

[53:11+] Add:
or DEFERRED

[53:18+] Add:
C454a (R444) NON_OVERRIDABLE and DEFERRED shall not both appear in the
same <binding-attr-list>.
C454b (R444) DEFERRED shall appear if and only if <interface-name> appears.

[53:22+] Add:
C456a (R444) An overriding binding shall have the DEFERRED attribute
only if the binding it overrides is deferred.
C456b (R444) A binding shall not override (4.5.6.2) an inherited binding
(4.5.6.1) that has the NON_OVERRIDABLE attribute.
{This is a constraint version of [58:11]}

[53:30] Add to end of paragraph:
A binding that specifies the DEFERRED attribute is a <<deferred binding>>.
A deferred binding shall appear only in an abstract type.

[53:31] Add to the end of the sentence:
or the interface specified by <interface-name>

[56:11+] Add:
An <<abstract type>> is a type that has the ABSTRACT attribute.

Add NOTE 4.52a:
A deferred bindings (4.5.4) defers the implementation of a type-bound
procedure to  extensions of the type, and can appear only in an abstract
type. The dynamic type of an object cannot be abstract; therefore,
a deferred binding cannot be invoked. An extension of an abstract type
need not be abstract if it has no deferred bindings. A short example
of an abstract type is:

TYPE, ABSTRACT :: FILE_HANDLE
CONTAINS
   PROCEDURE(OPEN_FILE), DEFERRED, PASS(HANDLE) :: OPEN
   ...
END TYPE

An extended example can be found in section C1.3a.

TYPE, ABSTRACT :: FILE_HANDLE
CONTAINS
   PROCEDURE(OPEN_FILE), DEFERRED, PASS(HANDLE) :: OPEN
   ...
END TYPE

[58:11] Delete
{This should be a constraint}

[59:9+] Add:
C453a (R451) The <derived-type-spec> shall not specify an abstract type
(4.5.3).

[67:24+] Add:
C503a (R503) The <type-spec> shall not specify an abstract type (4.5.3).
{This prohibits TYPE(abstract-type), but not CLASS(abstract-type).}

[103:3+] Add constraint:
C611a (R612) In a <data-ref>, if the rightmost <part-name> is of abstract
type, <data-ref> shall be polymorphic.

[415:6+] Add glossary entry:
<<abstract  type>> (4.5.7): A type that has the ABSTRACT attribute.
A nonpolymorphic object shall not be declared to be of abstract type.
A polymorphic object shall not be constructed or allocated to have a
dynamic abstract type.

[418:6+] Add glossary entry:
<<deferred binding>> (4.5.4): A binding with the DEFERRED attribute
(4.5.4). A deferred binding shall appear only in an abstract type
(4.5.3).

[433:37+] Add new section with example:

C1.3a Abstract Types.

The following defines an object that can be displayed in an X window:

	TYPE, ABSTRACT :: DRAWABLE_OBJECT
	   REAL, DIMENSION(3) :: RGB_COLOR=(/1.0,1.0,1.0/) ! White
	   REAL, DIMENSION(2) :: POSITION=(/0.0,0.0/) ! Centroid
	CONTAINS
   PROCEDURE(RENDER_X), PASS(OBJECT) :: RENDER
	END TYPE DRAWABLE_OBJECT

	ABSTRACT INTERFACE
	   SUBROUTINE RENDER_X(OBJECT, WINDOW)
	      CLASS(DRAWABLE_OBJECT), INTENT(IN) :: OBJECT
	      CLASS(X_WINDOW), INTENT(INOUT) :: WINDOW
	   END SUBROUTINE RENDER_X
	END INTERFACE

	We can declare a nonabstract type by extending the abstract type:

	TYPE, EXTENDS(DRAWABLE_OBJECT) :: DRAWABLE_TRIANGLE ! Not ABSTRACT
	   REAL, DIMENSION(2,3) :: VERTICES ! In relation to centeroid
	CONTAINS
	   PROCEDURE, PASS(TRIANGLE) :: RENDER=>RENDER_TRIANGLE_X ! Not shown here
	END TYPE DRAWABLE_TRIANGLE

The actual drawing procedure will draw a triangle in WINDOW with vertices
at x coordinates TRIANGLE%POSITION(1)+TRIANGLE%VERTICES(1,:) and y
coordinates TRIANGLE%POSITION(2)+TRIANGLE%VERTICES(2,:):

	SUBROUTINE RENDER_TRIANGLE_X(TRIANGLE, WINDOW)
	   CLASS(DRAWABLE_TRIANGLE), INTENT(IN) :: TRIANGLE
	   CLASS(X_WINDOW), INTENT(INOUT) :: WINDOW
	   ...
	END SUBROUTINE RENDER_TRIANGLE_X