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From: nosmo@eiffel.UUCP (Vince Kraemer)
Newsgroups: comp.lang.eiffel
Subject: Re: a problem in programming with Eiffel
Summary: solution enlightenment association types
Message-ID: <483@eiffel.UUCP>
Date: 22 Jan 91 18:52:11 GMT
References: <20022@hydra.gatech.EDU>
Organization: Interactive Software Engineering, Santa Barbara CA
Lines: 160

The solution is simple; depending upon the use of the language feature
"association types". Association types appear as declarations in Eiffel
code as ": like <anchor-object>".  By redefining the anchor object in a
class which takes advantage of this feature, all the associated objects
are effectively redeclared.

The library classes (specifically LINKED_LIST) were written with just
such a problem in mind and allow the user to take advantage of this
technique.  By reviewing the code for linked_list, it is easy to
determine that first and first_element are the anchors for the the
routines in linked list which are originally declared as FIGURES.

The sample code to illustrate the solution to this "problem" is
appended to this message as a shar-chive file.

(Note: this code has been tested using both the 2.2B and 2.3 releases.
You will have to modify the .eiffel file to compile it under 2.2B, though)

Vince Kraemer
ISE Tech. Support
(technical correspondence to: eiffel@eiffel.com)
---------------------------------------------------------------
# This is a shell archive.  Remove anything before this line,
# then unpack it by saving it in a file and typing "sh file".
#
# Wrapped by geneve!nosmo on Tue Jan 22 09:15:40 PST 1991
# Contents:  .eiffel special_c_f.e special_f.e special_test.e
 
echo x - .eiffel
sed 's/^@//' > ".eiffel" <<'@//E*O*F .eiffel//'
------------ EIFFEL SYSTEM DESCRIPTION FILE -------------------
ROOT: special_test
UNIVERSE: $EIFFEL/library/support
	$EIFFEL/library/structures
	$EIFFEL/library/graphics/figure
	$EIFFEL/library/graphics/window
EXTERNAL: $EIFFEL/files/graphiclib.a -lX11 -lm
PRECONDITIONS (y): ALL
----------------------------------------------------------------
@//E*O*F .eiffel//
chmod u=rw,g=rw,o=r .eiffel
 
echo x - special_c_f.e
sed 's/^@//' > "special_c_f.e" <<'@//E*O*F special_c_f.e//'
--|---------------------------------------------------------------
--|   Copyright (C) Interactive Software Engineering, Inc.      --
--|    270 Storke Road, Suite 7 Goleta, California 93117        --
--|                   (805) 685-1006                            --
--| All rights reserved. Duplication or distribution prohibited --
--|---------------------------------------------------------------
--| Author: Vincent Kraemer @ Interactive Software Engineering
--| Created: Tue Jan 22 07:59:21 1991

class SPECIAL_C_F export
	repeat complex_fig
inherit
	
	COMPLEX_FIG
		redefine
			first, first_element
feature

	first: SPECIAL_F is
		require
			not_empty: not empty
		do
			Result := first_element.item
		end;
	
	first_element: LINKABLE [special_f];
	
invariant

end -- class SPECIAL_C_F
@//E*O*F special_c_f.e//
chmod u=rw,g=rw,o=r special_c_f.e
 
echo x - special_f.e
sed 's/^@//' > "special_f.e" <<'@//E*O*F special_f.e//'
--|---------------------------------------------------------------
--|   Copyright (C) Interactive Software Engineering, Inc.      --
--|    270 Storke Road, Suite 7 Goleta, California 93117        --
--|                   (805) 685-1006                            --
--| All rights reserved. Duplication or distribution prohibited --
--|---------------------------------------------------------------
--| Author: Vincent Kraemer @ Interactive Software Engineering
--| Created: Tue Jan 22 08:04:34 1991

class SPECIAL_F export
	repeat FIGURE, special_call
inherit

	FIGURE
	
feature

	create is do closure.Create; end;
	
	special_call is do io.putstring ("Called special_call.\n"); end;
	
	recompute_closure is do end;

	convert_to_resolution (rv: REAL) is do end;

	origin: POINT;

	barycenter: POINT;

	duplicate: like Current is do end;

	translate (V: VECTOR) is do end;

	rotate (a: REAL; p: like origin) is do end;

	scale (f: REAL; a: like origin) is do end;

	display (w: graph_window) is do end;

	erase (w: GRAPH_WINDOW) is do end;

	contains (p: like origin): BOOLEAN is do end;

invariant

end -- class SPECIAL_F
@//E*O*F special_f.e//
chmod u=rw,g=rw,o=r special_f.e
 
echo x - special_test.e
sed 's/^@//' > "special_test.e" <<'@//E*O*F special_test.e//'
--|---------------------------------------------------------------
--|   Copyright (C) Interactive Software Engineering, Inc.      --
--|    270 Storke Road, Suite 7 Goleta, California 93117        --
--|                   (805) 685-1006                            --
--| All rights reserved. Duplication or distribution prohibited --
--|---------------------------------------------------------------
--| Author: Vincent Kraemer @ Interactive Software Engineering
--| Created: Tue Jan 22 08:13:14 1991

class SPECIAL_TEST

feature

	create is
		local
			sf: SPECIAL_F;
			scf: SPECIAL_C_F;
		do
			sf.create;
			scf.create;
			scf.put_right (sf);
			sf := scf.i_th (1);
			sf.special_call
		end;
	
end -- class SPECIAL_TEST
@//E*O*F special_test.e//
chmod u=rw,g=rw,o=r special_test.e
 
exit 0