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From: ka@sorry.no.email (Kenneth Almquist)
Newsgroups: comp.lang.ada
Subject: Re: Q: Endless loop by dispatching
Date: 15 Jul 2003 03:11:51 -0400
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Date: 2003-07-15T03:11:51-04:00
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Michael Erdmann <michael.erdmann@snafu.de> wrote:
> Thanks. In fact i have now selected this aproach. Each package
> derives from the base type (A.Object) is forced to implement
> a procedure Serialize by definint it in A as abstract.
>
> The bad thing about it, is that i wanted to add some code
> which is common to all implementation of A.Object in the
> Central Serialize procedure (e.g. writing the attrbiute
> name if ront of every field of B.Object) Now i have to duplicate
> this code in all implementations of A.Object.

The way to avoid duplicate code is to place the code in a generic
procedure which is instantiated for each type.  Something along
the line of:

    package A is

        type Object is tagged private;

        -- Write out all of the attributes of an object.
        procedure Serialize(This : Object);

        -- Write the value of a single attribute.
        procedure Write(This : Object; Attribute : Attribute_Id);

	-- Serialize_Extension for type T writes the attributes
	-- which appear in T type but not in the parent of type T.
	generic
	    type T is new Object with private;
	    Attributes : in Attribute_Array;
	    with procedure Write(This : Object; Attribute : Integer) is <>;
	procedure Serialize_Extension(This : Object);

    end A;


    package B is

        type Object is new A.Object with private;

        -- Write out all of the attributes of an object.
        procedure Serialize(This : Object);

        -- Write the value of a single attribute.
        procedure Write(This : Object; Attribute : Attribute_Id);

    end B;

    package body B is

        My_Attributes : constant Attribute_Array := (...);

        procedure Serialize(This : Object) is
            procedure SE is new Serialize_Extension(Object, My_Attributes);
        begin
            Serialize(A.Object(This));
            SE(This);
        end Serialize;

        procedure Write(This : Object; Attribute : Attribute_Id) is ...;

    end B;


Kenneth Almquist