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From: "Brian G. Holmes" <brian.holmes@gsc.gte.com>
Subject: General Access-To-Constant Parameters
Date: 1997/09/23
Message-ID: <34282934.3249@gsc.gte.com>#1/1
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Organization: GTE Government Systems
Reply-To: brian.holmes@gsc.gte.com
Newsgroups: comp.lang.ada
Date: 1997-09-23T00:00:00+00:00
List-Id: <comp.lang.ada>


Why doesn�t the Ada95 language support general access-to-constant
parameters?

General access parameters are a very useful construct of the Ada95
language.  Their use and advantages are discussed in several sections of
the Rationale.  I have also seen the construct abused in several
bindings to C software; a general access parameter - an access-to-object
parameter - is used where an access-to-constant parameter should have
been used.  An example of what I consider general access parameter abuse
follows.

Brian G. Holmes
GTE Government Systems Corporation
brian.holmes@gsc.gte.com


-----------------------------------------------------------------------------------------------------------------------
--
--  General_Access_Parameter_Abuse_Example...
--
--  Illustrates the abuse of general access parameters when creating
bindings.
--
--  The example is meant to show how parameters that might better accept
access-to-constant values are coded to accept
--  access-to-variable values.  The result is that the user must convert
constant objects into variable objects before
--  using the interface.  The example is somewhat contrived, but it is
easy to see how it might be extended to a more
--  complex and less familiar function.
--
--  The memory.h and string.h files both declare the memcpy function as
"void *memcpy(void *, const void *, size_t);".
--
--  19970923 BGH; created.
--
-----------------------------------------------------------------------------------------------------------------------

with Ada.Text_IO;
with Interfaces.C;
procedure General_Access_Parameter_Abuse_Example is


        type    Identification_Information is tagged record
                Number : Interfaces.C.int;
                Text   : Interfaces.C.char_array(0..80);
                end record;

        Bits_Per_Byte                        : constant := 8;

        Identification_Information_Byte_Size : constant
Interfaces.C.size_t :=
                                              
Interfaces.C.size_t(Identification_Information'size/Bits_Per_Byte);

        --

        procedure Bad_Interface( Name:  in out
Identification_Information;
                                 Given: in    
Identification_Information ) is

                procedure Memcpy( Target: access
Identification_Information;
                                  Source: access
Identification_Information;
                                  Length: in     Interfaces.C.size_t );

                pragma Import( C,Memcpy,"memcpy" );

                Local_Copy_Of_Given: aliased Identification_Information
:= Given;

        begin

                Memcpy(
Name'access,Local_Copy_Of_Given'access,Identification_Information_Byte_Size
);

        end Bad_Interface;

        --

        procedure Good_Interface( Name:  in out
Identification_Information;
                                  Given: in    
Identification_Information ) is

                type        Constant_Char_Array_Access is access
constant Identification_Information;

                procedure Memcpy( Target: access
Identification_Information;
                                  Source: in    
Constant_Char_Array_Access;
                                  Length: in     Interfaces.C.size_t );

                pragma Import( C,Memcpy,"memcpy" );

        begin

                Memcpy(
Name'access,Given'access,Identification_Information_Byte_Size );

        end Good_Interface;

        --

        Constant_Identifier : constant Identification_Information :=
(332211,(0..1=>'0',others=>Interfaces.C.nul));

        Identifier          : Identification_Information;


begin


        Ada.Text_IO.Put_Line( "*" );
        Ada.Text_IO.Put_Line( "* 
General_Access_Parameter_Abuse_Example..." );
        Ada.Text_IO.Put_Line( "* 
=========================================" );

        Ada.Text_IO.Put_Line( "*  Constant_Identifier is (" &
                             
Interfaces.C.int'image(Constant_Identifier.Number) & ",""" &
                             
Interfaces.C.To_Ada(Constant_Identifier.Text) & """ )." );

        Bad_Interface( Identifier,Constant_Identifier );

        Ada.Text_IO.Put_Line( "*  Identifier          is (" &
                              Interfaces.C.int'image(Identifier.Number)
& ",""" &
                              Interfaces.C.To_Ada(Identifier.Text) & """
)." );

        Good_Interface( Identifier,Constant_Identifier );

        Ada.Text_IO.Put_Line( "*  Identifier          is (" &
                              Interfaces.C.int'image(Identifier.Number)
& ",""" &
                              Interfaces.C.To_Ada(Identifier.Text) & """
)." );

        Ada.Text_IO.Put_Line( "* 
=========================================" );
        Ada.Text_IO.Put_Line( "*" );


end General_Access_Parameter_Abuse_Example;

-----------------------------------------------------------------------------------------------------------------------