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From: bradford@SKL-CRC.ARPA.UUCP
Newsgroups: comp.lang.ada
Subject: A Problem With Access Types
Message-ID: <8704081652.AA09954@skl-crc.ARPA>
Date: Wed, 8-Apr-87 11:52:36 EST
Article-I.D.: skl-crc.8704081652.AA09954
Posted: Wed Apr  8 11:52:36 1987
Date-Received: Sat, 11-Apr-87 10:41:55 EST
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List-Id: <comp.lang.ada>



I am writing a program which manipulates a large, complex data structure.
The data structure consists of many embedded records and arrays
of records. I wish to have a procedure update a particular record within
this structure, and what I tried to do originally was to simply create
an access type variable for the record type and then have this access
variable refer to the record and then pass the access variable to the
procedure. The following segment represents what I tried to do:

     type A_REC is
       record
          ...
       end;

     type A_ACCESS is access A_REC;

     A : A_REC;
     A_A : A_ACCESS;

     begin
         ...
         A_A := A;  -- Also tried  A_A := A_ACCESS(A);
         UPDATE(A_A);
         ...
     end;

Ada (VAX Ada 1.3) won't let me do this, and after re-reading the LRM,
it seems that this is consistent with the LRM. So I have to do it some
other way. Some options are: 

       1) Pass the whole structure and require the called procedure to
          know the structure (I really don't like this option).

       2) Copy the appropriate record, pass the copy as  an inout parameter,
          and update the record from the returned copy.

       3) Give up and quit (but that means I won't get paid and I really
          like getting paid).

Option 2 is workable for the problem I described above, but having access
types would be easier for a similar, related problem (which I won't go into
here). Can anyone supply another altrernative, or perhaps a way of using access
types like I wanted to originally. I would be most grateful.


Bob Bradford