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From: harrison@software.org (Tim Harrison)
Newsgroups: comp.lang.ada
Subject: Parameter passing mechanisms
Message-ID: <8902221418.AA16453@ajpo.sei.cmu.edu>
Date: 22 Feb 89 12:28:35 GMT
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List-Id: <comp.lang.ada>

Mats Weber asks:  

> Question: is the following program erroneous ?
> 
>    procedure Is_This_Program_Erroneous is
> 
>       type Vector is array (1..5) of Float;
> 
>       A_Vector : Vector := (0.35, 0.67, 1.8, 2.0, 7.7);
> 
>       procedure Add (V : in Vector; W : in out Vector) is
>       begin
>          for I in 1..5 loop
>             W(I) := V(I) + W(I);
>          end loop;
>       end Add;
> 
>    begin
>       Add(A_Vector, A_Vector);
>    end Is_This_Program_Erroneous;

> But according to LRM 6.2(13) - which is a note and as such is not part of
> the standard - the program is erroneous because there are multiple access
> paths to A_Vector during the call Add(A_Vector, A_Vector);, and therefore
> V is undefined after the assignment W(1) := V(1) + W(1);

The program in question is not erroneous.  LRM 6.2(13) says that "the value
of the formal is undefined after updating the actual other than by updating
formal."  In the example program the actual is A_Vector and the formals are
V and W. The example program is only updating the formal (W) not the actual
(A_Vector).  If the program were

   procedure Is_This_Program_Erroneous is
      type Vector is array (1..5) of Float;
      A_Vector : Vector := (0.35, 0.67, 1.8, 2.0, 7.7);
      procedure (V : in Vector) is
      begin
         for I in 1..5 loop
            A_Vector(I) := A_Vector(I) + V(I);
         end loop;
      end Add;
   begin
      Add(A_Vector);
   end Is_This_Program_Erroneous;

the program would be erroneous (according to LRM 6.2(13)), because the actual
(A_Vector) would be updated other than through the formal (V).

-- Tim Harrison
Software Productivity Consortium  Phone:   (703) 742-7113
SPC Building                      CSnet:   harrison@software.org
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