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From: madmats@elma.epfl.ch (Mats Weber)
Newsgroups: comp.lang.ada
Subject: Parameter passing mechanisms
Message-ID: <890220173849.21a00314@elcc.epfl.ch>
Date: 20 Feb 89 16:38:49 GMT
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List-Id: <comp.lang.ada>

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;

The effect of this program does not depend on the parameter passing
mechanism (copy or reference). Note that a similar example is said
to be correct in "Rationale for the design of the Ada language".

According to LRM 6.2(7), the program is correct because its effect does
not depend on the parameter passing mechanism.

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);

Mats Weber
Swiss Federal Institute of Technology
EPFL DI LITh
1015 Lausanne
Switzerland

e-mail : madmats@elma.epfl.ch