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From: Vincent Marciante <marciant@li.net>
Subject: Simpler Question (was Assigning to dereferenced null access variable)
Date: 2000/11/07
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--Okay, I cut down the question to:
--
--Why don't both blocks in the following code behave similarly?
--I get the following output using OS/2 GNAT 3.12p
--
--The Null_Characters(1..0) assignment succeeded
--The Null_Characters assignment caused Constraint_Error
--
--If the above indicates a compiler defect then what is the
--correct behavior?

with Ada.Text_IO; use Ada.Text_IO;
procedure Test_Null_Assignments is

    type Characters is array (Positive range <>) of Character;
    Null_Characters : Characters(1..0);

    type Reference is access Characters;
    type Character_Sequence is
        record
            The_Characters : Reference;
        end record;

    Null_Sequence : Character_Sequence;
begin
    begin
        Null_Sequence.The_Characters(1..0) := Null_Characters(1..0);  
        Put_Line("The Null_Characters(1..0) assignment succeeded");
        --OS/2 GNAT 3.12 succeeds
    exception
        when Constraint_Error => 
        Put_Line("The Null_Characters(1..0) assignment caused
Constraint_Error");
    end;

    begin
        Null_Sequence.The_Characters(1..0) := Null_Characters;  
        Put_Line("The Null_Characters assignment succeeded");
    exception
        when Constraint_Error => 
        Put_Line("The Null_Characters assignment caused
Constraint_Error");
        --OS/2 GNAT 3.12 raises Constraint_Error
    end;
end;