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From: Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>
Newsgroups: comp.arch.embedded,comp.lang.ada
Subject: Re: Certified C compilers for safety-critical embedded systems
Date: Sat, 27 Dec 2003 16:49:11 +0000 (UTC)
Organization: GMUGHDU
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In comp.lang.ada Ian Bell <ian@ruffrecordsdotworldonline.co.uk> wrote:
:James Rogers> foo : My_Array_Type;
:> 
:> for num in 0..99 loop
:>    foo(num) := num;
:> end loop;
:> 
:> All Ada compilers will correctly identify the error in the for loop.
:> [...]
: 
: I know nothing about ada so this is a genuine query rather than a ctiticism. 
: The above example is fine as long as literals are used - even a C compiler
: could be devised to make this check - but what happens when the array index
: is computed?

Wouldn't that become a famous compiler that finds out, at compile
time, which value a certain variable is going to have? :-)

If you want a hole in your foot, you can make one, though it might
not be easy:

with Interfaces;
with Ada.Integer_Text_IO; use Ada;

procedure t is
   --  read a positive value from standard input and create an
   -- array of that size, which is filled (hopping excessively)

   procedure rubber_buffer(limit: Positive) is

      subtype Index is Positive range 1 .. limit;
      -- a range constraint on Positive, determined at call time

      buffer: array(Index) of Interfaces.Unsigned_8;
      -- storage for 1 .. limit 8bit quantities

   begin
      --  demonstration of constraint_error

      off_buffer:  -- k grows too large for a buffer index

          for k in Index'first .. 2 * Index'last loop
             buffer(k) := 42;  -- index check failed, at run time
          end loop off_buffer;

      off_index_range:  -- k gets too large for Index subtype's range

          for
            k in Index'first .. Index(2 * Index'last)
                                --  range check failed, at run time
          loop
             buffer(k) := 42;
          end loop off_index_range;

   end rubber_buffer;

   n: Positive;
   --  upper limit of 1-based buffer, read at run time

begin
   Integer_Text_IO.get(n);
   rubber_buffer(n);
end t;


That's why language-defined array constructs such as
the 'range attribute are useful. You can write

  for k in buffer'range loop
     buffer(k) := 42;
  end loop;

(or in this case more simply, using language defined `others'

  buffer := (others => 42);)

no matter what the buffer's index range currently is.


-- Georg