Re: Q re pack & aliased

[tmoran@acm.org]

|   x : bunches (1 .. 10);
|   for x'Size use 240;
  Unfortunately, one rarely knows the width of an image file at
compile time.  What does your program print with and without
the "for x'Size use 240;"?

>a conversion between a 24-bit-type like you used before and the
>triples-record is quite easy without breaking too much...
  I'm sorry if that wasn't clear.  My "24-bit-type" is indeed
a record, just like your example.  The sole difference is the
addition of a specific layout for the color components:
    for Triple_Color_Type use record
        Blue  at 0 range 0 .. 7;
        Green at 1 range 0 .. 7;
        Red   at 2 range 0 .. 7;
    end record;

Dropping the word "aliased" and changing to:
--  type Triple_Color_Ptr_Type is access all Triple_Color_Type;
    package Gnat315_Workaround
      is new System.Address_To_Access_Conversions(Object=>Triple_Color_Type);
    subtype Triple_Color_Ptr_Type is Gnat315_Workaround.Object_Pointer;
    ...
    Image_Ptr : Triple_Color_Ptr_Type
--      := x(i)'Unchecked_Access;
      := Gnat315_Workaround.To_Pointer
           (x(i)'address);
makes the program work under Gnat, at the expense of ugliness, additional
special case documentation, and obscuring the fact that pixels are aliased.