Re: array conversion - how to do?

[Wes Groleau <no.such.user@does.not.exist.com>]

If I didn't misunderstand your intent, you could use 
Ada.Characters.Handling.To_Basic to eliminate most (not 
all) of the characters outside of the ASCII range, and
Ada.Characters.Handling.To_ISO_646 to convert the rest 
to something you specify.

To eliminate the non-printable characters--please note 
that Character'Val (127) is non-printable)--you could use
Ada.Strings.Fixed.Translate (in fact, you could use it for 
the whole thing).

Geert Bosch wrote:
> Here is a simple situation illustrating a problem I encountered
> in using arrays of pointers to tagged objects. I have an
> application that should only use printable ASCII characters in its
> string processing. So I declare the following:
> 
>    subtype ASCII_Character is Character
>      range Character'Val (32) .. Character'Val (127);
>    type ASCII_String is array (Positive range <>) of ASCII_Character;
> 
> Now this string is clearly convertible to the String type, since
> the components have the same type, the ASCII_Character is only
> more constrained. Thus when my application wants to output its
> ASCII_String results, it uses:
> 
>    with Ada.Text_IO; use Ada.Text_IO;
>    procedure Test is
>       Warning_Message : ASCII_String := "Warning: only use 7-bit ASCII";
>    begin
>       ...
>       Put_Line (String (Warning_Message));
>    end Test;
> 
> Although this is IMHO a solution with clear semantics, this is
> not legal Ada because of the rule that array conversions are only
> allowed between array types that have components of the same subtype.
> The workaround is both ugly and inefficient using a typical Ada
> implementation:
> 
>    procedure Put_Line (Item : in ASCII_String) is
>       Standard_String : String (Item'Range);
>    begin
>       for I in Item'Range loop
>          Standard_String (I) := Item (I);
>       end loop;
>    end Put_Line;
> 
>    ...  -- And so on for all subprograms that use strings
> 
> I could imagine why it would not be possible to convert String
> to ASCII_String, since that conversion might need time-consuming
> checks. The conversion in the example is completely safe though,
> which is statically checkable for the compiler.
> 
> Actually in my real program where I use arrays of
> access-to-classwide types it really makes sense to convert
> arrays of access B'Class to arrays of access A'Class, when
> B is an extension of A.
> 
> In some situations it is really useful to use arrays of
> (access-to) tagged types, but it is quite annoying that
> I cannot convert arrays to more general types but need
> to copy them or use Unchecked_Conversion.
> 
> Can somebody give me a reason why this limitation is
> in the language? A good work-around would also be very
> welcome.
> 
> Regards,
>    Geert

-- 
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    Wes Groleau, Hughes Defense Communications, Fort Wayne, IN USA
Senior Software Engineer - AFATDS                  Tool-smith Wanna-be
                    wwgrol AT pseserv3.fw.hac.com

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