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From: Stephen Leake <Stephen.Leake@gsfc.nasa.gov>
Subject: Re: [Q] : Unchecked conversion
Date: 1996/11/12
Message-ID: <32888B01.1961@gsfc.nasa.gov>#1/1
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references: <01bbce46$0f818680$829d6482@joy.ericsson.se>
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organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA
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reply-to: Stephen.Leake@gsfc.nasa.gov
newsgroups: comp.lang.ada
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Date: 1996-11-12T00:00:00+00:00
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Jonas Nygren wrote:
> 
> I have defined an IO buffer type which I intend to use for my own IO
> packages.
> The buffer type is defined as follows:
> 
>    subtype Byte is Integer range 0..255;
> 
>    type Buffer is array (Positive range <>) of Byte;
>    Pragma Pack(Buffer);
>    for Buffer'Component_Size use 8;
> 
> [snip]
>
>    function To_Buffer (S : String) return Buffer is
>       function Convert is
>          new Ada.Unchecked_Conversion(String, Buffer);
>    begin
>       return Convert(S);
>    end To_Buffer;
> 
> but Gnat gives me an error:
> 
>    unconstrained type "Standard.String" not allowed in unchecked conversion
> 
> [snip]

As the error message implies, you need to constrain the types before
instantiating Unchecked_Conversion:

   function To_Buffer (S : String) return Buffer is
      subtype Fixed_String is String (1 .. S'Length);
      subtype Fixed_Buffer is Buffer (1 .. S'Length);
      function Convert is
         new Unchecked_Conversion(Fixed_String, Fixed_Buffer);
   begin
      return Convert(S);
   end To_Buffer;

I just tried this with Gnat, and it works.

-- 
- Stephe