using the 'use at' feature inside a record?

using the 'use at' feature inside a record?

[PForan]

Hi all,

I have a question about the "use at" feature and how it can be used
inside a record to simulate C's unions. I often have an array of
bytes, as well as a string which is "use at"'ed on top of the byte
buffer, very useful as I can play with the data in any way I choose.
i.e.

byte_buf: array(1..100) of unsigned_8;
str_buf: string(1..100);
for str_buf use at byte_buf'address;

Is it possible to have something like this within a record?

TIA,
Phil

Re: using the 'use at' feature inside a record?

[Per Sandberg]

Well there are always unchecked unions:
    "ARM B.3.3 Pragma Unchecked_Union"
---------------------------
    type foo (dummy : Integer := 0) is record
       case dummy is
       when 1 =>
          as_integer              : integer;
       when 2 =>
          as_float                : Short_Float;
       when 3 =>
          as_String               : String (1 .. 4);
       when others =>
          as_Stream_Element_Array : Ada.Streams.Stream_Element_Array (1 
.. 4);
       end case;
    end record;
    pragma unchecked_Union(foo);
---------------------------

The above construct will is an exact equivalent of C's unions.

/Per


PForan wrote:
> Hi all,
> 
> I have a question about the "use at" feature and how it can be used
> inside a record to simulate C's unions. I often have an array of
> bytes, as well as a string which is "use at"'ed on top of the byte
> buffer, very useful as I can play with the data in any way I choose.
> i.e.
> 
> byte_buf: array(1..100) of unsigned_8;
> str_buf: string(1..100);
> for str_buf use at byte_buf'address;
> 
> Is it possible to have something like this within a record?
> 
> TIA,
> Phil

Re: using the 'use at' feature inside a record?

[Stephen Leake]

PForan <pforan@gmail.com> writes:

> I have a question about the "use at" feature and how it can be used
> inside a record to simulate C's unions. I often have an array of
> bytes, as well as a string which is "use at"'ed on top of the byte
> buffer, very useful as I can play with the data in any way I choose.
> i.e.
>
> byte_buf: array(1..100) of unsigned_8;
> str_buf: string(1..100);
> for str_buf use at byte_buf'address;
>
> Is it possible to have something like this within a record?

'use at' is obsolete (LRM J.7); you should now use:

for str_buf'address use byte_buf'address;


If you are implementing an Ada variant record type that must match a C
union type that has no field for the discriminant, use pragma
Unchecked_Union (LRM B.3.3).

-- 
-- Stephe

Re: using the 'use at' feature inside a record?

[PForan]

On Sep 10, 7:42 pm, Stephen Leake <stephen_le...@stephe-leake.org>
wrote:
> PForan <pfo...@gmail.com> writes:


Thanks a bunch for the replies all. I had no clue that 'use at' was
considered old, the new syntax is definetly clearer. Time to go
play :)

Cheers!
Phil