Re: Byte Swapping Constrained Data Types

[Jeffrey Carter <spam@spam.com>]

Brian Barker wrote:

>      type Terminal is ( Null_Terminal, Primary, Secondary, Console  );
>  
>      for Terminal use
>          ( Null_Terminal => 0,
>            Primary    => 1,
>            Secondary  => 2,
>            Console      => 3);

Get rid of this. It buys you nothing, and may cost you something. Some 
compilers generate extra code for enumeration types with a 
representation clause to deal with the possibility that the codes have 
holes and are used for indexing or for loop counters.

>      for Terminal'Size use 32;
>  
> I also have created a procedure based off of a generic byte-swap
> routine:
>  
>      procedure Swap is new Endian.Generic_Swap(Terminal);
>  
> I receive a record over TCP/IP and it is in Big Endian format, so I
> must byte swap it.  I have already copied the raw network data into my
> structure that contains the Terminal type.  When I try to byte swap it
> I receive a constraint error, because the big endian value of Terminal
> is out of range.

What you're receiving over the network is not of type Terminal. You need 
to receive it into a type that corresponds to all the possible values 
you can receive, and their byte-swapped equivalents (probably 
Unsigned_32), swap that, then unchecked convert it to your Terminal type.

Concerning the "efficiency" of Unchecked_Conversion, why are you 
worrying about a problem you have no evidence even exists? 
Unchecked_Conversion is the correct idiom to use. Use it. Measure the 
result. Only if that reveals efficiency problems directly related to the 
use of Unchecked_Conversion should you worry about.

Unchecked_Conversion usually does nothing. Given

type A is ...;
type B is ...;

pragma Assert (A'Size = B'Size);

function To_B is new Unchecked_Conversion (Target => B, Source => A);

X : A;
Y : B;
...
Y := To_B (X);

The assignment typically simply copies the bits of X into Y. The whole 
point of this being unchecked is that it AVOIDS the checks that the type 
system would usually require.

-- 
Jeff Carter
"Brave Sir Robin ran away."
Monty Python and the Holy Grail
59