Re: Data Overlays

[cis.ohio-state.edu!math.ohio-state.edu!howland.reston.ans.net!europa.eng. gtefsd.com!fs7.ece.cmu.edu!news.sei.cmu.edu!firth@ucbvax.Berkeley.EDU (Robert]

In article <1993Aug18.003940.2208@leeweyr.sccsi.com> bill@leeweyr.sccsi.com (Bi
ll Lee) writes:

>function Extract_Message ( A_Message : in Byte_Arrays ) return Messages is
>  Temp_Message : Messages;
>  for Temp_Message use at A_Message'address;
>begin
>  return Temp_Message;
>end Extract_Message;

As one who, when writing a compiler many years ago, took great pains to
ensure that this construct would work, I can sympathise with the above.

It is formally incorrect, but as far as I can see should work in all
reasonable circumstances, regardless of whether the parameters are
passed by value or by reference.  Of course, some compilers will
screw up when handling the return value, but that's their problem.

However, be warned that this approach might be a lot less efficient
than necessary - you could have upto two copies of the data in the
byte array.  That's the main reason I prefer to solve a problem such
as this by means of Unchecked_Conversion on addresses - I know that
there is no unnecessary copying; I'm keeping the data in the buffer
and simply looking at it through new spectacles, as it were.