Re: Packing Record Structures in Ada

[Corey Minyard <minyard@acm.org>]

dewar@merv.cs.nyu.edu (Robert Dewar) writes:
> 
> THe stream facility in Ada 95 is one of its very powerful features. I have
> the impression that a lot of Ada 95 programmers are not sufficiently aware
> of this very important capability, and its wide applicability to solving
> problems of this nature, either through explicit use, or implicit use via
> the distribution annex.
> 

From my reading of the ARM, it is not extremely clear to me that
streaming between different compilers is 100% compatible.  It may be
(I haven't read the AARM and I'm no language expert) and I would
expect it would be in general (assuming endian-ness conversion is
done, of course).

I have done tests of streaming vs direct layout and copying using
GNAT.  Streaming is rather slow, on the order of 20 times slower than
direct copying.  I would expect this is due to a polymorphic call on
every data item (every array element, etc.), so I wouldn't expect any
other compilers to be any better.  With a representation clause,
layouts are guaranteed except for endianness, and you can directly
copy the data structure.

If you were able to do a "for T'Bit_Order use ..." would that solve
the endianness problem?  I realize that compilers do not have to allow
this, but if they did...

-- 
Corey Minyard               Internet:  minyard@acm.org
  Work: minyard@nortel.ca       UUCP:  minyard@wf-rch.cirr.com