Re: An alternative to Unchecked Conversion

[Mark A Biggar <mark.a.biggar@lmco.com>]

Tom_Hargraves@Raytheon.com wrote:
> 
> Related to "Questions about Unchecked_Conversion".
> 
> There have been some good responses to the related topic. I agree with the
> sentiment that Unchecked_Conversion should be rarely if ever used. However if
> you HAVE TO use it, it does come with an overhead. It is a routine which has to
> be called, and it may end up copying data around and cost you cpu time.

Most compilers implement Unchecked_Conversion as an implisit inlined routine
with no more (actually less as there are no checks done) overhead then
an assignment statement.
 
> So when you HAVE TO, e.g. to copy Ada structures onto byte streams being
> transmitted over a 'hardware' interface, there's another technique which has
> been around for a while. In the fortran era it was implemented using the common
> block and equivalence statements.
> 
> The technique is simply to map the Ada structure directly onto the 'other' data
> structure.
> 
> It has the following advantages:
> 1. It's fast, there is no data transfer 'cost'.
> 2. The data structures can be different sizes.
> 
> It has the following disadvantages:
> 1. If your Ada structure contains constrained types, then a write to the
> 'other' data structure can subsequently cause a constraint error when you come
> to read the data.
> 2. Timing and synchronisation become a issue if the reader and writer are not
> in the same task/thread.

3. There is no guarentee in the LRM that it will work correctly (although
most implementations will "Do The Right Thing").  So it definitely not
protable, while Unchecked_conversion is Guarenteed to be.

--
Mark Biggar
mark.a.biggar@lmco.com