Re: Does this model work ?

[Marin David Condic <condicma@bogon.pwfl.com>]

Pascal Obry wrote:
> 
> I don't find this impertinent at all. In fact this is really a good
> question.
> Given that I do not use low level construct like rep clauses in my
> sources, I expect that all the Ada source on my Web page will
> compile and works fine under any 64 bits compiler.
> 
Avoiding representation clauses is probably a poor choice if portability
is a concern. Rep clauses might, in fact, guarantee that your software
works in a switch to a 64 bit architecture. For example, the current
implementation uses for the standard type Integer a 32 bit integer. On
some hypothetical 64 bit machine, an Ada implementation uses 64 bits for
the type Integer. No big problem for most internal computations, but
suppose you have a record type with an Integer in it and this is being
output to a file. Suddenly, your record consumes 32 more bits and your
software is not compatible with files produced earlier. Any new files
may not be compatible with other software products as well.

Whereas, if you scrupulously use representation clauses to specify the
exact size of things you need, you have a guarantee that the compiler
will give you that - or at least warn you that it can't & let you fix
it. I don't use precisely sized types for everything because obviously
there are plenty of coding situations where it just plain will never
matter, but where I think there may be an issue of portability or
compatibility with external systems, I use the rep clauses. You might
take a look at my web site where I have some utility code posted. I have
a package which defines a bunch of pre-fabricated types which I use in
building other things just to be sure I have control over the size - and
potentially the operations as well.

MDC
-- 
Marin David Condic
Real Time & Embedded Systems, Propulsion Systems Analysis
United Technologies, Pratt & Whitney, Large Military Engines
M/S 731-95, P.O.B. 109600, West Palm Beach, FL, 33410-9600
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