Re: Gem 39 - compiler specific?

[Maciej Sobczak <see.my.homepage@gmail.com>]

On 4 Wrz, 09:29, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:

> Do we need that semantics (reinterpret bit pattern as another type)? It is
> difficult to invent a case where an access value would be obtained from a
> bit patterns. It is never needed, IMO.

Hint: interfacing with other languages (notably C and C++) with
libraries that have some callback or object map functionality.

I have done it very recently and I used
System.Address_To_Access_Conversions. The access values to genuine Ada
objects (even of class-wide types) are converted to addresses and
handed to the C++ component, which at some later point in time calls
back into Ada providing the previously stored address that Ada
converts to the original access value.

A very important part of this business is that the C++ component
*never* uses the given address in any other way than to remember it.
It is just a "cookie".

This is an example of scenario where access values are obtained from
what is essentially a bit pattern. The conversions are from access to
address and then from address to access and both are performed by the
Ada code within a single compilation unit. As far as I understand it
is safe, AARM-wise.

--
Maciej Sobczak * www.msobczak.com * www.inspirel.com

Database Access Library for Ada: www.inspirel.com/soci-ada