Re: A question to the "access-type lifetime rule"

[David C. Hoos, Sr. <david.c.hoos.sr@ada95.com>]

In article <38E89AB9.7D33E88C@hamilton.physik.uni-konstanz.de>,
bernd.rinn@uni-konstanz.de wrote:
> Hi,
>
> I am new to Ada and I am looking for an elegant way to get around a
> restriction posed by the "access-type lifetime rule" for
> access-to-subprogram types. First of all: does anyone know, why the
> access-type lifetime matters and not the access-variable lifetime?
(This
> should prevent dangling pointers as well, I guess.)
>
> I want to write an numerical integration function, that is defined in
a
> separate package as
>
> type Integrand_Type is access function ( X : Long_Float ) return
> Long_Float;
> function Integrate(Integrand : Integrand_Type; Lower_Bound,
Upper_Bound
> : Long_Float) return Long_Float;
>
> Now I would like to call this from a main procedure and pass an access
> attribute of a nested function of the main procedure as `Integrand'.
Is
> there a way to reach this? (Unchecked_Access is not an allowed
attribute
> for subprograms and I do not want to generate a package of it's own
for
> the integrand function.)
>
> Thank you for your help.
>
> Best regards,
>
> Bernd
>
The fundamental problem here is that the compiler has no way of knowing
that your main procedure is the "main" procedure -- i.e. that your main
procedure will not return before the program ends -- and that therefore
your integrand function could have gone out of scope while a pointer
to it is still retained by your numerical integration package.
I believe the best way to deal with this is to make the integrand
function itself a library-level function -- i.e.., not encapsulated in
a package. It will therefore have the same lifetime as the program,
and the compiler will allow the 'Access attribute to be used anywhere
in the program.
A non-portable alternative (if you're using GNAT) is to use the
'Unrestricted_Access attribute -- which is jus that -- unrestricted.


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