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

[John English <je@bton.ac.uk>]

Bernd Rinn 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.)

If you declare the package where Integrate is declared like this:
  generic
  package X is ...

then in the same declaration section as the nested function you say
this:
  package Inner_X is new X;

and voila, you have smuggled the declarations from X into the inner
scope (the access level of Inner_X being the level at which the
instantiation occurs).

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