Re: accesibility level problem

[Thomas Handler <th@umundum.vol.at>]

Hi Steve,

steve quinlan wrote:
> 
> My version of GNAT (3.11b) gives a different message :
> 
>  Non-local pointer cannot point to local object
so does mine (3.11p) ;-)

> 
>  I think this is just standard accessibility rules. The type channelp is
> defined at a package level, the object you are aliasing is defined locally
> in a subprogram. It has a deeper level than the acces type -- it could go
> out of scope, but objects of that access type could still be in scope, and
> you could possibly assign the address of the object to a value of the access
Hmm, sounds as if I have missed something though I have read this
chapter more than twice ;-)

> type, thus creating the problem of an invalid reference. The accessibility
> rules prevent even the possibility of this happening by not allowing
> conversion of an access-to-object to a pointer-to-object when the pointer
> may outlive the object (I think). So, it's an appropriate case for
> unchecked_access.
OK, thank you very much for this description!

> 
> You'd just want to make sure that in AwaitChannels, you don't save any of
> those references in, say, some package body state variable. That could
> generate the invalid reference problem.
This explanation makes accessibility rules even more clear for me.

Once again thank you.

Ciao,
  Thomas Handler