Re: Default_Storage_Pool

[sbelmont700@gmail.com]

On Wednesday, February 21, 2018 at 8:14:03 PM UTC-5, Randy Brukardt wrote:
> Default_Storage_Pool only has an effect when an access type is declared (not 
> when it is used). Thus the allocators for the component of type T should use 
> Pool_1 regardless of what the Default_Storage_Pool is when the allocator is 
> written.
> 
> But I'd expect the local anonymous access allocator to use Pool_2. I don't 
> see any reason to use some other pool in this case - 13.11.3(6-6.3/4) is 
> pretty clear about this, and the rules specifically were designed so that it 
> would apply to anonymous access types.
> 
> Thus, this appears to be a bug, but I also fail to see any use for it (the 
> anonymous access type having to disappear long before anyone can used the 
> return value), so I would probably not give it much priority if it was 
> reported to me. (Of course, a fuller example could cause me to change my 
> mind on that.)
> 
>                                     Randy.


Thank you, that was what I thought it should do.  I had no legitimate use case, I was just trying to tease out counter-examples to confirm my understanding.  

I claim no in-depth comprehension, but because of either subsequent bugs or perhaps 2012 changes to AATs (or perhaps legitimately), GNAT also takes this:

package body O is

   janky : access Integer;
   
   function F return T is
   begin
      return Result : T do
         declare
            pragma Default_Storage_Pool(Result.pool_2);
            p2 : access Integer;
         begin
            p2 := new integer'(42);
            janky := p2;  --legal?
         end;
      end return;
   end F;
end;

When does janky become a dangling pointer?  Surely if the default pool is local to F (because the entire pool goes away after F ends), never if the default pool is pool_1 (since it has the same lifetime as janky), and 'possibly' if it's contained within Result.pool_2 (since it depends on where the client has creates the result). GNAT accepts all three variations (well, subject to the aforementioned bug presumably making case 3 the same as 1) and raises no exceptions for any of them.

Thank you for the continued explanations

-sb