Re: Custom Storage Pool questions

[Jere <jhb.chat@gmail.com>]

On Monday, September 13, 2021 at 7:12:43 AM UTC-4, J-P. Rosen wrote:
> Le 13/09/2021 à 02:53, Jere a écrit : 
> > I was learning about making user defined storage pools when 
> > I came across an article that made me pause and wonder how 
> > portable storage pools actually can be. In particular, I assumed 
> > that the Size_In_Storage_Elements parameter in the Allocate 
> > operation actually indicated the total number of storage elements 
> > needed. 
> > 
> > procedure Allocate( 
> > Pool : in out Root_Storage_Pool; 
> > Storage_Address : out Address; 
> > Size_In_Storage_Elements : in Storage_Elements.Storage_Count; 
> > Alignment : in Storage_Elements.Storage_Count) is abstract; 
> > 
> > But after reading the following AdaCore article, my assumption is now 
> > called into question: 
> > https://blog.adacore.com/header-storage-pools 
> > 
> > In particular, the blog there advocates for separately counting for 
> > things like unconstrained array First/Last indices or the Prev/Next 
> > pointers used for Controlled objects. Normally I would have assumed 
> > that the Size_In_Storage_Elements parameter in Allocate would account 
> > for that, but the blog clearly shows that it doesn't
> [...] 
> 
> That blog shows a special use for Storage_Pools, where you allocate 
> /user/ data on top of the requested memory. When called by the compiler, 
> it is up to the compiler to compute how much memory is needed, and your 
> duty is to just allocate that. 
> 
Yes, but if you look at that blog, they are allocating space for the /user/ data 
and for the Next/Prev for controlled types and First/Last for unconstrained
arrays in addition to the size specified by allocate.  

I agree I feel it is up to the compiler to provide the correct size to Allocate, 
but the blog would indicate that GNAT does not (or did not..old blog..so 
who knows?).  Does the RM require that an implementation pass the full
amount of memory needed to Allocate when new is called?