Re: converting pointer to value

[Shark8 <onewingedshark@gmail.com>]

On Thursday, March 4, 2021 at 10:35:01 AM UTC-7, Dmitry A. Kazakov wrote:
> On 2021-03-04 17:55, Shark8 wrote: 
> 
> >> type SQLPOINTER is private; 
> > .... 
> >> private 
> >> type SQLPOINTER is new System.Storage_Elements.Integer_Address; 
> > 
> > And here we have SQLPOINTER, which is private, but which is something of the same representation that Integer_Address has; let's assume that the ADDRESS type is implementation-defined, and a private type.
> SQLPOINTER is System.Address.

Which is implementation-defined.
Even though most programmers would assert that address and integer are the same thing, this isn't a given; segmented architectures would more naturally map to records.

> > What does this mean? 
> > It means that we don't know what the size of ADDRESS (and thus SQLPOINTER) actually is, but assuming you're on a 32- or 64-bit machine it's likely 2 to 4 times as large as the 16-bit SQLSMALLINT-- which is a good indication that a simple UNCHECKED_CONVERSION is the wrong answer.
> You need not to know that: 
> 
> declare 
> P : SQLPOINTER := ...; 
> A : System.Address; 
> pragma Import (Ada, A); 
> for A'Address use P'Address; 
> T : My_Fancy_Object; 
> pragma Import (Ada, My_Fancy_Object); 
> for T'Address use A; 
> begin 
> ... -- Use T at P

None of that addresses the issue that he asked: "how to get the Pvalue as an SQLRETURN?"
One of these is 16-bits, the other is derived from System.Storage_Elements.Integer_Address.
Sure, you can overlay, and that could be valid in some cases, but I simply don't have enough information here to say if that would be a valid solution and it's much more likely that it would be completely unintended.