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Newsgroups: comp.lang.ada
Date: Fri, 23 Feb 2018 14:30:04 -0800 (PST)
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Subject: Re: two questions on allocators
From: Shark8 <onewingedshark@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:50617
Date: 2018-02-23T14:30:04-08:00
List-Id: <comp.lang.ada>

On Friday, February 23, 2018 at 1:42:14 PM UTC-7, Mehdi Saada wrote:
> 2.2/3: for an allocator with a subtype indication, the subtype indication shall not specify a null exclusion
> why 
> type A is access INTEGER;
> type B is access not null A; 
> type C is access B;
> Object_C : C := new B;
> is allowed, but not
> type A is access INTEGER;
> type B is access A; 
> type C is access B;
> Object_C : C := new not null B; ?? What's the difference ?

Lots.
In the first one, A is an access to an integer, B is an access to A (with null exclusion), and C is an access to B... so in C/C++ something like int*, int**, and int***, respectively.

The second is the same insofar as the C/C++ analog is concerned; but it's different in that B doesn't exclude Null.

Finally, the phrase "new not null B" is in error, as you cannot say "new not null" in Ada.


> If the designated type of the allocator is class-wide, the accessibility level of the type determined by the subtype_indication or qualified_expression shall not be statically deeper than that of the type of the allocator.
> 
> -> what means the last "type of the allocator" ? Is it the same than "designed type of the allocator" ?

The type of the allocator refers to a concrete instance of System.Storage_Pools.Root_Storage_Pool which can be associated with an access-type via pragma/aspect.