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From: =?utf-8?Q?Yannick_Duch=C3=AAne_=28Hibou57?=
 =?utf-8?Q?=29?=
 <yannick_duchene@yahoo.fr>
Newsgroups: comp.lang.ada
Subject: Allocators and the requirement to raise `Storage_Error`
Date: Fri, 03 May 2013 07:35:29 +0200
Organization: Ada @ Home
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Date: 2013-05-03T07:35:29+02:00
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ARM 2012 @4.8 says:

http://www.ada-auth.org/standards/12rm/html/RM-4-8.html
> For an allocator with a subtype_indication, the
> subtype_indication shall not specify a null_exclusion

This may be interpreted as =E2=80=9Cnull may be returned by an allocator=
=E2=80=9D.

ARM 2012 @13.11 says:

http://www.ada-auth.org/standards/12rm/html/RM-13-11.html
> If Storage_Pool is not specified for a type defined
> by an access_to_object_definition, then the implementation
> chooses a standard storage pool for it in an
> implementation-defined manner. In this case, the exception
> Storage_Error is raised by an allocator if there is not
> enough storage.

This may be interpreted as =E2=80=9Cmay be a user defined storage pool=E2=
=80=9D is allowed  =

to return `null` when there is not enough storage in the pool.

ARM 2012 @13.11, again, says:

> If Storage_Pool is specified for an access type, then if Allocate
> can satisfy the request, it should allocate a contiguous block of
> memory, and return the address of the first storage element in
> Storage_Address. The block should contain Size_In_Storage_Elements
> storage elements, and should be aligned according to Alignment.
> The allocated storage should not be used for any other purpose
> while the pool element remains in existence. If the request cannot
> be satisfied, then Allocate should propagate an exception (such as
> Storage_Error). If Allocate behaves in any other manner, then the
> program execution is erroneous.

So finally, in no case at all it is allowed for an allocator, to return =
 =

`null` instead of raising `Storage_Error` when there is not enough  =

storage, even if the two quotes above had possibly provisional hope for =
 =

the other way.

I'm disappointed with the last =E2=80=9CIf Allocate behaves in any other=
 manner,  =

then the program execution is erroneous=E2=80=9D, which implies it's err=
oneous to  =

return `null`.

I may understand allocations failing in a declarative part, may be an  =

issue if some declarations coming next to it refers to this allocation. =
 =

But finally, one may also define a function doing something similar and =
 =

not being imposed this requirement, i.e. returning an access type after =
 =

some kind of allocation and returning `null` if it fails, except it won'=
t  =

work with non=E2=80=91constrained types and such a function would need t=
o be  =

defined for each constrained subtypes or derived types of the  =

non=E2=80=91constrained type (a pain).

I though there may be something in the RM, allowing an allocator to retu=
rn  =

`null`, and expect `Program_Error` to be raise instead, on any attempt t=
o  =

dereference an unallocated access.

I will review the relevant parts of the RM again for the reasons, but  =

right now, I don't see why it is this way.

(yes, I know I'm playing silly things, but I have my silly reasons).


-- =

=E2=80=9CSyntactic sugar causes cancer of the semi-colons.=E2=80=9D [1]
=E2=80=9CStructured Programming supports the law of the excluded muddle.=
=E2=80=9D [1]
[1]: Epigrams on Programming =E2=80=94 Alan J. =E2=80=94 P. Yale Univers=
ity