[J3] Specifying the PARAMETER and POINTER attributes on the same entity

[Rafik Zurob]

Thanks!

Rafik


"J3" <j3-bounces at mailman.j3-fortran.org> wrote on 18/07/2019 08:29:30 PM:

> From: Malcolm Cohen via J3 <j3 at mailman.j3-fortran.org>
> To: "'General J3 interest list'" <j3 at mailman.j3-fortran.org>
> Cc: Malcolm Cohen <malcolm at nag-j.co.jp>
> Date: 18/07/2019 08:29 PM
> Subject: [EXTERNAL] Re: [J3] Specifying the PARAMETER and POINTER 
> attributes on the same entity
> Sent by: "J3" <j3-bounces at mailman.j3-fortran.org>
> 
> A1.
> “The PARAMETER attribute specifies that an entity is a named constant
> . The entity has the value specified by its constant-expr,”
> 
> The BNF constant-expr only appears in the initialisation with “=”, 
> i.e. the constraint
> “C811 (R803) … If = appears in initialization, the entity shall not have 
the 
> POINTER attribute.”
> applies, which makes it a non-pointer.
> 
> QED.
> 
> Pointers can of course be constant. For example, a pointer component
> of a derived-type PARAMETER will be constant. Not to mention NULL(P)
> where P is a pointer that does not have non-constant length type 
parameters.
> 
> A2.
> Since PARAMETER and POINTER are incompatible here, the question is moot.
> 
> Cheers,
> -- 
> ..............Malcolm Cohen, NAG Oxford/Tokyo.
> 
> From: J3 <j3-bounces at mailman.j3-fortran.org> On Behalf Of Rafik Zurob 
via J3
> Sent: Friday, July 19, 2019 7:48 AM
> To: J3 Fortran <j3 at j3-fortran.org>
> Cc: Rafik Zurob <rzurob at ca.ibm.com>
> Subject: [J3] Specifying the PARAMETER and POINTER attributes on the
> same entity
> 
> Hello
> 
> I have a couple of questions on specifying the PARAMETER and POINTER 
> attributes on the same entity.
> 
> Question 1. Is it valid to specify both the PARAMETER and POINTER 
> attributes on the same entity?
> 
> For example, is the following program valid?
> 
> integer, target, save :: t
> integer, pointer, parameter :: p1 => t ! initial-data-target
> integer, pointer, parameter :: p2 => null() ! null-init
> end
> 
> As far as I can tell, the constraints forbidding mixing PARAMETER with 
> POINTER were removed starting in Fortran 2008.
> 
> Fortran 95 section 5.2.7 (POINTER statement) had:
> Constraint: The PARAMETER attribute shall not be specified for an 
> object-name.
> 
> Fortran 2003 similarly had:
> C514 (R501) The PARAMETER attribute shall not be specified for a dummy 
> argument, a pointer, an allocatable entity, a function, or an object in 
a 
> common block.
> 
> I can't find a similar constraint in Fortran 2008 or 2018. I also looked 

> into whether pointers had to be variables, and they don't appear to have 

> to be:
> 
> In Fortran 2008, 1.3.111 defines pointer as:
> data pointer (1.3) or procedure pointer (1.3)
> and then 1.3.111.1 defines data pointer as:
> data entity with the POINTER attribute (5.3.14)
> 1.3.46 then says that data entity is:
> data object, result of the evaluation of an expression, or the result of 

> the execution of a function reference
> which already gets us away from variable. Section 1.3.47 says that data 
> object is:
> constant (4.1.3), variable (6), or subobject of a constant (2.4.3.2.3)
> 
> I also couldn't find other references in the text requiring pointers to 
be 
> variables. The closest I found is the following in F2018:
> 
> 7.5.4.6 Default initialization for components
> 2 A pointer variable or component is data-pointer-initialization 
> compatible with a target if the pointer is type
> compatible with the target, they have the same rank, all nondeferred 
type 
> parameters of the pointer have the
> same values as the corresponding type parameters of the target, and the 
> target is contiguous if the pointer has
> the CONTIGUOUS attribute.
> 
> So, are pointers allowed to be parameters? (I don't think it's a very 
> useful thing to have and it would make it difficult to use techniques 
that 
> materialize the parameter's value when needed instead of allocating real 

> space for it. Now we either need to materialize a pointer descriptor on 
> the fly or allocate space for the parameter.) I also tried 3 compilers 
> and none accepted p1 or p2 above.
> 
> 
> Question 2: Assuming that the answer to question 1 is that POINTER and 
> PARAMETER are allowed on the same entity, is it intentional that data 
> pointer initialization is forbidden in the PARAMETER statement? The 
> parameter statement's syntax only allows the equal sign and requires 
> constant-expr.
> 
> e.g.
> 
> integer, target, save :: t
> integer, pointer :: p
> parameter (p = t)
> end
> 
> where the syntax is:
> 
> R551 parameter-stmt is PARAMETER ( named-constant-def -list )
> R552 named-constant-def is named-constant = constant-expr
> 
> Thanks
> 
> Rafik

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