Good/best way to enforce library-level instantiation a generic package

Good/best way to enforce library-level instantiation a generic package

[Vincent Marciante]

I made a generic package that I want only to be instantiated at library level.
I'm working on compile-time a way to enforce that desire which involves access 
type accessibility level checking but have not yet set it up correctly.
Is there a better/standard way?

Re: Good/best way to enforce library-level instantiation a generic package

[Randy Brukardt]

"Vincent Marciante" <vincent.marciante@l3harris.com> wrote in message 
news:6bcc6133-8c8b-4252-a8e6-fd64a5eec2ca@googlegroups.com...
>I made a generic package that I want only to be instantiated at library 
>level.
> I'm working on compile-time a way to enforce that desire which involves 
> access
> type accessibility level checking but have not yet set it up correctly.
> Is there a better/standard way?

For Ada 95, deriving from Controlled does the trick, but that was eliminated 
(at substantial cost) in Ada 2005 and later.

I suppose you could use type String_Access (which is a library-level access 
type) for this:

   with Ada.Strings.Unbounded;
   generic
       ...
   package My_Generic is
       -- Real stuff here.

       Library-Level : constant aliased String := "Library-Level";
       Check : Ada.Strings.Unbounded.String_Access := Library_Level'Access;
            -- 'Access is illegal if My_Generic is not instantiated at the 
library level.
   end My_Generic;

String_Access is a silly type that isn't used in the spec of 
Ada.Strings.Unbounded, and thus shouldn't be there, but it does work for 
this use. :-)

You can of course use any library-level access type in your program for this 
purpose; I picked this one 'cause it is already sitting around.

Hope this helps.

                   Randy.

Re: Good/best way to enforce library-level instantiation a generic package

[Vincent Marciante]

On Monday, March 16, 2020 at 9:21:18 PM UTC-4, Randy Brukardt wrote:
> "Vincent Marciante" wrote in message 
> news:6bcc6133-8c8b-4252-a8e6-fd64a5eec2ca@googlegroups.com...
> >I made a generic package that I want only to be instantiated at library 
> >level.
> > I'm working on compile-time a way to enforce that desire which involves 
> > access
> > type accessibility level checking but have not yet set it up correctly.
> > Is there a better/standard way?
<snip>
> You can of course use any library-level access type in your program for this 
> purpose; I picked this one 'cause it is already sitting around.
> 
> Hope this helps.
> 
>                    Randy.
Thanks for the suggestion. It was along that lines of what I was doing
and it hepled. 

Re: Good/best way to enforce library-level instantiation a generic package

[Jeffrey R. Carter]

On 3/17/20 2:21 AM, Randy Brukardt wrote:
> "Vincent Marciante" <vincent.marciante@l3harris.com> wrote in message
> news:6bcc6133-8c8b-4252-a8e6-fd64a5eec2ca@googlegroups.com...
>> I made a generic package that I want only to be instantiated at library
>> level.
>> I'm working on compile-time a way to enforce that desire which involves
>> access
>> type accessibility level checking but have not yet set it up correctly.
>> Is there a better/standard way?
> 
> For Ada 95, deriving from Controlled does the trick, but that was eliminated
> (at substantial cost) in Ada 2005 and later.
> 
> I suppose you could use type String_Access (which is a library-level access
> type) for this:
> 
>     with Ada.Strings.Unbounded;
>     generic
>         ...
>     package My_Generic is
>         -- Real stuff here.
> 
>         Library-Level : constant aliased String := "Library-Level";
>         Check : Ada.Strings.Unbounded.String_Access := Library_Level'Access;
>              -- 'Access is illegal if My_Generic is not instantiated at the
> library level.
>     end My_Generic;

Check should be constant, too.

For the OP:

You can also use Ada.Tags.Expanded_Name on a tagged type declared in the generic:

type T is abstract tagged null record;
Name : constant String := Ada.Tags.Expanded_Name (T);
pragma Assert
    (Ada.Strings.Fixed.Index (Name (Name'First .. Name'Last - 2), ".") = 0,
     "Generic_Name must be instantiated at library level");

-- 
Jeff Carter
"Gentlemen, you can't fight in here. This is the War Room!"
Dr. Strangelove
30

Re: Good/best way to enforce library-level instantiation a generic package

[Randy Brukardt]

"Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message 
news:r4qbs8$192$1@dont-email.me...
...
> You can also use Ada.Tags.Expanded_Name on a tagged type declared in the 
> generic:
>
> type T is abstract tagged null record;
> Name : constant String := Ada.Tags.Expanded_Name (T);
> pragma Assert
>    (Ada.Strings.Fixed.Index (Name (Name'First .. Name'Last - 2), ".") = 0,
>     "Generic_Name must be instantiated at library level");

That would reject an instance in a library-level child package, which 
probably isn't what the OP wants.

                Randy.

Re: Good/best way to enforce library-level instantiation a generic package

[Jeffrey R. Carter]

On 3/18/20 2:03 AM, Randy Brukardt wrote:
> "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message
> news:r4qbs8$192$1@dont-email.me...
>>
>> type T is abstract tagged null record;
>> Name : constant String := Ada.Tags.Expanded_Name (T);
>> pragma Assert
>>     (Ada.Strings.Fixed.Index (Name (Name'First .. Name'Last - 2), ".") = 0,
>>      "Generic_Name must be instantiated at library level");
> 
> That would reject an instance in a library-level child package, which
> probably isn't what the OP wants.

An instance IN a child pkg is not a library-level instance, so the OP would want 
to reject it. But an instance AS a library-level child pkg would be rejected, so 
this doesn't work.

AFAICT, a child pkg can only be a library-level pkg, so "library-level child 
pkg" seems like overqualification.

-- 
Jeff Carter
"My mind is aglow with whirling, transient nodes of
thought, careening through a cosmic vapor of invention."
Blazing Saddles
85

Re: Good/best way to enforce library-level instantiation a generic package

[Randy Brukardt]

"Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message 
news:r4sp9u$m59$1@dont-email.me...
> On 3/18/20 2:03 AM, Randy Brukardt wrote:
>> "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message
>> news:r4qbs8$192$1@dont-email.me...
>>>
>>> type T is abstract tagged null record;
>>> Name : constant String := Ada.Tags.Expanded_Name (T);
>>> pragma Assert
>>>     (Ada.Strings.Fixed.Index (Name (Name'First .. Name'Last - 2), ".") = 
>>> 0,
>>>      "Generic_Name must be instantiated at library level");
>>
>> That would reject an instance in a library-level child package, which
>> probably isn't what the OP wants.
>
> An instance IN a child pkg is not a library-level instance, so the OP 
> would want to reject it. But an instance AS a library-level child pkg 
> would be rejected, so this doesn't work.

??? Any declaration in a library-level *package* is a library-level 
declaration. That's not true for other kinds units, of course. 3.10.2(22) 
explains that "library-level" is determined by the accessibility of a 
declaration, and a declaration in a package has the same accessibility as 
the package.

                          Randy.

Re: Good/best way to enforce library-level instantiation a generic package

[Vincent Marciante]

On Tuesday, March 17, 2020 at 9:03:51 PM UTC-4, Randy Brukardt wrote:
 
> That would reject an instance in a library-level child package, which 
> probably isn't what the OP wants.
> 
>                 Randy.

Correct: I would want to instantiate into a library-level package, including a child.  I just want to avoid repeated execution of elaboration code of an instance and have any such arrangement that would cause that be at least flagged/warned at compile-time.

Re: Good/best way to enforce library-level instantiation a generic package

[briot.emmanuel]

On Monday, March 16, 2020 at 7:51:29 PM UTC+1, Vincent Marciante wrote:
> I made a generic package that I want only to be instantiated at library level.
> I'm working on compile-time a way to enforce that desire which involves access 
> type accessibility level checking but have not yet set it up correctly.
> Is there a better/standard way?

The way I do this is using gnat-specific pragmas and attributes:

generic
package Generics is
   pragma Compile_Time_Error
     (not Generics'Library_Level, "must be at library level");
   ...
end Generics;

Re: Good/best way to enforce library-level instantiation a generic package

[Vincent Marciante]

On Tuesday, March 17, 2020 at 2:29:58 AM UTC-4, briot....@gmail.com wrote:
> On Monday, March 16, 2020 at 7:51:29 PM UTC+1, Vincent Marciante wrote:
> > I made a generic package that I want only to be instantiated at library level.
> > I'm working on compile-time a way to enforce that desire which involves access 
> > type accessibility level checking but have not yet set it up correctly.
> > Is there a better/standard way?
> 
> The way I do this is using gnat-specific pragmas and attributes:
> 
> generic
> package Generics is
>    pragma Compile_Time_Error
>      (not Generics'Library_Level, "must be at library level");
>    ...
> end Generics;

'Library_Level is nice and clean!  It should be part of the Standard!
I am using GNAT but still have to be compatible with other compiler so
will have to go with something along the lines of Randy's suggestion.
Thanks.