renaming as classwide type

renaming as classwide type

[Simon Belmont]

is it really illegal to rename an object as it's classwide parent?  gnat claims so.  Similar results using a type extension as a generic formal "in out" object of classwide type.

  declare  
    type P is interface;
    type C is new P with null record;
  
    o : C;
    r : P'Class renames o;  --error
    begin
      null;
    end;

expected type "P'Class" defined at line
found type "C" defined at line

Re: renaming as classwide type

[G.B.]

On 21.09.23 21:29, Simon Belmont wrote:
> is it really illegal to rename an object as it's classwide parent?  gnat claims so.  Similar results using a type extension as a generic formal "in out" object of classwide type.
> 
>    declare
>      type P is interface;
>      type C is new P with null record;
>    
>      o : C;
>      r : P'Class renames o;  --error
>      begin
>        null;
>      end;
> 
> expected type "P'Class" defined at line
> found type "C" defined at line

O does not denote an object of a class-wide type,
I think. However, P'Class (o) does, so that renaming
that would make r be of its declared type P'Class.

Re: renaming as classwide type

[Simon Belmont]

On Thursday, September 21, 2023 at 5:03:06 PM UTC-4, G.B. wrote:
> O does not denote an object of a class-wide type, 
> I think. However, P'Class (o) does, so that renaming 
> that would make r be of its declared type P'Class.

That seems needlessly pedantic, considering that assignment is fine (o2 : P'Class := o) as well as assigning it to an intermediate pointer (po : access P'Class := o'Access; r : P'Class renames po.all)

Re: renaming as classwide type

[Randy Brukardt]

It's not needless; there is an implicit conversion on an assignment, and it 
only needs to work one way. When you rename a variable, the conversion has 
to work both ways. But that's problematic, since conversions often involve 
checks and we don't want checks popping up in unexpected places (both for 
implementation reasons and for readability reasons).

Also, renaming does not change the nominal subtype of an object, regardless 
of what subtype is given in the renames. Ada 2022 allows you to omit the 
type name altogether since it provides little value.

                        Randy.

"Simon Belmont" <sbelmont700@gmail.com> wrote in message 
news:5ab7e1d5-1491-49c3-877d-32bb6e12b43an@googlegroups.com...
On Thursday, September 21, 2023 at 5:03:06?PM UTC-4, G.B. wrote:
> O does not denote an object of a class-wide type,
> I think. However, P'Class (o) does, so that renaming
> that would make r be of its declared type P'Class.

That seems needlessly pedantic, considering that assignment is fine (o2 : 
P'Class := o) as well as assigning it to an intermediate pointer (po : 
access P'Class := o'Access; r : P'Class renames po.all) 

Re: renaming as classwide type

[Simon Wright]

Simon Belmont <sbelmont700@gmail.com> writes:

>     r : P'Class renames o;  --error

This compiles OK with GCC 13.1.0:

   r : P'Class renames P'Class (o);