Re: Type in allocator has deeper level than designated class-wide type

[Adrian Hoe <abyhoe@gmail.com>]

On Jun 14, 2:04 pm, Adrian Hoe <aby...@gmail.com> wrote:
> On Jun 14, 9:24 am, Adam Beneschan <a...@irvine.com> wrote:
>
>
>
>
>
> > On Jun 13, 6:07 pm, Adrian Hoe <aby...@gmail.com> wrote:
>
> > > I changed the line in procedure Add from
>
> > >       New_Element := new Element_Record'
> > >                         ( Constructor.Init ( Name, Color, Data) ) ;
>
> > > to
>
> > >       New_Element := new Element_Record;
>
> > > The compiler is still pointing the same warning to this line.
>
> > > What exactly does the warning message "Type in allocator has deeper
> > > level than designated class-wide type" mean? Is the compiler unable to
> > > determine the size of the array during allocation? Or the array is a
> > > level deeper than the List.Access_Node?
>
> > It has nothing to do with arrays.  When you declare a type as "access
> > all T'Class", an object of that type can't be made to point to a type
> > declared in a more deeply nested subprogram.  A simple example:
>
> > package Pack1 is
> >     type Root is tagged null record;
> >     type Acc is access all Root'Class;
> > end Pack1;
>
> > package Pack2 is
> >     procedure Proc1;
> > end Pack2;
>
> > with Pack1;
> > package body Pack2 is
> >     procedure Proc1 is
> >         type Child is new Pack1.Root with null record;
> >         A : Pack1.Acc;
> >     begin
> >         A := new Child;   -- ILLEGAL
> >     end Proc1;
> > end Pack2;
>
> > This assignment statement is illegal, because somewhere later, A could
> > be copied to, say, another variable of type Pack1.Acc declared
> > globally in the body of Pack2, and then you'd have a pointer to a
> > Child that would exist after Proc1 has exited and the Child type no
> > longer exists, which is a big no-no.  4.8(5.1) prohibits this.
>
> > Since in this case, the record type (Element_Record) is declared in a
> > generic, that complicates things, because the generic *could* be
> > instantiated inside a procedure, which would lead to the same sort of
> > problem as above.  I don't remember all the rules, but depending on
> > where the type is declared and where the allocator occurs (generic
> > specification or body), it might be illegal to instantiate this
> > generic inside a procedure, or it might cause Program_Error to be
> > raised.  I just tried our compiler again and modified the source to
> > instantiate High_Cloud; if there's an instance of High_Cloud inside a
> > procedure, it *does* give a warning that Program_Error will be raised
> > (if the "new Element_Record" expression is reached).  If your compiler
> > gives the warning when the generic is first seen, before an instance
> > of the generic is seen, that's just a choice your compiler makes.  But
> > unless your program is arranged in a different order than I'm
> > guessing, or unless there are some new Ada 2012 rules that I'm not yet
> > familiar with, the compiler should not reject your generic, and it
> > should not reject it or cause Program_Error to be raised if there's an
> > instance of the generic that is *not* inside a subprogram.
>
> >                                   -- Adam
>
> I see and I followed this post by Robert Duff:https://groups.google.com/group/comp.lang.ada/browse_frm/thread/8fc03...
>
> Hence,
>
> generic
>    type Index is ( <> ) ;
>    type Item is private;
> package High_Cloud is
>
>    package List renames Heterogeneous_Doubly_Link_List;
>    use List;
>
>    type Element_Record           is new List.Node with private;
>    type Series_Access            is new List.Access_List;
> private
>    type Element_Record is new List.Node with
>       record
>          Name           : Unbounded_String;
>          Color          : Unbounded_String;
>          Data           : Item;
>       end record;
> ...
> end High_Cloud;
> In the package body of High_Cloud, I have a procedure:
>    procedure Add
>       ( Cloud          : in out High_Cloud_Record;
>         Name            : in     Unbounded_String;
>         Color           : in     Unbounded_String;
>         Data            : in     Item )
>    is
>       New_Element : List.Access_Node;
>    begin
>       New_Element := new Element_Record'
>                         ( Constructor.Init ( Name, Color, Data) ) ;
>       Concatenate ( Cloud.Series, New_Element ) ;
>    end Add;
>
> To use High_Cloud:
>
> with High_Cloud;
>
> package Hen is
>
>    type I_Array is array ( 1 .. 31 ) of Integer;
>    package G is new High_Cloud ( Positive, I_Array ) ;
>    use G;
>
> end Hen;
>
> and in the procedure, I have:
>
> with High_Cloud;
> with Hen;
>
> procedure smalltest is
>
>    package H renames Hen;
>    use H;
>
>    Data_Op : H.I_Array := ( 3, 4, 2, 3, 3, 3, 0, 3, 6, 3, 2, 6, 8, 7,
>                             3, 4, 3, 5, 1, 3, 1, 2, 2, 0, 2, 6, 8, 7,
>                             3, 3, 3
>                           ) ;
> begin
>
>    H.G.Init ( Chart ) ;
>    H.G.Add  ( Chart, "OP", "#000000", Data_Op ) ;
>
> end smalltest;
>
> I don't know if this is the most elegant and effective way, but it
> works.
> --
> Adrian Hoehttp://adrianhoe.com/adrianhoe


But this defeats my implementation intention which the size and the
type of its data array can only be determined during runtime. Any
suggestion?

Thanks.
--
Adrian Hoe
http://adrianhoe.com/adrianhoe