Cloning a Class Wide Data Type with "new"?

Cloning a Class Wide Data Type with "new"?

[Warren W. Gay VE3WWG]

I havn't had much time to research this one yet, but I've
got stuck trying to clone a class wide type this morning,
on my way to work. A sample program is provided below.

I need to be able to allocate and then copy an arbitrary
tagged type (based upon a base type) into the newly
allocated object. There's gotta be a way to do this..
I just can't remember it!

The problem is the operation of the form of :

   Cloned : Base_Access := new Base_Class'(Orig);

Here is a sample program that distills the problem:


with Ada.Text_IO;
use Ada.Text_IO;

procedure TProg is

    type Base_Type is tagged
       record
          V :      Integer;
       end record;

    type Base_Access is access all Base_Type'Class;
    subtype Base_Class is Base_Type'Class;

    type Derived_Type is new Base_Type with
       record
          Z :      Natural;
       end record;

    -- This is the original object to be cloned
    Orig :   Derived_Type := ( V => 32, Z => 99 );

    -- This is a local copy (easy)
    Copied : Base_Class  := Orig;

    -- Allocate Cloned to be same as Orig (the problem)
    Cloned : Base_Access := new Base_Class'(Orig);
begin

    Put_Line("Cloned.V = " & Cloned.V'Img);

end TProg;


TIA, Warren.
-- 
Warren W. Gay VE3WWG
http://home.cogeco.ca/~ve3wwg

Re: Cloning a Class Wide Data Type with "new"?

[Jean-Pierre Rosen]

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"Warren W. Gay VE3WWG" <ve3wwg@cogeco.ca> a �crit dans le message news:
3EE097D5.2010901@cogeco.ca...
> I havn't had much time to research this one yet, but I've
> got stuck trying to clone a class wide type this morning,
> on my way to work. A sample program is provided below.
>
> I need to be able to allocate and then copy an arbitrary
> tagged type (based upon a base type) into the newly
> allocated object. There's gotta be a way to do this..
> I just can't remember it!
>
> The problem is the operation of the form of :
>
>    Cloned : Base_Access := new Base_Class'(Orig);
>

   Cloned : Base_Access := new Base_Class'class'(Orig);

Hey, in Ada types must conform, and Base_Access is an access to
Base_Access'Class!

--
---------------------------------------------------------
           J-P. Rosen (rosen@adalog.fr)
Visit Adalog's web site at http://www.adalog.fr

Re: Cloning a Class Wide Data Type with "new"?

[Warren W. Gay VE3WWG]

Jean-Pierre Rosen wrote:
> "Warren W. Gay VE3WWG" <ve3wwg@cogeco.ca> a �crit dans le message news:
> 3EE097D5.2010901@cogeco.ca...
> 
>>I havn't had much time to research this one yet, but I've
>>got stuck trying to clone a class wide type this morning,
>>on my way to work. A sample program is provided below.
>>
>>I need to be able to allocate and then copy an arbitrary
>>tagged type (based upon a base type) into the newly
>>allocated object. There's gotta be a way to do this..
>>I just can't remember it!
>>
>>The problem is the operation of the form of :
>>
>>   Cloned : Base_Access := new Base_Class'(Orig);
>>
> 
>    Cloned : Base_Access := new Base_Class'class'(Orig);

BZZZzzt!

gcc -c -gnata -gnatf -gnato tprog.adb
tprog.adb:26:50: expected type "Base_Type'Class" defined at line 6
tprog.adb:26:50: found type "Derived_Type" defined at line 14
gnatmake: "tprog.adb" compilation error

> Hey, in Ada types must conform, and Base_Access is an access to
> Base_Access'Class!

You do understand what I am trying to do, right?  I just cannot
seem to figure out how to specify it.

-- 
Warren W. Gay VE3WWG
http://home.cogeco.ca/~ve3wwg

Re: Cloning a Class Wide Data Type with "new"?

[David C. Hoos]

"Warren W. Gay VE3WWG" <ve3wwg@cogeco.ca> wrote in message
news:3EE097D5.2010901@cogeco.ca...
> I havn't had much time to research this one yet, but I've
> got stuck trying to clone a class wide type this morning,
> on my way to work. A sample program is provided below.
>
> I need to be able to allocate and then copy an arbitrary
> tagged type (based upon a base type) into the newly
> allocated object. There's gotta be a way to do this..
> I just can't remember it!
>
> The problem is the operation of the form of :
>
>    Cloned : Base_Access := new Base_Class'(Orig);
>
> Here is a sample program that distills the problem:
>
>
> with Ada.Text_IO;
> use Ada.Text_IO;
>
> procedure TProg is
>
>     type Base_Type is tagged
>        record
>           V :      Integer;
>        end record;
>
>     type Base_Access is access all Base_Type'Class;
>     subtype Base_Class is Base_Type'Class;
>
>     type Derived_Type is new Base_Type with
>        record
>           Z :      Natural;
>        end record;
>
>     -- This is the original object to be cloned
>     Orig :   Derived_Type := ( V => 32, Z => 99 );
>
>     -- This is a local copy (easy)
>     Copied : Base_Class  := Orig;
>
>     -- Allocate Cloned to be same as Orig (the problem)
>     Cloned : Base_Access := new Base_Class'(Orig);

The operand of the qualified expression must first be converted to the
Base_Class -- i.e.:
    Cloned : Base_Access := new Base_Class'(Base_Class (Orig));

See RM95 4.7 (3)

> begin
>
>     Put_Line("Cloned.V = " & Cloned.V'Img);
>
> end TProg;
>
>
> TIA, Warren.
> -- 
> Warren W. Gay VE3WWG
> http://home.cogeco.ca/~ve3wwg
>
> _______________________________________________
> comp.lang.ada mailing list
> comp.lang.ada@ada.eu.org
> http://ada.eu.org/mailman/listinfo/comp.lang.ada
>

Re: Cloning a Class Wide Data Type with "new"?

[Marc A. Criley]

"Warren W. Gay VE3WWG" <ve3wwg@cogeco.ca> wrote in message news:<3EE097D5.2010901@cogeco.ca>...
> I need to be able to allocate and then copy an arbitrary
> tagged type (based upon a base type) into the newly
> allocated object. There's gotta be a way to do this..
> I just can't remember it!
> 
> The problem is the operation of the form of :
> 
>    Cloned : Base_Access := new Base_Class'(Orig);

Here's what I did to get it to compile...

     Cloned : Base_Access := new Base_Class'Class'(Base_Class(Orig));

...AND it seemed to work--though it'd be a little iffy for me to
explain in detail what's going on.

Marc A. Criley

Modified sample program follows:

with Ada.Text_IO;
use Ada.Text_IO;

procedure TProg is

    type Base_Type is tagged
       record
          V :      Integer;
       end record;

    type Base_Access is access all Base_Type'Class;
    subtype Base_Class is Base_Type'Class;

    type Derived_Type is new Base_Type with
       record
          Z :      Natural;
       end record;

    -- This is the original object to be cloned
    Orig :   Derived_Type := ( V => 32, Z => 99 );

    -- This is a local copy (easy)
    Copied : Base_Class  := Orig;

    -- Allocate Cloned to be same as Orig (the problem)
    Cloned : Base_Access := new Base_Class'Class'(Base_Class(Orig));
begin

    Put_Line("Cloned.V = " & Cloned.V'Img);
    Put_Line("Cloned.Z = " & Derived_Type(Cloned.all).Z'Img);

end TProg;

Re: Cloning a Class Wide Data Type with "new"?

[Anh_Vo]

"Warren W. Gay VE3WWG" <ve3wwg@cogeco.ca> wrote in message news:<3EE097D5.2010901@cogeco.ca>...
> I havn't had much time to research this one yet, but I've
> got stuck trying to clone a class wide type this morning,
> on my way to work. A sample program is provided below.
> 
> I need to be able to allocate and then copy an arbitrary
> tagged type (based upon a base type) into the newly
> allocated object. There's gotta be a way to do this..
> I just can't remember it!
> 
> The problem is the operation of the form of :
> 
>    Cloned : Base_Access := new Base_Class'(Orig);
> 
> Here is a sample program that distills the problem:
> 
> 
> with Ada.Text_IO;
> use Ada.Text_IO;
> 
> procedure TProg is
> 
>     type Base_Type is tagged
>        record
>           V :      Integer;
>        end record;
> 
>     type Base_Access is access all Base_Type'Class;
>     subtype Base_Class is Base_Type'Class;
> 
>     type Derived_Type is new Base_Type with
>        record
>           Z :      Natural;
>        end record;
> 
>     -- This is the original object to be cloned
>     Orig :   Derived_Type := ( V => 32, Z => 99 );
> 
>     -- This is a local copy (easy)
>     Copied : Base_Class  := Orig;
> 
>     -- Allocate Cloned to be same as Orig (the problem)
>     Cloned : Base_Access := new Base_Class'(Orig);
> begin
> 
>     Put_Line("Cloned.V = " & Cloned.V'Img);
> 
> end TProg;
> 
> 
> TIA, Warren.

Base_Class is not a real type. Base_Type and Derived_Type are real
types. Thefore, either one of them should be used. I assume
Derived_Type was your intention in this case. Otherwise, explicit
conversion is needed. Both cases are shown below.

Cloned : Base_Access := new Derived_Type' (Orig);  -- case 1

Cloned : Base_Access := new Base_Type' (Base_Type (Orig));  -- case 2

A. Vo

Re: Cloning a Class Wide Data Type with "new"?

[Anh_Vo]

> Base_Class is not a real type. Base_Type and Derived_Type are real
> types. Thefore, either one of them should be used. I assume
> Derived_Type was your intention in this case. Otherwise, explicit
> conversion is needed. Both cases are shown below.
> 
> Cloned : Base_Access := new Derived_Type' (Orig);  -- case 1
> 
> Cloned : Base_Access := new Base_Type' (Base_Type (Orig));  -- case 2
> 
> A. Vo

My fingers were ahead of my thought. What I mean was that Base_Class
is class wide type while Base_Type and Derived_Type are not. That
means there should be case 3 for it.

Cloned : Base_Access := new Base_Class' (Base_Class (Orig));  -- case
3

By the way, I made another error while typing the word Therefore. But
it turned out to be Thefore.

Re: Cloning a Class Wide Data Type with "new"?

[Warren W. Gay VE3WWG]

Thank you everyone that replied to the "challenge".

Anh_Vo wrote:
>>Base_Class is not a real type. Base_Type and Derived_Type are real
>>types. Thefore, either one of them should be used. I assume
>>Derived_Type was your intention in this case. Otherwise, explicit
>>conversion is needed. Both cases are shown below.
>>
>>Cloned : Base_Access := new Derived_Type' (Orig);  -- case 1
>>
>>Cloned : Base_Access := new Base_Type' (Base_Type (Orig));  -- case 2
>>
>>A. Vo
> 
> 
> My fingers were ahead of my thought. What I mean was that Base_Class
> is class wide type while Base_Type and Derived_Type are not. That
> means there should be case 3 for it.
> 
> Cloned : Base_Access := new Base_Class' (Base_Class (Orig));  -- case
> 3
> 
> By the way, I made another error while typing the word Therefore. But
> it turned out to be Thefore.

Yes, as pointed out by yourself and others, the above solution works.

Boiling it down to avoid using Base_Class (which is just Base_Type'Class),
this may be more easily(?) understood as:

  Cloned : Base_Access := new Base_Type'Class'(Base_Type'Class(Orig));

which I guess makes sense. "Orig" is a specific type, so you need to
"convert" it to a class wide type to be used as the "prototype" to
be cloned. Then the "new" operator needs to be told to return a class
wide type, so I guess as clumsy as it looks, this is perhaps the only
conceivable way that it should be specified (ie. it makes sense).

Thanks everyone, Warren.
-- 
Warren W. Gay VE3WWG
http://home.cogeco.ca/~ve3wwg