package Ada.Tags

package Ada.Tags

[RasikaSrinivasan]

Friends

I have two types declared as follows:

with Ada.Calendar ;

package events is
    type events_record_type is abstract tagged 
      record
         time : ada.Calendar.time ;
      end record ;
   procedure show(event : events_record_type) ;
   
   type basal_delivery_event_type is new events_record_Type with
      record
         amount : float ;
      end record ;
   overriding
   procedure show( event : basal_delivery_event_type ) ;
   
   type bolus_delivery_event_type is new events_record_Type with
      record
         amount : float ;
      end record ;
   overriding
   procedure show( event : bolus_delivery_event_type ) ;
   
   type SMBG_event_type is new events_record_type with
      record
         amount : short_integer ;
      end record ;
   overriding
   procedure show( event : SMBG_event_type ) ;
   
end events ;

The following implementation in the body:



   procedure show (event : events_record_type) is
      use GNAT.Calendar.Time_IO;  
   begin      
      put( ada.Tags.Expanded_Name( event'tag ) );      
      put( gnat.Calendar.time_io.Image( event.time , gnat.Calendar.Time_IO.ISO_Date & " %T")) ;      
   end show;

gives me a compilation error:

"tag" attribute can only be applied to objects of class - wide type.

In the above, isnt event an object meeting this criterion?

clearly i am missing something. The question is I want to find out the equivalent of RTTI from C++. ie Which one if the derived types is it.

clues appreciated. srini

Re: package Ada.Tags

[Simon Wright]

"RasikaSrinivasan@gmail.com" <RasikaSrinivasan@gmail.com> writes:

>    procedure show (event : events_record_type) is
>       use GNAT.Calendar.Time_IO;
>    begin
>       put( ada.Tags.Expanded_Name( event'tag ) );
>       put( gnat.Calendar.time_io.Image( event.time , gnat.Calendar.Time_IO.ISO_Date & " %T")) ;
>    end show;
>
> gives me a compilation error:
>
> "tag" attribute can only be applied to objects of class - wide type.
>
> In the above, isnt event an object meeting this criterion?
>
> clearly i am missing something. The question is I want to find out the
> equivalent of RTTI from C++. ie Which one if the derived types is it.

You can get it to compile by converting the object 'event' of a specific
type to the class-wide type:

  Events_Record_Type'Class (Event)'Tag

But, how are you going to get Show(Events_Record_Type) to be called?

Re: package Ada.Tags

[RasikaSrinivasan]

Thanks. It worked.

The question of exact application is valid of course and requires thinking.

cheers, srini

On Thursday, June 7, 2012 5:35:11 PM UTC-4, Simon Wright wrote:
> "RasikaSrinivasan@gmail.com" <RasikaSrinivasan@gmail.com> writes:
> 
> >    procedure show (event : events_record_type) is
> >       use GNAT.Calendar.Time_IO;
> >    begin
> >       put( ada.Tags.Expanded_Name( event'tag ) );
> >       put( gnat.Calendar.time_io.Image( event.time , gnat.Calendar.Time_IO.ISO_Date & " %T")) ;
> >    end show;
> >
> > gives me a compilation error:
> >
> > "tag" attribute can only be applied to objects of class - wide type.
> >
> > In the above, isnt event an object meeting this criterion?
> >
> > clearly i am missing something. The question is I want to find out the
> > equivalent of RTTI from C++. ie Which one if the derived types is it.
> 
> You can get it to compile by converting the object 'event' of a specific
> type to the class-wide type:
> 
>   Events_Record_Type'Class (Event)'Tag
> 
> But, how are you going to get Show(Events_Record_Type) to be called?

Re: package Ada.Tags

[Adam Beneschan]

On Thursday, June 7, 2012 1:36:32 PM UTC-7, RasikaSr...@gmail.com wrote:
> Friends
> 
> I have two types declared as follows:
> 
> with Ada.Calendar ;
> 
> package events is
>     type events_record_type is abstract tagged 
>       record
>          time : ada.Calendar.time ;
>       end record ;
>    procedure show(event : events_record_type) ;
>    
>    type basal_delivery_event_type is new events_record_Type with
>       record
>          amount : float ;
>       end record ;
>    overriding
>    procedure show( event : basal_delivery_event_type ) ;
>    
>    type bolus_delivery_event_type is new events_record_Type with
>       record
>          amount : float ;
>       end record ;
>    overriding
>    procedure show( event : bolus_delivery_event_type ) ;
>    
>    type SMBG_event_type is new events_record_type with
>       record
>          amount : short_integer ;
>       end record ;
>    overriding
>    procedure show( event : SMBG_event_type ) ;
>    
> end events ;
> 
> The following implementation in the body:
> 
> 
> 
>    procedure show (event : events_record_type) is
>       use GNAT.Calendar.Time_IO;  
>    begin      
>       put( ada.Tags.Expanded_Name( event'tag ) );      
>       put( gnat.Calendar.time_io.Image( event.time , gnat.Calendar.Time_IO.ISO_Date & " %T")) ;      
>    end show;
> 
> gives me a compilation error:
> 
> "tag" attribute can only be applied to objects of class - wide type.
> 
> In the above, isnt event an object meeting this criterion?

No.  Ada makes a distinction between specific types and class-wide types.  If a parameter is declared using a specific type, only objects of that type can be passed to it.  (This is different from what happens in, say, Java.)  What's happening here is that when you declare your derived types, the compiler inherits "show" from them, and declares a new version of "show" with a new specific type as a parameter--either basal_delivery_event_type, bolus_delivery_event_type, or SMBG_event_type.  If you don't override them, the code for the new routines will be the same as the code for the original routine, but using the new specific type.  When you dispatch, the program will call whichever routine has that specific type as a parameter.  But the type is still a specific type.  (Note: Although I'm describing this as if a new copy of the "show" routine is being created, the code should still be in your program only once.)

This is different from a routine with a class-wide parameter:

   procedure classwide_show (events : events_record_type'class);

Now, you can call this routine with any of your derived types as a parameter.  But this time, there's only one copy of the routine, not multiple copies as above.  Now, the routine can use events'tag to get information about the actual type of the object.  

But you can't do this in your example, because the show routine uses a specific type as a parameter.  However, you can get around it like this:

  put( ada.Tags.Expanded_Name( events_record_type'class(event)'tag ) );

In other words, tell the compiler to treat event as though it were a class-wide type, and it will be happy.

One other thing: You've declared overriding routines for show, but you didn't provide the bodies.  If you it to do the exact same thing as the "show" you declared for events_record_type, then don't override it--just let the program use the inherited copy.  If you want to have a body that performs the original "show" and does some other functions, then override it and have the body call this:

   events_record_type(event).show;  --or
   show (events_record_type (event));

That tells the compiler to treat event as though it had type events_record_type, and then it will pick the version of "show" that has an events_record_type parameter, i.e. the original one.  The 'Tag will still be the tag of whatever type you want, because 'Tag tells you what type the object really is, not what type you're telling the compiler to pretend it is.  

Hope this helps.  The paradigm in Ada is different from C++ or Java, so I think it takes some getting used to if you're coming from one of those languages.  But it's pretty simple to grasp once you get used to it.

                           -- Adam

Re: package Ada.Tags

[Yannick Duchêne (Hibou57)]

Le Thu, 07 Jun 2012 23:35:11 +0200, Simon Wright <simon@pushface.org> a  
écrit:

> "RasikaSrinivasan@gmail.com" <RasikaSrinivasan@gmail.com> writes:
>
>>    procedure show (event : events_record_type) is
>>       use GNAT.Calendar.Time_IO;
>>    begin
>>       put( ada.Tags.Expanded_Name( event'tag ) );
>>       put( gnat.Calendar.time_io.Image( event.time ,  
>> gnat.Calendar.Time_IO.ISO_Date & " %T")) ;
>>    end show;
>>
>> gives me a compilation error:
>>
>> "tag" attribute can only be applied to objects of class - wide type.
>>
>> In the above, isnt event an object meeting this criterion?
>>
>> clearly i am missing something. The question is I want to find out the
>> equivalent of RTTI from C++. ie Which one if the derived types is it.
>
> You can get it to compile by converting the object 'event' of a specific
> type to the class-wide type:
>
>   Events_Record_Type'Class (Event)'Tag

Forgive me for being pedantic, but it's important to note this is not a  
type conversion, that's a *view conversion*.

-- 
“Syntactic sugar causes cancer of the semi-colons.” [1]
“Structured Programming supports the law of the excluded muddle.” [1]
[1]: Epigrams on Programming — Alan J. — P. Yale University

Re: package Ada.Tags

[Dmitry A. Kazakov]

On Fri, 08 Jun 2012 11:50:30 +0200, Yannick Duch�ne (Hibou57) wrote:

> Forgive me for being pedantic, but it's important to note this is not a  
> type conversion, that's a *view conversion*.

More pedantic: view conversion is a type conversion. The reverse is untrue.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: package Ada.Tags

[Adam Beneschan]

On Friday, June 8, 2012 2:50:30 AM UTC-7, Hibou57 (Yannick Duchêne) wrote:

> Forgive me for being pedantic, but it's important to note this is not a  
> type conversion, that's a *view conversion*.

There are two kinds of type conversions: "view conversions" and "value conversions".  So, yes, this is a type conversion, and yes, it's a view conversion. 

The important thing to note (for the OP) is that since this is not a value conversion, that means the tag doesn't change.  Similarly with one of the examples I posted earlier, where I wrote Events_Record_Type(Event) [without the 'Class].  This is also a view conversion, which means that while it causes the compiler to look at the object differently, it's not a *value* conversion, so the value doesn't change to a value of a different type--which means 'Tag will still be the same, which is what the OP wanted.

                          -- Adam

Re: package Ada.Tags

[Simon Wright]

"Yannick Duchêne (Hibou57)" <yannick_duchene@yahoo.fr> writes:

> Le Thu, 07 Jun 2012 23:35:11 +0200, Simon Wright <simon@pushface.org>
> a écrit:

>> You can get it to compile by converting the object 'event' of a specific
>> type to the class-wide type:
>>
>>   Events_Record_Type'Class (Event)'Tag
>
> Forgive me for being pedantic, but it's important to note this is not
> a type conversion, that's a *view conversion*.

Forgiven!