Re: A bad counterintuitive behaviour of Ada about OO

[Victor Porton <porton@narod.ru>]

Simon Wright wrote:

> Victor Porton <porton@narod.ru> writes:
> 
>> http://freesoft.portonvictor.org/binaries/ada-obj-test.tar.gz
>>
>> contains a tiny Ada program which prints 0 despite intuition suggests
>> that it should print 123, because it just copies (using Adjust) an object
>> holding 123.
>>
>> In this program Handle_Type models some handle provided by an external
>> API, where 0 is an undefined handle. Base_Object is a tagged record which
>> holds a handle inside itself and can copy or destroy the handle
>> automatically (as Base_Object is a controlled object).
>>
>> Example_Record is a derived type of Base_Object which imitates a behavior
>> of a simple library which operates over some handles. For example, in
>> this example copying a handle preserves it unchanged.
>>
>> This tiny program was created by me as a model of my real Ada bindings
>> for Raptor C library, which produces a wrong behavior.
>>
>> Why Ada behaves in this counter-intuitive way?
>>
>> What should I do in similar situations when developing real Ada software?
> 
> I think what is happening is that in
> 
>    function From_Handle(Handle: Handle_Type) return Base_Object is
>    begin
>       return (Ada.Finalization.Controlled with Handle=>Handle);
>    end;
> 
> the aggregate is created and copied to the function's result as a
> Base_Object, so that when
> 
>    procedure Adjust(Object: in out Base_Object) is
>    begin
>       if Object.Handle /= 0 then
>          Object.Handle := Copy_Handle(Base_Object'Class(Object),
>          Object.Handle);
>       end if;
>    end;
> 
> is called during the copy,
> 
>    function Copy_Handle(Object: Base_Object; Handle: Handle_Type) return
>    Handle_Type is begin
>       return 0;
>    end;
> 
> returns 0, as you've seen.
> 
> 
> (1) If all you wanted to do was copy the Handle over, you don't need to
> do anything, because that's the default behaviour (i.e., copies the
> bits).
> 
> (2) Normally, From_Handle wouldn't be inherited, so you'd have had to
> write your own; because Example_Object has a null extension, it can be
> inherited.

So now I understand what I should do. To eliminate side effects when copying 
subobjects of Base_Object type, I should remove overriding Adjust from 
Base_Object and define it manually for particular object types. Copy_Handle 
then becomes unnecessary and should be removed altogether.

To make possible to implement Adjust in derived types, I need to define:

-- Don't call this procedure unless you really need it.
procedure Set_Handle_Hack(Object: in out Base_Object; Handle: Handle_Type) 
is
begin
   Object.Handle := Handle;
end;

It is an acceptable solution.

-- 
Victor Porton - http://portonvictor.org