Re: Conversion of Access Types Question

["David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>]

Paul S. Heidmann wrote in message <369E4ACA.B8B07871@neta.com>...
>David C. Hoos, Sr. wrote:
>>
>> Paul S. Heidmann wrote in message <369DFFFC.A160D47C@neta.com>...
>> >Greetings fellow Ada lovers!
>> >
>> >I'm an experienced Ada83 programmer that is learning to use Ada95.
>> >Things are going very well, but I don't understand the problems
>> >that I'm having converting access types that point to variables of
>> >class wide types.  Consider the following piece of code:
>> >
>> >=======================================
>> >
>> >procedure Types is
>> >
>> >   type T1 is tagged
>> >      record
>> >         I : Integer;
>> >      end record;
>> >   type A_T1 is access all T1'Class;
>> >
>> >   type T2 is new T1 with
>> >      record
>> >         I2 : Integer;
>> >      end record;
>> >   type A_T2_Classwide is access all T2'Class;
>> >   type A_T2 is access T2;
>> >
>> >   Thing1 : A_T1;
>> >   Thing2 : A_T2_Classwide;
>> >   Thing3 : A_T2;
>> >
>> >begin
>> >   Thing1 := new T2;
>> >   Thing2 := A_T2_Classwide (Thing1);
>> >   Thing3 := A_T2 (Thing1);  -- illegal, types are not convertable.
>> >end Types;
>> >
>> >=======================================
>> >
>> >My question is, why is the last line an illegal conversion, but the
>> >second to last line not?
>> >
>> Compiling your code with gnat results in the following message:
>>
>>   [hoosd@VISNT021 d:/gnat_examples]$ gcc -c types.adb
>>   types.adb:23:14: target type must be general access type
>>   types.adb:23:14: add "all" to type "A_T2" defined at line 14
>>
>> Making the change the compiler directs solves the problem.
>>
>> i.e., change line 14 to read:
>>    type A_T2 is access all T2;
>
>I already understood that could solve the problem by making A_T2
>a generalized access type (that's why I have type A_T2_Classwide,
>above).  I question is why is this the case?  I could have been
>clearer...  Let me give it another try.
>
>A_T2_Classwide is an access type to a group of types rooted at
>T2.  In my understanding, the 'class, when applied to T2, refers
>to a set of types rooted at T2.  T1, in the above code, would not be
>in T2'class, right?  T2 would be in T1'class, however.  Therefore,
>A_T1 could point to an object of type T1 or of type T2 (and can, see
>first line of code).  If Thing1, above, points to an object of type
>T2 (and it does), then I should be able to covert it to A_T2 (but I
>can't).  Why is it that adding a 'class, which only refers to things
>derived from T2, makes the conversion from A_T1 (which has nothing to
>do with T2'Class) legal?
>
It's not the 'class which made the conversion from A_T1 to A_T2_Classwide
legal, it's the "all" in the declaration of A_T2_Classwide which makes it
legal.


Without the "all" (i.e. the target type is not a general access type), then
the rule of LRM 4.6(21) comes into play:
   Specifically, there is no type which is an ancestor of both the target
type (A_T1), and of the operand type (A_T2).

Making the target type a general access type brings into play the
less-restrictive rules of 4.6(13-17).

Hope this helps.

David C. Hoos, Sr.