Re: operation can be dispatching in only one type

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Mon, 23 Nov 2015 06:16:38 -0800 (PST), Serge Robyns wrote:

> On Monday, 23 November 2015 14:48:45 UTC+1, Dmitry A. Kazakov  wrote:
>> On Mon, 23 Nov 2015 05:05:02 -0800 (PST), Serge Robyns wrote:
>> 
>>> On Monday, 23 November 2015 12:29:55 UTC+1, Dmitry A. Kazakov  wrote:
>>>> On Mon, 23 Nov 2015 02:23:57 -0800 (PST), Serge Robyns wrote:
>>>> 
>>>>> I'm facing an issue with Ada when it comes to use an operation on two
>>>>> tagged types.
>>>> 
>>>> You can use tagged types, they cannot be dispatching arguments though. That
>>>> is a big difference.
>>>> 
> 
>> C++ code is broken, always broken, because C++ confuses T and T'Class and
>> hence re-dispatches each time and every place (except for ctors). Ada's
>> exact equivalent to
>> 
>> void account::update_balance (account_balance& ab, money amount)
>> 
>> is
>> 
>> procedure Update_Balance (X : in out Account;
>>     AB : in out Account_Balance'Class, ...)
> 
> This one did the trick.  I do feel very frustrated with regards to the Ada
> idiosyncrasies, if I may say so. Recalls me the "'" notation I had to use
> when declaring a part of a constant that was from a parent class.

It depends on what you believe is "right." If you programmed C++ in Ada you
would indeed face difficulties, and conversely. But again C++ is
inconsistent in its treatment of specific and class-wide types. Ada merely
requires you to specify the exact type of the operation's argument, that
is.

> I'm wondering the best way to "learn" these without falling asleep while
> reading for the 10th time basic programming stuff and then missing that
> one important comment in the text.

Regarding OO features, the best way is start thinking of it in terms of
types rather than of [untyped] data structures and procedures. You will
better understand both Ada and C++:

class = set of types (closed upon inheritance)
class-wide type = the closure of the class
specific type = member of the class

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