Re: operation can be dispatching in only one type

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

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.

> However in C++ it does work as I expected.

C++ has no multiple dispatch either.
 
> Let's say I've the following tagged types.
> 
>    type T_Balance is tagged record
>       Id       : T_Balance_Id;
>       Name     : T_Balance_Name;
>       Balance  : T_Amount;
>    end record;
> 
>    package Balance_Map is new
>      Ada.Containers.Ordered_Maps (Key_Type   => T_Balance_Id,
>                                   Element_Type => T_Balance);
> 
>    type T_Account is tagged record
>       Name             : T_Account_Name;
>       Total_Balance    : T_Money;
>       Balances         : Balance_Map.Map;
>    end record;
> 
> And I want to have an operation increasing a balance and the total in the account.
> The following declaration is refused:
> 
>    procedure Update_Balance
>      (Self   : in out T_Account;
>       AB     : in out T_Balance;
>       Amount : in     T_Money);
> 
> My hope was that it gets dispatched to the right operation on Self which
> in turns will call the right operation on AB.  But the compiler seems not
> to agree.

It is too late to declare a dispatching operation on T_Balance when you
already used T_Balance as a type somewhere else, e.g. to instantiate a
package.

I don't use Ada containers, but the pattern is:

1. Instantiate the package with T_Balance'Class

2. Declare the container-specific callback, e.g. Update_Balance with
T_Balance'Class

3. Dispatch from Update_Balance, to an [abstract] primitive operation of
T_Balance you declare in advance. E.g. On_Update.

[ This is what happens in C++, because in C++ T and T'Class are
[in]consistently confused. ]

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