Re: What makes a procedure call 'dispatching' in Ada?

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

Markus Schoepflin wrote:
> Hello,
> 
> I'm trying to fell may way around object oriented Ada programming, and I 
> think I must be missing something absolutely basic. Please consider the 
> following package:
> 
> ---%<---
> package FOOS is
> 
>    type FOO is abstract tagged null record;
> 
>    procedure P (THIS : in FOO);
>    procedure A (THIS : in FOO) is abstract;
> 
> end FOOS;

No problems there (well, Dmitry will have philosophical objections to 
this design :-)

> package body FOOS is
> 
>    procedure P (THIS : in FOO)
>    is
>    begin
>       A (THIS);

At this point, the compiler knows that THIS is a FOO object, or is to be 
seen as a FOO object. But you have said that the procedure A on FOO is 
abstract -- not implemented -- so you cannot call it.

I assume that your intention is to call the procedure A that is 
implemented (overriden) for the actual object THIS, which is of some 
type derived from FOO .. some type in FOO'Class for which you have 
implemented A. To do so, you must ask the compiler to make this call 
dispatching, by converting the parameter to FOO'Class:

         A (FOO'Class (THIS));

This is called a "redispatching" call, because the procedure P may have 
been reached as the result of a dispatching call on P, and now we are 
asking to dispatch again.

One way to understand this is that in Ada by default calls are 
statically bound, not run-time bound (dispatching). Only calls that have 
  parameters of class type are dispatching.

Dmitry has philosophical objections to redispatching, which is why he 
suggests that you should declare P with a class-type parameter THIS in 
the first place, so that A (THIS) will dispatch. Either will work, but 
in Dmitry's approach you cannot override P for derived types, because it 
is then a class-wide operation, not a primitive operation of FOO. In 
both approaches you can override A for derived types (and you must 
override it because it is abstract in FOO).

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
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