Re: dispatching query

Re: dispatching query

[John English]

Mark Rutten (Mark.Rutten@dsto.defence.gov.au) wrote:
: Is it only possible to dispatch on subprograms which can be inherited
: (i.e. specified in the same declarative block)?

You're talking about primitives. Dispatching happens when you have a
class wide type (X'Class).

: package messages is
:   type message is abstract tagged null record;
:   type message_ptr is access all message'Class;
:   procedure copy(msg: access message) is abstract;
: end messages;

: Then (in some other package) I declare a type (or several types) which
: extend message. What I would like to do is to define a set of procedures
: in a third package (separate to the declarations of the types) which can
: be dispatched upon
: e.g.
:   procedure handle(msg: access boolean_message);
:   procedure handle(msg: access integer_message);

So, make handle a primitive of Message (abstract if necessary) and
then hand it a class-wide pointer (an access Message'Class) to make
it dispatch. That is, add
  procedure Handle (Msg : access Message) is abstract;
to Messages, and then call Handle(This_Message'Access)
(where This_Message is a Message'Class object).

-----------------------------------------------------------------
 John English              | mailto:je@brighton.ac.uk
 Senior Lecturer           | http://www.it.bton.ac.uk/staff/je
 Dept. of Computing        | ** NON-PROFIT CD FOR CS STUDENTS **
 University of Brighton    |    -- see http://burks.bton.ac.uk
-----------------------------------------------------------------

Re: dispatching query

[Stephen Leake]

Mark Rutten wrote:
> 
> Is it only possible to dispatch on subprograms which can be inherited
> (i.e. specified in the same declarative block)?

This is correct; see LRM 3.9.2. A dispatching operation is a primitive
operation, which is declared in the same block as the tagged type.

> 
> e.g.
> 
> package messages is
> 
>   type message is abstract tagged null record;
>   type message_ptr is access all message'Class;
> 
>   procedure copy(msg: access message) is abstract;
> 
> end messages;
> 
> Then (in some other package) I declare a type (or several types) which
> extend message. What I would like to do is to define a set of procedures
> in a third package (separate to the declarations of the types) which can

why do you want the operations in a third package?

If you want to hide the operations from some parts of the application,
you can declare the operations in the private part of the package spec.

> be dispatched upon
> 
> e.g.
> 
>   procedure handle(msg: access boolean_message);
>   procedure handle(msg: access integer_message);
> 
> where boolean_message and integer_message have been inherited from
> message and are declared in the second package. Then I would like to
> call
> 
>   handle(msg);
> 
> (msg of type message_ptr) and have this dispatch on the above two
> procedures.
> 
> I guess what I'm really trying to do is to replace what might be a case
> statement over the different inherited types with this set of
> procedures. I've tried the above with gnat3.09 on Solaris, but it
> complains at having too many choices when trying to resolve the
> procedure call, rather than recognising a dispatchable operation. Is
> there a way around this?

As you guessed, you must declare dispatching operations with the types.

-- 
- Stephe