Re: Ada Interfaces and the Liskov Substitution Principle

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

On 24 May 2007 00:29:29 -0700, Maciej Sobczak wrote:

> On 23 Maj, 23:58, "Randy Brukardt" <r...@rrsoftware.com> wrote:
> 
>>> My humble opinion: assignment of class-wide types should be prohibited
>>> in the first place. I mean - at compile time.
>>> It's just asking for troubles.
>>
>> That doesn't make much sense. Yes, it is a bit weird that assignment is
>> potentially a dispatching operation, but otherwise there is no semantic
>> problem with it.
> 
> Can it be dispatching on both arguments?

It shall be. It is a pity that Ada does not have MD.

> If not, how can you reasonably implement it?
> 
> And even if it could, still, how can you *reasonably* implement it?
> 
> Let's take a classic example with the hierarchy of geometric objects
> (Object, Rectlangle, Triangle, Circle, and so on) and this:
> 
> procedure Do_Something(X : in Object'Class; Y : out Object'Class) is
> begin
>   Y := X; -- ?
> end Do_Something;

In the current version of Ada Y is constrained to its actual type. So in
all cases of different types you will get Constraint_Error - no full MD.
But if there were MD, then you could call Do_Something (Circle, Ellipse).
And don't forget:

declare
   X : T'Class := Read_From_File; -- T is non-limited, Adjust is called
begin
   ...

> Coming from C++, I have never seen any single example where assignment
> made sense with polymorphic hierarchies.

Real-life example. Consider a threaded library. Let you have an Ada-like
rendezvous. You wanted to propagate an exception raised in the callee to
the caller. Similarly, let an exception is propagated out of a child
thread. You want to continue its propagation to the parent.

> Do you know any such example? Can you show it?

Marshaling objects.

(All types of, like in the example above, like in view change of a small
by-value object, like in making an object persistent or broadcasting it
over the network)

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