Re: Runtime type selection

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

On Tue, 13 Oct 2009 10:45:41 +0000, Colin Paul Gloster wrote:

> On Sun, 11 Oct 2009, Dmitry A. Kazakov wrote:
> 
>|----------------------------------------------------------------------------|
>|"                                                                           |
>|On Sun, 11 Oct 2009 09:41:54 -0700 (PDT), xorque wrote:                     |
>|                                                                            |
>|> I'd like to develop a program that processes a graph. The contents of     |
>|> the graph are a random selection of objects of different types (all       |
>|> elements may be of the same type or may all be different - assuming       |
>|> there are enough types defined). By 'type' in this case, I am             |
>|> referring to Ada types.                                                   |
>|>                                                                           |
>|> The basic problem is that I want to create an object of a type picked     |
>|> at random, at runtime. I'm not entirely sure what my options are here.    |
>|                                                                            |
>|The problem is that graph nodes have nothing in common. Thus there is       |
>|nothing except for the graph traversal operations you could implement on    |
>|the graph as a whole.                                                       |
>|                                                                            |
>|[..]"                                                                       |
>|----------------------------------------------------------------------------|
> 
> You made it sound as if a Visitor Pattern might work.

In which case nodes would be descendants of some common root with the
visitor's accept operation. A common interface would change everything,
however. 

Interesting is that Ada's pool-specific pointers (to class-wide targets)
are a kind of doubly dispatching (pool class x type class), which in the
end allows visitor-like implementations without any common root.

There is another case of hidden double dispatch in Ada. It is stream
attributes T'Class'Input and T'Class'Output (stream class x type class).

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