Re: Derived private interface

["Randy Brukardt" <randy@rrsoftware.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:1nvqqny2226ro$.1ixvoiht8zu8l$.dlg@40tude.net...
> On Mon, 1 Aug 2011 16:56:17 -0500, Randy Brukardt wrote:
>
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
>> news:1rlxo1uthv5xt.1agapd9q0mek4$.dlg@40tude.net...
>> ...
>>> BTW, Generic_Dispatching_Constructor of Ada 2005 did not change much to
>>> Ada
>>> 95 because of tags. You still need a registering layer to maintain
>>> external name to tag mapping.
>>
>> It did eliminate one thing, the need to have a scheme to create an object 
>> of
>> each tagged type. That forced the use of either a case statement or an
>> access-to-subprogram, either of which is error-prone.
>
> I don't consider the case statement because it is just a wrong pattern.
> Regarding mappings type ID to either a constructing function or tag, they
> are equivalent. Both require the same amount of maintenance.

There are more sources of error when using access-to-subprogram; it's easy 
to forget to create a routine or use the wrong one. Neither of those are 
possible with generic dispatching constructor (or with a case statement, for 
that matter, presuming that you use Ada's completeness checks). This is a 
minor advantage, but it is real; it's the major reason that I don't even 
think about using access-to-subprogram (and have to be reminded of the 
possibility).

 >> There is no possibility of eliminating the need for some sort of 
registry --
>> although in some cases you can use the one built-into the language (the
>> External_Tag). There has to be a mapping somewhere of some sort of key to 
>> a
>> tag, since it isn't possible to export tags to the "real world".
>
> It is possible, and even required for distributed weakly coupled
> applications, but in order to do that you have to export the types
> themselves rather than their tags.

It might be possible for some non-Ada language, but it's not possible for 
Ada (and this is an Ada site and I was only talking about Ada here). There 
is no way in Ada to "export the types"; you have to export some type 
indication -- and that requires some sort of mapping of indication to type.

                             Randy.