Re: Derived private interface

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

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 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.

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