Re: Registries [2]

Sorry I didn't answer this earlier; it's tough because I don't feel I
have good answers.

Daniel Wallin <[email protected]> writes:

> At 21:30 2003-07-31, David Abrahams wrote:
>>Daniel Wallin <[email protected]> writes:
>>
>> >>
>> >>Hmm, you're suggesting importing cast graph components?  I don't know;
>> >>it seems like a waste for several extensions which are known to
>> >>collaborate to have to each maintain a separate copy of the cast
>> >>graph.  I think we can do better.
>> >
>> > Ok, do you have any ideas? Would it be impossible to just share
>> > subgraph's?  Meaning having edges that point to vertices in another
>> > local cast-graph.
>>
>>I honestly have no clue.  How would you know which other cast graph
>>to link up to?
>
> What's wrong with explicit importing? It's the duplicated storage that's
> bad, no?

Yes, I suppose.  Of course polymorphic classes have edges in both
directions, so importing converters and defining new conversions might
have an effect on the imported graph.  Then there's a cache which
records reachability and casting shortcuts, and that couldn't be
shared because different information might be visible on either side
of the extension boundary.

Of course, I haven't proven that the cache is really important to
speed -- it's another premature optimization -- but I'm guessing
you'll want it.  Certainly some people had problems with the speed of
stepwise up/downcasting in v1.

> How about revisiting hierarchical implicit importing? Didn't we only
> abandon that because we thought local registries wouldn't work
> on unix platforms?

Argh.  I don't remember.  However, it seems like it could only
complicate the cast graph issue.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com



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