Faster Virtual Method Dispatch

[Sean Kelly]

Craig Black wrote:
> "Mike Capp" <mike.capp at gmail.com> wrote in message 
> news:e2l4g1$2n5s$1 at digitaldaemon.com...
>> In article <e2k12i$19bi$1 at digitaldaemon.com>, Walter Bright says...
>>> 1) for every allocation, shared_ptr<> allocates an *extra* block of
>>> memory to hold its accounting information. So you've got, out of the
>>> box, twice as many allocations/deallocations happening.
>> Not true for intrusive refcounting. (This isn't always feasible if you're
>> dealing with third-party classes, which is why shared_ptr isn't intrusive, 
>> but
>> if you _can_ use it it's very close to ideal.)
> 
> What's intrusive refcounting?  Please use small words. My brain is almost 
> full right now. :)

Maintaining the reference count within the class being pointed to 
instead of within the shared pointer code.

>>> 2) for every copy of the shared_ptr<>, like passing one to a function,
>>> returning one, etc., the reference count has to be incremented, then
>>> decremented.
>> Again, not true for intrusive refcounting. You can assign an intrusive
>> smartpointer from a raw pointer or reference, so most of the time you can 
>> use
>> those to pass/return. Refcount twiddling is only needed when changing an
>> "owning" pointer, plus the rare "safety" case where you have a raw pointer 
>> but
>> are calling code which might drop the last "real" owning ref.
>>
>>> And even with all that falderol, shared_ptr<> still can't do slicing,
>>> which are an important efficiency improvement.
>> Agreed, but couldn't you do something very similar with a simple ptr class
>> containing a pointer to the (refcounted) base string plus a couple of 
>> range end
>> indices? That's worked well for me in the past, but there may be benefits 
>> of D
>> slices that I'm missing.
> 
> If you think this is doable, then you should write a C++ version of 
> wordcount based on this principal.  If you can get comparable performance to 
> the D version then you may be on to something.

I'd meant to do this and never got around to it.  However, the wordcount 
example is meant to compare performance using typical programming 
techniques for each language, and writing your own specialized 
containers isn't typical :-)


Sean