on Thu Apr 26 2012, "Stephan T. Lavavej" <stl-AT-exchange.microsoft.com> wrote:
[STL]
Each is optimally sized (in particular, make_shared/allocate_shared implement the "we know where you live" optimization that I have previously described
[Dave Abrahams]
I can't find that description. Pointer please?
See http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/STL11-Magic-Sec... (which also has links to my slides - viewable online even without PowerPoint), in particular Slide 6.
9:30 or so in the video.
I described the optimization in detail, but without code, so you can pick it up without reading my sources. Basically, the "traditional" control blocks need to keep a pointer to the object so they can delete it (because of conversions, the shared_ptr's own pointer cannot be used for this purpose - it could have the wrong address with no way to restore it at runtime). But if you're willing to write dedicated control blocks for make_shared/allocate_shared (instead of trying to stuff the object in a special "deleter"), then they can just destroy the object in place. "We know where you live", so we don't need to store a pointer to the object.
I was surprised when I learned that I was apparently the first one to implement this - I just assumed that everyone would write it this way.
Me too :-)
See slide 7 for measurements - picking up this optimization should save you 8 bytes on x86 and 16 bytes (!!!) on x64. That's per object, so if you have a lot of them, it adds up.
Yep.
Consider it a gift - my thanks for all of the wonderful things that Boost has given TR1/C++11.
Thanks kindly.
You don't need a special case for emptiness if you make them base classes when they're classes (a.k.a. use boost::compressed_pair)
I should look into implementing that from scratch in VC12. Sometimes I dream about making the STL dependent on Boost, and causing the universe to recursively implode. It would be fun!
Yeah, who needs an LHC to destroy the universe as we know it? -- Dave Abrahams BoostPro Computing http://www.boostpro.com