On 21/11/2010 4:17 PM, Mike Capp wrote:
On 21 November 2010 20:48, Graydon Hoare<[email protected]> wrote:
It's not clear to me yet that the boxing and vtbl'ing costs on objs will
really hurt, but we're not venturing into objc territory on dispatch. We're
doing more like "C++ with the pimpl idiom".
And presumably your objs aren't so mutable that an optimizer couldn't
cache the result of a vtbl lookup if it was going to be called in a
tight loop?
There's no "lookup" to cache; vtbl dispatch is a register+immediate
indirect branch. For a dynamic callee, you can't cache it any more than
that. The only way in which C++ is "faster" is by statically binding
most of its methods (it's not virtual-by-default; you can't *override*
most C++ methods).
Rust does something that's a little more conventional in the world of
languages-outside-of-C++: object methods are all virtual and object
bodies are all heap allocated. But dispatch is still just an indirect
branch through a static vtbl. This removes perhaps 90% of the complexity
and fragility of the C++ object model while retaining a very good -- not
perfect, but good -- dispatch speed.
To get static binding in rust, you have to use a static function call,
not an object-method call.
-Graydon
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