Re:For Better Computing, Liberate CPUs from Garbage Collection
by Shikadi on Thursday May 09, 2019 @03:16PM
(#19871291)
RAII has nothing to do with reference counting, it stands for resource acquisition is initialization. In C++ that means the lifetime of a resource is tied to the creation and destruction of an object. Often when people talk about RAII and C++ they are also referring to destructors being called when an object goes out of scope, so if an object that contains an object containing some allocated resource goes out of scope, the resource is released as the destructor path is called.
If an object with a smart pointer goes out of scope, then of course there's the overhead of the smart pointer, but the previous comment wasn't referring to that situation
Re:For Better Computing, Liberate CPUs from Garbage Collection
by kazinator on Thursday May 09, 2019 @03:47PM
(#19871626)
RAII is commonly used for representing smart pointers that manage references to objects that themselves have otherwise pervasive lifetimes.
RAII not managing any shared resources is trivial and uninteresting. The lifecycle of an object that is confined to a single lexical scope can be handled manually and verified by visual inspection (though of course it is more reliable to automate that with RAII or macros, not to mention less verbose).
Because you do not have an exclusive reference to all of them; we could be freeing something that is still in use somewhere.
So why not reference-count? Because reference counting is slow, and must be meticulously maintained (here, languages help with constructs like smart pointers), and doesn't handle cycles in the object structure.
> I find RAII a lot easier to model and think about than "you drop this reference and one day maybe your object is destroyed, but don't really rely on that".
The RAII model doesn't do away with this "maybe" in any shape or form. We drop this reference, and the smart pointer decrements the refcount; maybe it has hit zero so the object is destroyed, maybe not. Maybe the zero refcount triggers a whole cascade of thousands of other objects hitting their zero refcount, which takes many cycles to execute, or maybe it doesn't.