Comment by pjmlp
13 years ago
> The caching misbehavior of referencing counting has been greatly exaggerated, especially in the context of UI where responsiveness is much more important than raw CPU speed. Also, the ref counting tradeoff seems to work better for device (e.g. all the cool kids [1] are doing it).
Apple's ARC is a compiler hack. It only works if you happen to use set of Cocoa libraries that are recognized by the compiler and allow for some magic incantation.
You cannot make any Objective-C library ARC aware.
ARC is a good solution for Objective-C, because Apple never managed to have their GC working safely in all situations, given the underlying C type system.
> .NET still does GC, it is only the WinRT APIs (something like COM) that manage resources through ref counting. There is some cool interop magic that makes this somewhat transparent to the programmer.
True, but C++ does not and you still have the performance penalty of reference counting.
The only way to have reference counting with good performance is do what Parasol or Objective-C do, by removing unnecessary increment/decrement operations thanks to dataflow analysis.
Which is something that you cannot do just by using the WinRT runtime and need to rely on the cleverness of the compilers.
Actually, are you aware that reference counting tends to be presented on the first chapter of many CS books about garbage collection as poor man's GC?
> Apple's ARC is a compiler hack. It only works if you happen to use set of Cocoa libraries that are recognized by the compiler
I don't understand. How does ARC rely on more than what NSObject provides? I'm coming from the C++ shared_ptr world and ARC does not feel much different so far (less explicit, but easier to optimise).
No, it is an hack.
Just read this document.
https://developer.apple.com/library/mac/#releasenotes/Object...
You need to write code with lots of special cases to be able to have ARC active and not fall into strange compiler errors.
The compiler errors are actually quite clear.
Most of the restrictions (bridge casts and naming methods) are interoperation with non-ARC code, and from working on Java/JNI I can assure you GC<>manual interaction was worse.
It is of course still quite possible to get abandoned memory through retain cycles.
What part of that is supposed to explain your rather bizarre claim that ARC only works with special Apple libraries?
2 replies →
It doesn't even rely on NSObject. Any root class that provides a compliant implementation of retain, release, and autorelease will work with ARC.
> True, but C++ does not and you still have the performance penalty of reference counting.
Not very relevant. For UI code, responsive resource allocation and de-allocation is much more important than saving a few % CPU cycles.
> Actually, are you aware that reference counting tends to be presented on the first chapter of many CS books about garbage collection as poor man's GC?
Yes! Sometimes the conventional wisdom doesn't hold in the end, or it doesn't hold for all use cases. But I should have been more clear that ref counting is GC, whereas what I refer to as GC involves sweeping reference roots periodically to identify garbage.
> > True, but C++ does not and you still have the performance penalty of reference counting.
> Not very relevant. For UI code, responsive resource allocation and de-allocation is much more important than saving a few % CPU cycles.
I beg to differ. I consider missile control radar systems of military applications a bit more important than UIs.
There are military applications using real time GC systems, which I can list here if you want.
I also have a German magazine here (Making Games Magazin 1/13) that explains how Witcher2 for the Xbox 360 makes use of a GC.
Anyway one thing I do agree, automatic memory management, regardless of the form, should be the any of any programming language.
Pure manual memory management should be treated like Assembly. It will never go away and should only be used for the very few cases where there is no other way around the problem at hand.
I'm not anti-GC, I just think that reference counting is appropriate for UI code. .NET still supports GC, and the collector is very good compared to our competitors; I'm not saying we should give that up!
I should have mentioned that the primary reason WinRT seems to use reference counting is for interop reasons: its much easier to great a UI toolkit that is programmable in both C# and C++. Note that WPF never had a C++ interface, while WinRT has one without the need for any bridge code.
Reference counting has some interesting applications where it is more appropriate trace-based GC. Heck, sometimes we want old-fashioned arenas (Pascal!), it would be nice if our languages could give us some choices.
Manual memory management is annoying but sometimes necessary. I can see why native programmers don't want to give it up as it constitutes the primary performance difference these days between native and managed languages.