I'm studying ARC. And now about zeroing weak pointer.
OK I understood all the features. The semantic of weak reference is just same with weak reference of GC system, but you know, Objective-C doesn't use GC (except special case) so I can't understand how this works.
I'm a little complicated guy, so I need to know underlying implementation principal to accept the feature to use. But the problem is it's very hard to find document which describes the principal of zeroing-weak pointer :(
IMO, the only way to make this work is tracking and keeping all pointers referencing itself at runtime, and setting them all nil when its referencing count becomes 0. But this looks too heavy and stupid. I believe a lot better solution used in actual ARC implementation.
Can you help me to find the documentation? Or direct description would be more great!
It's explained here:
http://mikeash.com/pyblog/friday-qa-2010-07-16-zeroing-weak-references-in-objective-c.html
Spoiler:
It works pretty much how you'd think. Every class maintains a set of addresses of weak pointers that are pointing to it, and when its dealloc is called it sets them all to zero.
It may seem dumb, but it's fast because there's no "tracking" involved, ARC just inserts the code to add a pointer to that set every time the object is assigned to a new weak reference.
It's actually much more efficient than the trickery involved in Garbage collection, which basically involves wading through the heap on a background thread looking for pointers and keeping stock of what they're pointing to.
Implemented by a global hash table in runtime.
Apple source: https://opensource.apple.com/source/objc4/objc4-647/runtime/objc-weak.mm
Related
Today I read book about ARC. So there are two type points both strong and weak points.
I already searched the property about them and got it.
But I couldn't see or understand why we use weak point instead of strong?
This is simple question. Please let me know easily.
Thanks.
First of all its not weak points, its weak property. Lets say if you don't want owner ship of a particular object you can use weak property. If the actual owner of this reference release this and its retain count becomes zero, a weak reference will be automatically assigned to nil. Which will save you from crashes.
You can get more information here : https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/EncapsulatingData/EncapsulatingData.html
This question has been answered very well on apple's page !
I will just link it in hope it really helped you to get the required info !
link is here :) arc
also I think this SO question is very helpful for understanding the things :[link]here (Objective-C ARC: strong vs retain and weak vs assign)
To be simple, you use "weak property" and or "weak variable" for following:
You do not want to take the ownership of object. Like delegate/dataSource are generally declared weak.
You may say you can also use "assign" instead of "weak". When the variable is freed from memory, using "weak" automatically sets it to nil, whereas "assign" now is referring to deallocated instance and code can crash if you try to do something on it.
To avoid retain cycles causing memory leaks. For example avoid passing "self" in blocks. use "__weak id weakSelf = self", and now pass weakSelf in blocks.
ARC is your friend, but can do potential harm if not taken care of things mentioned above.
OK - newbie Objective-C question:
When declaring properties there are attributes such as below:
#property (weak, nonatomic)
I realize I need read up on this to fully understand it but most of what I found was reference material, so a link to a good article that could explain best practices / usage scenarios (when to use which attribute for primitives, reference types, outlets, etc) or a couple of examples would be appreciated.
Thanks!
Even though it is waaaay to late for an answer i've found this question while googleing the same issue and also found this article by apple which explains the whole thing perfectly.
Hope it helps to anyone who are researching the same thing,
http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/ProgrammingWithObjectiveC/EncapsulatingData/EncapsulatingData.html
From a recent class I gave on this (inspired by Paul Hegarty)
nonatomic - NOT thread safe see link Justin pointed out in the comments above.
strong (or retain) - keep this object allocated until I don’t point to it anymore (set it to nil). Compiler will also throw this out of the heap (deallocate it) if I am not pointed to strongly anymore (I get dealloc’d)
weak - keep this object allocated as long as something still points to it strongly. IBOutlets are usually declared as weak since they will be retained automatically by the view controller.
Primitive types are not allocated on the heap and don't use strong or weak
Atomicity has to do with threading, and is a pretty advanced topic for a newbie. The short answer, however, is that iOS properties are always declared as nonatomic. Here's some more detailed information about it.
The weak/strong keyword has to do with memory management with ARC and preventing what's called a retain cycle. This can also be a bit of a tough concept for a newbie, but the high-level overview is that a retain cycle happens when two objects have strong references to each other and thus neither object will be destroyed by ARC. This is a form of a memory leak, as you may have an object that's no longer in use but is still taking up memory. By declaring a property as weak, it will ensure that it's not automatically destroyed so long as something still has a strong reference to it. For instance, let's say you have an array with a couple of objects in it. Two of the objects have strong references to each other. Then, the array loses its owner and is destroyed. BUT, the two objects in that array that point to each other are NOT destroyed since they have strong references. Thus, you have two objects which you cannot access since the owning array is destroyed, but they're still taking up memory.
Can someone briefly explain to me how ARC works? I know it's different from Garbage Collection, but I was just wondering exactly how it worked.
Also, if ARC does what GC does without hindering performance, then why does Java use GC? Why doesn't it use ARC as well?
Every new developer who comes to Objective-C has to learn the rigid rules of when to retain, release, and autorelease objects. These rules even specify naming conventions that imply the retain count of objects returned from methods. Memory management in Objective-C becomes second nature once you take these rules to heart and apply them consistently, but even the most experienced Cocoa developers slip up from time to time.
With the Clang Static Analyzer, the LLVM developers realized that these rules were reliable enough that they could build a tool to point out memory leaks and overreleases within the paths that your code takes.
Automatic reference counting (ARC) is the next logical step. If the compiler can recognize where you should be retaining and releasing objects, why not have it insert that code for you? Rigid, repetitive tasks are what compilers and their brethren are great at. Humans forget things and make mistakes, but computers are much more consistent.
However, this doesn't completely free you from worrying about memory management on these platforms. I describe the primary issue to watch out for (retain cycles) in my answer here, which may require a little thought on your part to mark weak pointers. However, that's minor when compared to what you're gaining in ARC.
When compared to manual memory management and garbage collection, ARC gives you the best of both worlds by cutting out the need to write retain / release code, yet not having the halting and sawtooth memory profiles seen in a garbage collected environment. About the only advantages garbage collection has over this are its ability to deal with retain cycles and the fact that atomic property assignments are inexpensive (as discussed here). I know I'm replacing all of my existing Mac GC code with ARC implementations.
As to whether this could be extended to other languages, it seems geared around the reference counting system in Objective-C. It might be difficult to apply this to Java or other languages, but I don't know enough about the low-level compiler details to make a definitive statement there. Given that Apple is the one pushing this effort in LLVM, Objective-C will come first unless another party commits significant resources of their own to this.
The unveiling of this shocked developers at WWDC, so people weren't aware that something like this could be done. It may appear on other platforms over time, but for now it's exclusive to LLVM and Objective-C.
ARC is just play old retain/release (MRC) with the compiler figuring out when to call retain/release. It will tend to have higher performance, lower peak memory use, and more predictable performance than a GC system.
On the other hand some types of data structure are not possible with ARC (or MRC), while GC can handle them.
As an example, if you have a class named node, and node has an NSArray of children, and a single reference to its parent that "just works" with GC. With ARC (and manual reference counting as well) you have a problem. Any given node will be referenced from its children and also from its parent.
Like:
A -> [B1, B2, B3]
B1 -> A, B2 -> A, B3 -> A
All is fine while you are using A (say via a local variable).
When you are done with it (and B1/B2/B3), a GC system will eventually decide to look at everything it can find starting from the stack and CPU registers. It will never find A,B1,B2,B3 so it will finalize them and recycle the memory into other objects.
When you use ARC or MRC, and finish with A it have a refcount of 3 (B1, B2, and B3 all reference it), and B1/B2/B3 will all have a reference count of 1 (A's NSArray holds one reference to each). So all of those objects remain live even though nothing can ever use them.
The common solution is to decide one of those references needs to be weak (not contribute to the reference count). That will work for some usage patterns, for example if you reference B1/B2/B3 only via A. However in other patterns it fails. For example if you will sometimes hold onto B1, and expect to climb back up via the parent pointer and find A. With a weak reference if you only hold onto B1, A can (and normally will) evaporate, and take B2, and B3 with it.
Sometimes this isn't an issue, but some very useful and natural ways of working with complex structures of data are very difficult to use with ARC/MRC.
So ARC targets the same sort of problems GC targets. However ARC works on a more limited set of usage patterns then GC, so if you took a GC language (like Java) and grafted something like ARC onto it some programs wouldn't work any more (or at least would generate tons of abandoned memory, and may cause serious swapping issues or run out of memory or swap space).
You can also say ARC puts a bigger priority on performance (or maybe predictability) while GC puts a bigger priority on being a generic solution. As a result GC has less predictable CPU/memory demands, and a lower performance (normally) than ARC, but can handle any usage pattern. ARC will work much better for many many common usage patterns, but for a few (valid!) usage patterns it will fall over and die.
Magic
But more specifically ARC works by doing exactly what you would do with your code (with certain minor differences). ARC is a compile time technology, unlike GC which is runtime and will impact your performance negatively. ARC will track the references to objects for you and synthesize the retain/release/autorelease methods according to the normal rules. Because of this ARC can also release things as soon as they are no longer needed, rather than throwing them into an autorelease pool purely for convention sake.
Some other improvements include zeroing weak references, automatic copying of blocks to the heap, speedups across the board (6x for autorelease pools!).
More detailed discussion about how all this works is found in the LLVM Docs on ARC.
It varies greatly from garbage collection. Have you seen the warnings that tell you that you may be leaking objects on different lines? Those statements even tell you on what line you allocated the object. This has been taken a step further and now can insert retain/release statements at the proper locations, better than most programmers, almost 100% of the time. Occasionally there are some weird instances of retained objects that you need to help it out with.
Very well explained by Apple developer documentation. Read "How ARC Works"
To make sure that instances don’t disappear while they are still needed, ARC tracks how many properties, constants, and variables are currently referring to each class instance. ARC will not deallocate an instance as long as at least one active reference to that instance still exists.
To make sure that instances don’t disappear while they are still needed, ARC tracks how many properties, constants, and variables are currently referring to each class instance. ARC will not deallocate an instance as long as at least one active reference to that instance still exists.
To know Diff. between Garbage collection and ARC: Read this
ARC is a compiler feature that provides automatic memory management of objects.
Instead of you having to remember when to use retain, release, and autorelease, ARC evaluates the lifetime requirements of your objects and automatically inserts appropriate memory management calls for you at compile time. The compiler also generates appropriate dealloc methods for you.
The compiler inserts the necessary retain/release calls at compile time, but those calls are executed at runtime, just like any other code.
The following diagram would give you the better understanding of how ARC works.
Those who're new in iOS development and not having work experience on Objective C.
Please refer the Apple's documentation for Advanced Memory Management Programming Guide for better understanding of memory management.
Or, Why I Didn't Use retainCount On My Summer Vacation
This post is intended to solicit detailed write-ups about the whys and wherefores of that infamous method, retainCount, in order to consolidate the relevant information floating around SO.*
The basics: What are the official reasons to not use retainCount? Is there ever any situation at all when it might be useful? What should be done instead?** Feel free to editorialize.
Historical/explanatory: Why does Apple provide this method in the NSObject protocol if it's not intended to be used? Does Apple's code rely on retainCount for some purpose? If so, why isn't it hidden away somewhere?
For deeper understanding: What are the reasons that an object may have a different retain count than would be assumed from user code? Can you give any examples*** of standard procedures that framework code might use which cause such a difference? Are there any known cases where the retain count is always different than what a new user might expect?
Anything else you think is worth metioning about retainCount?
*
Coders who are new to Objective-C and Cocoa often grapple with, or at least misunderstand, the reference-counting scheme. Tutorial explanations may mention retain counts, which (according to these explanations) go up by one when you call retain, alloc, copy, etc., and down by one when you call release (and at some point in the future when you call autorelease).
A budding Cocoa hacker, Kris, could thus quite easily get the idea that checking an object's retain count would be useful in resolving some memory issues, and, lo and behold, there's a method available on every object called retainCount! Kris calls retainCount on a couple of objects, and this one is too high, and that one's too low, and what the heck is going on?! So Kris makes a post on SO, "What's wrong with my memory management?" and then a swarm of <bold>, <large> letters descend saying "Don't do that! You can't rely on the results.", which is well and good, but our intrepid coder may want a deeper explanation.
I'm hoping that this will turn into an FAQ, a page of good informational essays/lectures from any of our experts who are inclined to write one, that new Cocoa-heads can be pointed to when they wonder about retainCount.
** I don't want to make this too broad, but specific tips from experience or the docs on verifying/debugging retain and release pairings may be appropriate here.
***In dummy code; obviously the general public don't have access to Apple's actual code.
The basics: What are the official reasons to not use retainCount?
Autorelease management is the most obvious -- you have no way to be sure how many of the references represented by the retainCount are in a local or external (on a secondary thread, or in another thread's local pool) autorelease pool.
Also, some people have trouble with leaks, and at a higher level reference counting and how autorelease pools work at fundamental levels. They will write a program without (much) regard to proper reference counting, or without learning ref counting properly. This makes their program very difficult to debug, test, and improve -- it's also a very time consuming rectification.
The reason for discouraging its use (at the client level) is twofold:
The value may vary for so many reasons. Threading alone is reason enough to never trust it.
You still have to implement correct reference counting. retainCount will never save you from imbalanced reference counting.
Is there ever any situation at all when it might be useful?
You could in fact use it in a meaningful way if you wrote your own allocators or reference counting scheme, or if your object lived on one thread and you had access to any and all autorelease pools it could exist in. This also implies you would not share it with any external APIs. The easy way to simulate this is to create a program with one thread, zero autorelease pools, and do your reference counting the 'normal' way. It's unlikely that you'll ever need to solve this problem/write this program for anything other than "academic" reasons.
As a debugging aid: you could use it to verify that the retain count is not unusually high. If you take this approach, be mindful of the implementation variances (some are cited in this post), and don't rely on it. Don't even commit the tests to your SCM repository.
This may be a useful diagnostic in extremely rare circumstances. It can be used to detect:
Over-retaining: An allocation with a positive imbalance in retain count would not show up as a leak if the allocation is reachable by your program.
An object which is referenced by many other objects: One illustration of this problem is a (mutable) shared resource or collection which operates in a multithreaded context - frequent access or changes to this resource/collection can introduce a significant bottleneck in your program's execution.
Autorelease levels: Autoreleasing, autorelease pools, and retain/autorelease cycles all come with a cost. If you need to minimize or reduce memory use and/or growth, you could use this approach to detect excessive cases.
From commentary with Bavarious (below): a high value may also indicate an invalidated allocation (dealloc'd instance). This is completely an implementation detail, and again, not usable in production code. Messaging this allocation would result in a error when zombies are enabled.
What should be done instead?
If you're not responsible for returning the memory at self (that is, you did not write an allocator), leave it alone - it is useless.
You have to learn proper reference counting.
For a better understanding of release and autorelease usage, set up some breakpoints and understand how they are used, in what cases, etc. You'll still have to learn to use reference counting correctly, but this can aid your understanding of why it's useless.
Even simpler: use Instruments to track allocs and ref counts, then analyze the ref counting and callstacks of several objects in an active program.
Historical/explanatory: Why does Apple provide this method in the NSObject protocol if it's not intended to be used? Does Apple's code rely on retainCount for some purpose? If so, why isn't it hidden away somewhere?
We can assume that it is public for two primary reasons:
Reference counting proper in managed environments. It's fine for the allocators to use retainCount -- really. It's a very simple concept. When -[NSObject release] is called, the ref counter (unless overridden) may be called, and the object can be deallocated if retainCount is 0 (after calling dealloc). This is all fine at the allocator level. Allocators and zones are (largely) abstracted so... this makes the result meaningless for ordinary clients. See commentary with bbum (below) for details on why retainCount cannot be equal to 0 at the client level, object deallocation, deallocation sequences, and more.
To make it available to subclassers who want a custom behavior, and because the other reference counting methods are public. It may be handy in a few cases, but it's typically used for the wrong reasons (e.g. immortal singletons). If you need your own reference counting scheme, then this family may be worth overriding.
For deeper understanding: What are the reasons that an object may have a different retain count than would be assumed from user code? Can you give any examples*** of standard procedures that framework code might use which cause such a difference? Are there any known cases where the retain count is always different than what a new user might expect?
Again, a custom reference counting schemes and immortal objects. NSCFString literals fall into the latter category:
NSLog(#"%qu", [#"MyString" retainCount]);
// Logs: 1152921504606846975
Anything else you think is worth mentioning about retainCount?
It's useless as a debugging aid. Learn to use leak and zombie analyses, and use them often -- even after you have a handle on reference counting.
Update: bbum has posted an article entitled retainCount is useless. The article contains a thorough discussion of why -retainCount isn’t useful in the vast majority of cases.
The general rule of thumb is if you're using this method, you better be damn sure you know what you're doing. If you are using it for debugging a memory leak you're doing it wrong, if you're doing it to see what is going on with an object, you're doing it wrong.
There is one case where I have used it, and found it useful. That is in doing a shared object cache where I wanted to flush the object when nothing had a reference to it anymore. In this situation I waited until the retainCount is equal to 1, and then I can release it knowing that nothing else is holding onto it, this will obviously not work properly in garbage collected environments and there are better ways to do it. But this is still the only 'valid' use case I've seen for it, and isn't something a lot of people will be doing.
We all know an object's properties should be released through its dealloc method, but often for objects with many properties this can be pretty cumbersome. It's kind of a headache especially when adding or removing new properties to remember to go back to dealloc and add and remove release calls.
Is there any method of releasing all of an object's properties generically? I wasn't able to find anything while looking through the docs, but could this be done through reflection if it's not already implemented?
I guess another simple option might be to just place all the properties in an array or other container object and always just release the container. Any other options?
I saw one once (and even used it). It involves using the Objective-C Runtime to loop through the properties of a class, check which ones have either a retain or copy flag, and then set them to nil. Then, your -dealloc implementation can be reduced to something like [self cleanupProperties] or something.
The long story short, however, I've stopped using that because of really wacky problems that I can't explain. I don't know for sure that this is what caused it, but it just seems clever enough that it would have some sort of nasty, unforeseen side-effects.
So, in answer to your question: it's definitely possible, but I'd advise you don't. Use garbage collection if possible! :)
Unless you can turn on garbage collection you're pretty much down to two options. As you suggested you could stuff all the property references into a single NSDictionary (which you would release in -dealloc). Otherwise you're stuck with the way it's usually done.
You can see more about garbage collection in Objective-C 2.0 here.