When I have a TList (so, a list of "reference to procedure"), and I Clear it, do all the captured variables used in the anonymous methods get freed, so no leaking occurs?
Ie. is reference counting in effect upon clearing the TList?
Delegate types are reference counted like interfaces (in fact they are implemented as interfaces). That means if they run out of scope the object behind the scenes (you might have seen that ArcRec$xxxx thing mentioned somewhere - that is the class name the compiler generates) gets destroyed. Captured variables are implemented as fields inside that class so they also run out of scope and are getting freed.
However you might pay attention to some circular referencing which might cause a memory leak with captured variables because of some important fact:
If you have multiple anonymous methods inside a single routine/method they all are implemented by one single class (that ArcRec$xxxx thing). So in this case the anonymous method with the longest lifetime might keep another one alive even if that already is out of scope.
Related
It is hard to find a good heading for this, but i think my problem comes clear if i post a small code snipped:
SomeObject instance = SomeObject(importantParameter);
// -> "instance" is now a reference to the instance somewhere in RAM
instance = SomeObject(anotherImportantParameter);
// -> "instance" is now a reference to a different instance somewhere in RAM
My question is now, is the used RAM that was allocated at the first construction reused at the second construction? Or is the RAM of the first instance marked as unused for the garbage collector and the second construction is done with a completely new instance with a different portion of RAM?
If the first is true, what with this:
while(true) {
final SomeObject instance = SomeObject(importantParameter);
}
Will then, each time the while is repeated, the RAM be reused?
It's unspecified. The answer is a resounding "maybe".
The language specification never says what happens to unreachable objects, since it's unobservable to the program. (That's what being unreachable means).
In practice, the native Dart implementation uses a generational garbage collector.
The default behavior would be to allocate a new object in "new-space" and overwrite the reference to the previous object. That makes the previous object unreachable (as long as you haven't store other references to it), and it can therefore be garbage collected. If you really go through objects quickly, that will be cheap, since the unreachable object is completely ignored on the next new-space garbage collection.
Allocating a lot of short-lived objects still has an overhead since it causes new-space GC to happen more often, even if the individual objects don't themselves cost anything.
There is also a number of optimization that may change this behavior.
If your object is sufficiently simple and the compiler can see that no reference to it ever escapes, or is used in an identical check, or ... any other number of relevant restrictions, then it might "allocation sink" the object. That means it never actually allocates the object, it just stores the contents somewhere, perhaps even on the stack, and it also inlines the methods so they refer to the data directly instead of going through a this pointer.
In that case, your code may actually reuse the memory of the previous object, because the compiler recognizes that it can.
Do not try to predict whether an optimization like this happens. The requirements can change at any time. Just write code that is correct and not unnecessarily complex, then the compiler will do its best to optimize in all the ways that it can.
According to Cocoa's "Create Rule" a method that returns a newly instantiated object must begin with either new, create, or copy if ownership is transferred to the caller.
Let's say I have a class that implements a factory method called (NSObject *)getCorrectObject.
This method will return a new instance of the correct subclass of the callee (determined by some internal state of the callee). Technically this method does not follow the "Create Rule" and could lead to memory leaks in non-ARC environments.
Would it be possible to instead use (NSObject *__autoreleasing)getCorrectObject to avoid using new or create in this case?
In non-ARC I would return an autoreleased object, but I'm not entirely sure if __autoreleasing works for anything other than In/Out parameters.
According to Cocoa's "Create Rule" a method that returns a newly instantiated object must begin with either new, create, or copy if ownership is transferred to the caller.
This isn't called the Create Rule (and isn't correct). The Create Rule is a Core Foundation rule related to the words Create and Copy. Cocoa has a different rule related to “alloc”, “new”, “copy”, or “mutableCopy”.
Let's say I have a class that implements a factory method called (NSObject *)getCorrectObject.
Then it would be incorrectly named. Starting a method with get indicates that it returns a value by reference. The correct signature would be:
+ (BOOL)getCorrectObject(NSObject**)result;
This method will return a new instance of the correct subclass of the callee (determined by some internal state of the callee). Technically this method does not follow the "Create Rule" and could lead to memory leaks in non-ARC environments.
That is not based on whether it is a new instance. It's based on whether it includes an unbalanced retain.
Methods that begin with “alloc”, “new”, “copy”, or “mutableCopy” should return an object with one unbalanced retain. Other methods should return an object that has an equal number of retain and autorelease attached to it.
If for some reason you have to violate these naming rules (and you really don't want to violate these rules), the correct way to indicate it to ARC is by using NS_RETURNS_RETAINED (see Foundation/NSObjCRuntime.h for all the macros). This only assists ARC in correctly fixing your memory management when dealing with mixed ARC/Retain code in cases where it is impossible to fix your incorrect naming. Pure ARC code doesn't need this; it'll balance out either way. Pure Retain code won't be helped by this. The caller has to do the right thing (and so has to just know that you did it wrong and compensate).
Note that as long as you're using ARC throughout the project (and you really, really should be), most of this won't matter a lot in practice. ARC will balance everything out for you pretty much no matter what you name things, since you're not in charge of adding the retains, releases, and autoreleases anyway.
I have a collection which maintains weak references to its objects. I'd like it to conform to NSFastEnumeration, but the buffer provided by countByEnumeratingWithState:objects:count: uses unsafe_unretained references. That creates a gap during which a returned reference could become invalid but not zeroed.
That's fine in the general case -- if the collection stuffs its (currently valid but weakly-referenced) object into the buffer and returns it, then the caller will presumably create its own strong reference if needed. But that leaves two problems:
(1) I don't see any guarantee that the for(){} iteration construct itself creates a temporary strong reference to the object, so if the contents of the {x} block changes something outside the collection in a way that causes the object to be released, then it'll have a dangling reference.
(2) There's still a small gap while returning from countByEnumeratingWithState: during which activity on another thread could invalidate the reference. My collection isn't meant to be thread-safe, but it would be nice if it could at least safely store references to objects which could be referenced on another thread, as there's really no way to prevent that in any multi-threaded application.
You can't return a strong reference directly to the caller. The caller won't release it, and the fast enumeration protocol does not guarantee that you will get a chance to release it yourself when the caller is done.
Instead you can retain+autorelease the objects before you store them into the buffer. That would guarantee the objects stay alive while the caller uses them. It may hurt the "fast" part of fast enumeration, but you would still get the "convenient syntax" part. If you add a nil check after you read the weak variable then you can avoid storing nil pointers into the buffer.
I haven't found a simple answer for these two questions:
do I have to remove a listener before deleting the property instance (the listener is not used anywhere else)?
BooleanProperty bool = new SimpleBooleanProperty();
bool.addListener(myListener);
bool.removeListener(myListener); // is it necessary to do this?
bool = null;
do I have to unbind a uni-directional bounded property before deleting the property instance?
BooleanProperty bool = new SimpleBooleanProperty();
bool.bind(otherBool);
bool.unbind(); // is it necessary to do this?
bool = null;
Case 1
Given that myListener "is not used anywhere else" and therefore I assume, a [method-] local variable, the answer is no. In the general case though, the answer is mostly a no but can sometimes be a yes.
As long as myListener is strongly reachable, then it will never become eligible for finalization and it will continue to consume memory. For example, this would be the case if myListener is a "normally" declared static variable (*all "normal" references in Java are strong references*). However, if myListener is a local variable, then the object will not be reachable anymore after the return of the current method call and bool.removeListener(myListener) is a bit meaningless over-engineering. Both the observer and the Observable goes out of scope and will eventually be finalized. A quote from my own blog post about this answer might paint a better picture:
It doesn’t matter if the box know about the cat inside of it, if you
throw the box into the ocean. If the box isn't reachable, nor is the
cat.
Theory
To fully understand the situation here, we have to remind ourselves of the life-cycle of a Java object (source):
An object is strongly reachable if it can be reached by some thread
without traversing any reference objects. A newly-created object is
strongly reachable by the thread that created it. [..] An object is
weakly reachable if it is [not] strongly [..] reachable but can be
reached by traversing a weak reference. When the weak references to a
weakly-reachable object are cleared, the object becomes eligible for
finalization.
In the case of static variables, these will always be accessible as long as the class is loaded, thus reachable. If we didn't want a static reference to be the one that hinder the garbage collector to do his job, then we could declare the variable to use a WeakReference instead. JavaDoc says:
Weak reference objects [..] do not prevent their referents from being
made finalizable, finalized, and then reclaimed. [..] Suppose that the
garbage collector determines at a certain point in time that an object
is weakly reachable. At that time it will atomically clear all weak
references to that object [..]. At the same time it will declare all
of the formerly weakly-reachable objects to be finalizable.
Explicit management
For illustration, let's assume that we write a JavaFX space simulation game. Whenever an Observable planet moves into the view of a spaceship observer, the game engine register the spaceship with the planet. It is quite apparent that whenever the planet goes out of view, the game engine should also remove the spaceship as an observer of the planet by using Observable.removeListener(). Otherwise, as the spaceship continues to fly through space, memory will leak. Eventually, the game cannot handle five billion observed planets and it will crash with an OutOfMemoryError.
Do note that for the vast majority of JavaFX listeners and event handlers, their life-cycle is parallel to that of their Observable so the application developer has nothing to worry about. For example, we might construct a TextField and register with the text field's textProperty a listener that validate user input. As long as the text field sticks around, we want the listener to stick around. Sooner or later, the text field is not used anymore and when he is garbage collected, the validation listener is also garbage collected.
Automatic management
To continue on the space simulation example, assume that our game has limited multiplayer support and all the players need to observe each other. Perhaps each player keep a local score board of kill metrics or perhaps they need to observe broadcasted chat messages. The reason is not the important point here. What would happen when a player quit the game? Clearly, if the listeners are not explicitly managed (removed), then the player who quit will not become eligible for finalization. The other player's will keep a strong reference to the offline player. Explicit removal of the listeners would still be a valid option and probably the most preferred choice for our game, but let's say that it feels a bit obtrusive and we want to find a more slick solution.
We know that the game engine keep strong references to all players online, for as long as they are online. So we want the spaceships to listen for changes or events of each other only for as long as the game engine keep the strong references. If you read the "theory" section, then surely a WeakReference sounds like a solution.
However, just wrapping something in a WeakReference is not the entire solution. It seldom is. It is true that when the last strong reference to the "referent" is set to null or otherwise become unreachable, the referent will be eligible for garbage collection (assuming that the referent cannot be reached using a SoftReference). But the WeakReference is still hanging around. The application developer need to add some plumbing so that the WeakReference itself is removed from the data structure he was put in. If not, then we might have reduced the severity of the memory leak but a memory leak will still be present because dynamically added weak references consume memory too.
Lucky for us, JavaFX added interface WeakListener and class WeakEventHandler as a mechanism for "automatic removal". The constructors of all related classes accept the real listener/handler as provided by client code, but they store the listener/handler using a weak reference.
If you look at the JavaDoc of WeakEventHandler, you'll notice that the class implement EventHandler, so the WeakEventHandler can be used wherever an EventHandler is expected. Likewise, a known implementation of a WeakListener can be used wherever an InvalidationListener or a ChangeListener is expected.
If you look into the source code of WeakEventHandler, you'll notice that the class is basically only a wrapper. When his referent (the real event handler) is garbage collected, the WeakEventHandler "stop working" by not doing anything at all when WeakEventHandler.handle() is called. The WeakEventHandler doesn't know about which object he has been hooked up with, and even if he did, the removal of an event handler is not homogeneous. All known implementing classes of WeakListener has a competitive advantage though. When their callbacks are invoked, they are implicitly or explicitly provided a reference to the Observable they are registered with. So when the referent of a WeakListener is garbage collected, eventually the WeakListener implementation will make sure that the WeakListener itself is removed from the Observable.
If it is isn't already clear, the solution for our space simulation game would be to let the game engine use strong references to all online spaceships. When a spaceship goes online, all other online spaceships are registered with the new player using a weak listener such as WeakInvalidationListener. When a player goes offline, the game engine remove his strong reference to the player and the player will become eligible for garbage collection. The game engine doesn't have to bother about explicit removal of the offline player as a listener of the other players.
Case 2
No. To better understand what I'll say next, please read my case 1 answer first.
BooleanPropertyBase store a strong reference to otherBool. This in itself does not cause otherBool to always be reachable and thus potentially cause a memory leak. When bool becomes unreachable, then so do all its stored references (assuming they are not stored anywhere else).
BooleanPropertyBase also works by adding itself as an Observer of the property you bind it to. However, it does so by wrapping itself in a class that works almost exactly like the WeakListeners described in my case 1 answer. So once you nullify bool, it will be only a matter of time before it is removed from otherBool.
I completely agree with the case 1 answer, but the case 2 is a bit more tricky. The bool.unbind() call is necessary. If ommitted, it does cause a small memory leak.
If you run the following loop, the application will eventually run out of memory.
BooleanProperty p1 = new SimpleBooleanProperty();
while(true) {
BooleanProperty p2 = new SimpleBooleanProperty();
p2.bind(p1)
}
The BooleanPropertyBase, intenally, does not use a real WeakListener (an implementation of the WeakListener interface), it is using a half-baked solution. All the "p2" instances get eventually garbage-collected, but a listener holding an empty WeakReference remains in the memory forever for each "p2". The same holds for all properties, not only BooleanPropertyBase. It's explained here in detail, and they say it is fixed in Java 9.
In most cases, you do not notice this memory leak, because it leaves only a few dozen bytes for every binding that has not been unbound. But in some cases it caused me real trouble. An good example are table cells of a table that gets frequently updated. The cells then re-bind to different properties all the time, and these left-overs in the memory accumulate quickly.
~ Will ARC always release an object the line after the last strong pointer is removed? Or is it undetermined and at some unspecified point in the future it will be released? Similarly, assuming that you don't change anything with your program, will ARC always be the same each time you run and compile your program?
~ How do you deal with handing an object off to other classes? For example, suppose we are creating a Cake object in a Bakery class. This process would probably take a long time and involve many different methods, so it may be reasonable for us to put the cake in a strong property. Now suppose we want to hand this cake object off to a customer. The customer would also probably want to have a strong pointer to it. Is this ok? Having two classes with strong pointers to the same object? Or should we nil out the Bakery's pointer as soon as we hand off?
Your code should be structured so the answer to this doesn't matter - if you want to use an object, keep a pointer to it, don't rely on ARC side effects to keep it around :) And these side effects might change with different compilers.
Two strong pointers is absolutely fine. ARC will only release the object when both pointers are pointing to something else (or nothing!)
ARC will implement the proper retains and releases at compile time. It will not behave any different than if you put them in there yourself so it will always do the same compilation and to answer your question should always behave the same. But that said it does not mean that your object will always be released immediately after the pointer is removed. Because you never call dealloc directly in any form of objective C you are only telling it that there is no reference count and that it is safe to release. This usually means that it will be released right away though.
If you pass an object from one class to another and the receiving class has a strong property associated with it and the class that passes it off eventually nils its pointer it will still have a reference count of at least 1 and will be fine.
Ok, first this answer might helpt you also a little bit: ARC equivalent of autorelease?
Generally after the last strong variable is nilled, the object is released immediately. If you store it in a property, you can nil the property, assign it to something like __strong Foo *temp = self.bar; before you nil, and return that local __strong variable (although arc normally detects the return, and inferes the __strong byitself).
Some more details on that: Handling Pointer-to-Pointer Ownership Issues in ARC
DeanWombourne's answer is correct; but to add to (1).
In particular, the compiler may significantly re-order statements as a part of optimization. While method calls will always occur in the order written in code (because any method call may have side effects), any atomic expression may be re-ordered by the compiler as long as that re-order doesn't impact behavior. Same thing goes for local variable re-use, etc...
Thus, the ARC compiler will guarantee that a pointer is valid for as long as it is needed, no more. But there is no guarantee when the pointed to object might be released other than that it isn't going to happen beyond the scope of declaration. There is also no guarantee that object A is released before B simply because A is declared and last used before B.
IN other words, as long as you write your code without relying on side effects and race conditions, it should all just work.
Please keep you code proper as it has diffrent behaviour on diffrent complier.