NSAutoreleasePool EXC_BAD_ACCESS和有关Objective-C中的内存管理的问题

Question

I have difficulties understanding how NSAutoReleasePool works.

1) Does NSAutoReleasePool keep track of each allocation separately or does it work variable dependent? In other words, does this leak memory or release both?:

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];  
 NSArray* myObj = [NSObject alloc];  
 myObj = [NSObject alloc];  
 [pool release];

2) Why does the following code work:

 NSAutoreleasePool *pool1 = [[NSAutoreleasePool alloc] init];

 NSArray* myObj = [NSObject alloc];

 for(int i = 0; i < 100; i++) {
  [myObj release];
  myObj = [NSObject alloc];
 }

 [pool1 release];

but the following gives EXC_BAD_ACCESS at [pool1 release] :

 NSAutoreleasePool *pool1 = [[NSAutoreleasePool alloc] init];

 NSArray* myObj = [NSObject alloc];

 NSAutoreleasePool *pool2 = [[NSAutoreleasePool alloc] init]; 

 for(int i = 0; i < 100; i++) {
  [myObj release];
  myObj = [NSObject alloc];
 }

 [pool2 release];
 [pool1 release];

3) How can I make the code above work(the one with the 2 pools)?

Answer 1

Autorelease pools only works for objects that you autorelease. None of the objects in your code are autoreleased and so none are affected by the pools that you created.

Therefore, the answers are:

1. It leaks both objects (since neither are either released or autoreleased)
2. It doesn't. It leaks the last object allocated
3. The documentation says that you can nest them . If I assume that where you have [NSObject alloc] you really mean [[[NSObject alloc] init] autorelease] then your second code sample will fail because the very first time you release myObj it will be for an object declared outside pool2 . This means that when you release pool1 it tries to release an object that has already been released. The way to get it working is to not release (or autorelease) an object more times than it's retained or allocated.

Answer 2

NSAutoreleasePool does no tracking of allocations. When you send the message autorelease to an object, it searches for an autorelease pool on its thread (one is created for you automatically at the top of each trip through the run loop, and deallocated at the bottom; in most other cases, you need to manually create them). If it finds one, then it is added to a list of objects that the autorelease pool maintains.

When an autorelease pool is deallocated, it sends the release message to each item in its list. If an item has been added to the list multiple times, it is released multiple times.

As to why your code gets an EXC_BAD_ACCESS, I am unsure. It may have something to do with the fact that you have not called init on any of your NSObjects, or it may have to do with code that you have not yet shown us.

Answer 3

A few things:

:When creating objects always make use of an init method. (ie, [[NSObject alloc] init])

:Objects get added to an AutoreleasePool when they are autoreleased. The objects in this example are simply being released and so will not use the autorelease pools at all. You can see this by removing all the autorelease pools; the code still functions fine.

:The first example will leak memory. You are allocating two objects and never releasing either. If you were to autorelease them, then you would probably be getting the behavior you are looking for and there would not be a leak.

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSArray* myObj = [[[NSObject alloc] init] autorelease];
myObj = [[[NSObject alloc] init] autorelease];
[pool release];

:Example 2 works, because you are just allocating, than releasing objects; none of which are making use of the autorelease pool. You can remove the autorelease code to see this.

:I have been unable to replicate the behavior you are seeing on Example 3. Perhaps there is something else going on inside your code.