ARC中的@autoreleasepool还有作用吗?

QUESTION

For the most part with ARC (Automatic Reference Counting), we don't need to think about memory management at all with Objective-C objects. It is not permitted to create NSAutoreleasePools anymore, however there is a new syntax:

在ARC中,我们不再需要手动管理内存.ARC已经不允许你创建NSAutoreleasePools了,然而有一种新的语法格式:

@autoreleasepool{…}

My question is, why would I ever need this when I'm not supposed to be manually releasing/autoreleasing ?

我的问题是,为什么@autoreleasepool 可以在ARC中使用而不出错呢?

ANSWER

There seems to be a lot of confusion on this topic (and at least 80 people who probably are now confused about this and think they need to sprinkle @autoreleasepool around their code).

这个主题看起来相当迷惑人(至少80人搞不清楚状况,或许他们会将@autoreleasepool从他们的代码中移除掉)

If a project (including its dependencies) exclusively uses ARC, then @autoreleasepool never needs to be used and will do nothing useful. ARC will handle releasing objects at the correct time. For example:

如果一个工程完全由ARC编译(包括它依赖的工程项目),那么@autoreleasepool 完全没有用处,ARC会自己管理好对象的内存释放问题,例如:

@interfaceTesting:NSObject+(void) test;

@end

@implementation Testing

-(void) dealloc 
{
　　NSLog(@"dealloc");
}

+(void) test
{
　　while(true)
　　　　NSLog(@"p = %p",[Testing new]);
}

@end

displays:

p =0x17696f80
dealloc
p =0x17570a90
dealloc

Each Testing object is deallocated as soon as the value goes out of scope, without waiting for an autorelease pool to be exited. (The same thing happens with the NSNumber example; this just lets us observe the dealloc.) ARC does not use autorelease.

每一个Testing对象,在出了值的作用域之后都会被立马释放掉了,完全不需要有一个 autorelease 存在,ARC中使用autorelease完全没有用处.

The reason @autoreleasepool is still allowed is for mixed ARC and non-ARC projects, which haven't yet completely transitioned to ARC.

你之所以可以继续使用@autoreleasepool 的原因是因为如果你的工程是ARC与MRC混合的,工程中并没有完全的转换成ARC而已.

If you call into non-ARC code, it may return an autoreleased object. In that case, the above loop would leak, since the current autorelease pool will never be exited. That's where you'd want to put an @autoreleasepool around the code block.

如果你在MRC代码中调用了,它会给你返回一个autoreleased 的对象.在那种情形中,上面的循环就会泄露,所以,为了兼容MRC,你需要给你的相关代码添加@autoreleasepool.

But if you've completely made the ARC transition, then forget about autoreleasepool.

如果你的项目完全由ARC编译的,那么请忘掉这个autoreleasepool :).