“原子”和非原子之间有什么区别？ [重复]

Possible Duplicate:
Atomic vs nonatomic properties

可能重复:原子与非原子属性

I just want to know what is the differneve between theses two lines of code :

我只是想知道这两行代码之间的区别是什么:

@property(nonatomic, retain) NSString *str;

and

@property(atomic, retain) NSString *str;

Thanx, Regards, tek3

Thanx,问候,tek3

2 个解决方案

#1

Atomic properties are necessary in a reference counted multi threaded environment in order to stop objects from disappearing before a thread has a chance to retain them.

在引用计数的多线程环境中,必须使用原子属性,以便在线程有机会保留对象之前阻止对象消失。

Consider the naive implementation of a get accessor:

考虑一下get访问器的天真实现:

@interface MyObject : NSObject 
{
    id myPropertyIVar;
}
-(id) myProperty;

@end

@implementation MyObject

-(id) myProperty
{
    return myPropertyIvar;
}

// other stuff

@end

This is all fine except that if you release the instance of MyObject before retaining the returned value from -myProperty the returned value may well be deallocated. For this reason, it is safer to implement -myProperty like this:

这很好,除非你在保留-myProperty的返回值之前释放MyObject的实例,否则很可能会释放返回的值。因此,像这样实现-myProperty更安全:

-(id) myProperty
{
    return [[myPropertyIvar retain] autorelease];
}

This is now completely safe in a single threaded environment.

现在,这在单线程环境中是完全安全的。

Unfortunately, in a multithreaded environment there is a race condition. If the thread is interrupted at any time before the retain has incremented the retain count, either of the following will cause you to receive a garbage pointer:

不幸的是,在多线程环境中存在竞争条件。如果在保留增加保留计数之前的任何时间线程被中断,则以下任何一种情况都会导致您收到垃圾指针:

the instance of MyObject is released and deallocated by another thread causing the ivar to be released and deallocated

MyObject的实例被另一个线程释放并释放,导致ivar被释放和释放

myProperty is reassigned by another thread causing the old version to be released and deallocated

myProperty由另一个线程重新分配,导致旧版本被释放和释放

For this reason, all accesses to the property must be protected by a lock. The get accessor looks something like this.

因此,必须通过锁保护对属性的所有访问。 get访问器看起来像这样。

-(id) myProperty
{
    // lock
    return [[myPropertyIvar retain] autorelease];
    // unlock
}

The set accessor is similarly protected and so is the release in -dealloc

set访问器受到类似的保护,-dealloc中的版本也是如此

#2

The Apple docs explain all this very well. To learn about properties, including their atomicity, read this page.

Apple文档很好地解释了这一切。要了解属性,包括其原子性,请阅读此页面。

#1