这些对象什么时候在 ARC 下被释放？

我有几个关于ARC（自动引用计数）的问题：

CFURLRef url = (__bridge CFURLRef)[NSURL fileURLWithPath:appPath];
//Question 1: Here, I would expect the NSURL object to be autoreleased and
//therefore the CFURLRef will also be available for “a while.” Is this correct?

url = NULL;
//Question 2: Will this cause the NSURL to be released immediately?

NSURL *url = [NSURL fileURLWithPath:appPath];
url = nil;
//Question 3: Does the “url = nil” result in an immediate release of the NSURL?

NSURL *url = [[NSURL alloc] initWithString:@"/something"];
url = nil;
//Question 4: What about this?

CFURLRef url = (__bridge CFURLRef)[NSURL fileURLWithPath:appPath];
//Question 1: Here, I would expect the NSURL object to be autoreleased and
//therefore the CFURLRef will also be available for “a while.” Is this correct?

是的，因为它们是同一个对象。仅类型发生变化；它仍然是对同一对象的相同引用。

也就是说，在您释放某个对象的最后所有权后，期望该对象继续“可用一段时间”是很脆弱的。如果您打算继续使用该对象，请至少拥有它那么长时间。当你用完之后就释放它，而且不要越早。

url = NULL;
//Question 2: Will this cause the NSURL to be released immediately?

NSURL *url = [NSURL fileURLWithPath:appPath];
url = nil;
//Question 3: Does the “url = nil” result in an immediate release of the NSURL?

是和不是。

对强变量的赋值（除非另有说明，变量是隐式强的）立即释放以前的值并保留新值。每次分配 NSURL 对象都会导致该对象被保留；每个任务的nil导致先前持有的物体被释放。

然而，在这两种情况下，URL 对象可能已被自动释放fileURLWithPath:。与往常一样，自动释放不会早于自动释放池结束时到期，因此该对象可能会继续存活到那时。这通常不是问题，但如果您制作大量对象（例如，在紧密循环中），它偶尔会弹出。

就你而言，是的，每个任务nil释放之前驻留在该处的对象，履行并结束您对该所有权的责任。

NSURL *url = [[NSURL alloc] initWithString:@"/something"];
url = nil;
//Question 4: What about this?

此处相同：您的分配释放了该对象。由于这个没有自动释放（你alloc自己动手），该对象将立即死亡。