Objective C - ARC - 何时使用@autoreleasepool

Question

我读了一些关于 ARC 的书，看到了这个：

@interface Address : NSObject {
    @public
    NSString *city;
}
@end

@implementation Address
- (Address*) init: (NSString*) c {
    city = c;

    return self;
}

- (void) dealloc {
    NSLog(@"Destroying address: %@", city);
}
@end

@interface Customer : NSObject {
    NSString *name;
    Address *addr;
}
@end

@implementation Customer
- (Customer*) init: (NSString*) n withAddress: (Address*) a {
    //Note 1: Automatic retain on assignment
    name = n;
    addr = a;

    return self;
}

- (void) dealloc {
    NSLog(@"Destroying: %@", name);
    //Note 2: Automatic release of member variables
}

@end

Customer* objectReturnTest() {
    NSString * n = [[NSString alloc] initWithString: @"Billy Bob"];
    Address * a = [[Address alloc] init: @"New York City"];      

    Customer *c = [[Customer alloc] init: n withAddress: a];

    //Note 3: ARC will put the returned object in autorelease pool.
    return c;
}

A couple of basic things to note here. As "Note 1"  says, when an object is assigned to a variable, a call to retain is made automatically. This increments the reference count. As "Note 2" says, when an object is destroyed, all member variable objects are released for you. You no longer have to do that from the dealloc method.

Finally, when a method returns a newly created object, ARC will put the returned object in an autorelease pool. This is stated in "Note 3".

Now, let’s use the code.

int main (int argc, const char * argv[])
{
    NSString * n = [[NSString alloc] initWithString: @"Johnny Walker"];
    Address * a = [[Address alloc] init: @"Miami"];
    Customer *c = [[Customer alloc] init: n withAddress: a];

    NSLog(@"Before force release");
    c = nil; //Force a release
    NSLog(@"After force release");

    @autoreleasepool {

        Customer *c2 = objectReturnTest();

    }
    NSLog(@"After autorelease pool block.");

    return 0;
}

The log output from this code will be:

Before force release

Destroying: Johnny Walker

After force release

Destroying: Billy Bob

Destroying address: New York City

After autorelease pool block.

Destroying address: Miami

A couple of things to note here. See how force release works. We set a variable to nil. ARC immediately releases the reference count. This causes the Customer object "Johnny Walker" to get destroyed. But, the member Address object "Miami" doesn’t get destroyed. This object gets destroyed at the very end of the main method. This is an extremely odd and non-intuitive behavior. Technically, this is not a memory leak, but, in reality member variables can pile up and take up a lot of memory. This is just as bad as memory leak.

The object return test works as expected. Customer "Billy Bob" is put in auto release pool. At the end of the @autoreleasepool block, the pool is drained and the object is released.

看着这一部分；

int main (int argc, const char * argv[])
{
    NSString * n = [[NSString alloc] initWithString: @"Johnny Walker"];
    Address * a = [[Address alloc] init: @"Miami"];
    Customer *c = [[Customer alloc] init: n withAddress: a];

    NSLog(@"Before force release");
    c = nil; //Force a release
    NSLog(@"After force release");

    @autoreleasepool {

        Customer *c2 = objectReturnTest();

    }
    NSLog(@"After autorelease pool block.");

    return 0;
}

当他这样做时 c = nil; ca和n不应该全部被销毁吗？然而它说输出只是 n 被破坏了.. 有人可以解释为什么吗？

他说结果就像内存泄漏一样糟糕，那么如何修复呢？

最后一个问题，什么时候应该使用@autoreleaasepool？

Answer 1

逐行取消

c = nil; //Forces a release

分配 Customer 实例，因为没有人保留它，因此输出为

摧毁：约翰尼·沃克

，但 n 和 a 尚未被释放，因为它们仍然保留在作用域中，并且 nil 尚未分配给它们。

我不认为这是任何类型的内存泄漏，

---

你通常不需要使用@autorelasepool，除非你正在做这样的事情，

- (void)myMethod {
    for (int i = 0; i < 1000000; i++) {
        NSString *string = [NSString stringWithFormat:@"%d", i];
        // do something with string
    }
}

在循环期间将分配 1000000 NSString。它们将在方法返回后（实际上是在此运行循环之后）被释放，但已经消耗了太多内存。因此应该替换为

- (void)myMethod {
    for (int i = 0; i < 1000000; i++) {
        @autoreleasepool {
            NSString *string = [NSString stringWithFormat:@"%d", i];
            // do something with string
        }
    }
}

您应该阅读此内容以了解有关内存管理的更多信息
https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmAutoreleasePools.html#//apple_ref/doc/uid/20000047-CJBFBEDI

Answer 2

名称和地址之间的明显区别在于，您为地址创建一个地址对象，为名称创建 NSString。在地址对象中它是@public。这意味着当客户被释放时，NSString 超出了范围，但不是地址对象，它仍然会记住当您释放客户时给@public NSString *city 的地址。

因此，当您调用地址的公共值时，它仍然存在，但名称的 NSString 不存在。要解决此问题，您可以删除地址对象的接口，这会释放这两个值，或者为名称创建一个接口而不是使用 NSString。