Question

我试图了解ARC和NSHashTable weakObjectsHashTable的工作原理。我添加到哈希表的对象应该被删除/归零，或者一旦对象被释放后它们变成了什么。下面的NSLog中的代码示例显示该对象仍然存在于哈希表中。我做错了什么？

#import <Foundation/Foundation.h>

int main(int argc, char *argv[])
{
    @autoreleasepool
    {
        NSHashTable *hashTable = [NSHashTable weakObjectsHashTable];

        @autoreleasepool
        {
            NSString *str = @"Hello World!";
            [hashTable addObject:str];
            str = nil;
        }

        NSLog(@"hashTable:%@", [hashTable allObjects]);
        // prints: hashTable:("Hello World!") – but should be empty?
    }
}

Answer 1

要指定nil对象，它不会影响结果。 NSString * str = @＆＃34; Hello World！＆＃34 ;;对象引用并不弱！ Apple的文档说：如果在这样的哈希表中没有对对象的强引用，那么这些对象将被释放。

https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Collections/Articles/Sets.html#//apple_ref/doc/uid/20000136-CJBDHAJD

Answer 2

尝试使用OS X控制台应用程序的这段代码：

//
//  main.m
//  

#import <Foundation/Foundation.h>

uint objectsAlive = 0;
uint totalObjects = 0;

@interface TestObject : NSObject
@end

@implementation TestObject
{
    uint myIx;
}

- (id)init
{
    // NSAssert([NSThread currentThread] == [NSThread mainThread], @"Not on the main thread");

    self = [super init];
    if (self)
    {
        objectsAlive++;
        totalObjects++;

        myIx = totalObjects;

        NSLog(@"Object with id=%u created; alive objects %u", myIx, objectsAlive);
    }

    return self;
}

- (void)dealloc
{
    objectsAlive--;
    NSLog(@"Object  with id=%u is being destroyed by ARC; alive objects will become %u", myIx,objectsAlive);
}

@end

int main(int argc, const char * argv[]) {

    // default global autorelease pool
    @autoreleasepool {

        NSHashTable * testHashMap = [NSHashTable weakObjectsHashTable];
        // weakObjectsHashTable - according to Apple docs, entries are not necessarily purged right away when the weak object is reclaimed, and we can observe this behavior here - some entries stay alive until last autorelease

        // comment out the line above and uncomment the line below to observe different behavior with strong references in NSHashTable
        // NSHashTable * testHashMap = [[NSHashTable alloc] init];

        // nested autoreleasepool to ensure that the objects added to the testHashMap are released by ARC
        @autoreleasepool {

            for(int i = 0; i < 10;i++) {
                TestObject * obj = [[TestObject alloc] init];
                [testHashMap addObject: obj];
                NSLog(@"Count in hash table inside additions scope is %lu",
                      (unsigned long)testHashMap.count);
            }

            NSLog(@"Count in hash table inside @autoreleasepool is %lu",
                  (unsigned long)testHashMap.count);
            NSLog(@"Objects in hash table inside of @autoreleasepool are %@",
                  testHashMap.allObjects);
            NSLog(@"Exiting inner autorelease pool...");

        }

        // objects in NSHashTable also were released, according to dealloc logs in TestObject, but count is still lagging behind (shows 2)
        NSLog(@"Count in hash table outside of @autoreleasepool is %lu",
              (unsigned long)testHashMap.count);

        // this should indeed show that NSHashTable with weakObjectsHashTable is empty, despite count=2
        NSLog(@"Objects in hash table outside of @autoreleasepool are %@",
              testHashMap.allObjects);

        NSLog(@"Exiting outer autorelease pool, ecpect all objects in strong ref NSHashTable to die...");
    }

    return 0;
}

你应该看到hashmap确实是空的，对象在for循环内被正确销毁，其中强引用被放弃。但问题在于.count属性 - 它存在，告诉我有条目，尽管allObjects没有返回。

Answer 3

正如文件所说：

主要选择是提供＆＃34;弱＆＃34;自动删除的引用，但在将来的某个无限期点。

因此，自动删除的要点是未知的，并且无法保证在下一个运行循环中将其删除，因此您无法依赖AutoReleasePool。

NSHashTable weakObjectsHashTable - 添加的对象未归零

3 个答案: