NSFastEnumeration在使用ARC的C阵列上崩溃

Question

我正在初始化一个对象的C数组，并设置第一个元素：

id __strong *_objs =  (id __strong *)calloc(16,sizeof(*_objs));
_objs[0] = @1;
_count++;

然后我使用NSFastEnumeration的以下实现：

- (NSUInteger) countByEnumeratingWithState: (NSFastEnumerationState*)state
                                   objects: (id __unsafe_unretained*)stackbuf
                                     count: (NSUInteger)len
{
    NSUInteger size = _count;
    NSInteger count;
    state->mutationsPtr = (unsigned long *)size;
    count = MIN(len, size - state->state);
    if (count > 0)
    {
        IMP imp = [self methodForSelector: @selector(objectAtIndex:)];
        int p = state->state;
        int i;
        for (i = 0; i < count; i++, p++) {
            stackbuf[i] = (*imp)(self, @selector(objectAtIndex:), p);
        }
        state->state += count;
    }
    else
    {
        count = 0;
    }
    state->itemsPtr = stackbuf;
    return count;
}

不幸的是，当我运行它时，它与EXC_BAD_ACCESS一起崩溃：

for (id object in array){  // EXC_BAD_ACCESS
    NSLog(@"%@",object)
}

知道为什么吗？

如果您有CodeRunner，则here是一个可执行版本。

Answer 1

问题出在mutationsPtr，它指向您不允许访问的内存地址1(也不是4字节对齐的)：

state->mutationsPtr = (unsigned long *)size;

将其替换为有效的启动器指针(小心:下面的指针在您的场景中可能没有任何意义，但至少它修复了EXC_BAD_ACCESS)：

state->mutationsPtr = (unsigned long *)&_count;

Answer 2

@Jano以防您想要摆脱在Xcode (4.6)的最新版本中出现的编译器警告

   - (NSUInteger) countByEnumeratingWithState: (NSFastEnumerationState*)state
                               objects: (id __unsafe_unretained*)stackbuf
                                 count: (NSUInteger)len

因为它与objects:..stackbuf的原始原型不匹配

    - (NSUInteger) countByEnumeratingWithState: (NSFastEnumerationState*)state
                               objects: (__autoreleasing id *)stackbuf
                                 count: (NSUInteger)len

请参阅Automatic Reference Counting: Error with fast enumeration中的托德·莱曼斯回答