objective-c - NSFastEnumeration 在带有 ARC 的 C 数组上崩溃

Question

我正在初始化对象的 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，它因 EXC_BAD_ACCESS 而崩溃:

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

知道为什么吗？

如果你有CodeRunner , here是可执行版本。

Answer 1

问题是 mutationsPtr 指向您不允许访问的内存地址 1(并且也不是 4 字节对齐的):

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

将其替换为适合初学者的有效指针(注意:下面的指针在您的场景中可能毫无意义，但至少它修复了 EXC_BAD_ACCESS):

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