iOS weak的实现原理

static NSMutableDictionary *weaks;

struct OjcP {
    Class isa;
    //...
};

static NSNumber *objHeapAddress(id obj) {
    struct OjcP *heapObj = (__bridge struct OjcP *)(obj);
    return @((long)&(heapObj->isa));
}

@interface ReleasableObject : NSObject

@end
@implementation ReleasableObject

-(void)dealloc {
    [super dealloc];
    NSLog(@"--- ReleasableObject dealloc ---");
    id p = [weaks objectForKey:objHeapAddress(self)];
    id *p1 = (id *)[p longValue];
    NSLog(@"读取弱指针地址：%p", p1);
    *p1 = nil;
}

@end

int main(int argc, const char * argv[]) {
    NSObject *obj;
    NSObject *weak_p;
    @autoreleasepool {
        
        // 弱引用
        obj = [[ReleasableObject alloc] init];
        weak_p = obj;
        NSLog(@"存储弱指针地址：%p", &weak_p);
//        NSLog(@"%@",  weak_p);
        // 方法1
//        weak_p = nil;
        // 方法2
//        id *p = &weak_p;
//        *p = nil;
        // 方法3
        weaks = [[NSMutableDictionary alloc] init];
        // 弱指针地址
        long p11 = (long)(&weak_p);
        // 堆内存中对象isa地址
        [weaks setObject:@(p11) forKey:objHeapAddress(obj)];
        
        // 释放对象
        [obj release];
    }
    // 崩溃代码
    NSLog(@"%@",  weak_p);
    return 0;
}