struct SideTable {
    spinlock_t slock;// 用于給原子性操作加鎖
    RefcountMap refcnts;// 引用計數(shù)hash表
    weak_table_t weak_table;// weak對象指針hash表
}

/**
* The global weak references table. Stores object ids as keys,
* and weak_entry_t structs as their values.
*/
struct weak_table_t {
    weak_entry_t *weak_entries;// 存儲 weak 對象信息的 hash 數(shù)組
    size_t    num_entries;// 數(shù)組中元素的個數(shù)，數(shù)組初始化的時候默認(rèn)4個，占用達(dá)到3/4會翻倍擴(kuò)容
    uintptr_t mask;// 計數(shù)輔助量
    uintptr_t max_hash_displacement;// hash 元素最大偏移值
};

// objc-weak.mm
/** 
 * Called by dealloc; nils out all weak pointers that point to the 
 * provided object so that they can no longer be used.
 * 
 * @param weak_table 
 * @param referent The object being deallocated. 
 */
void 
weak_clear_no_lock(weak_table_t *weak_table, id referent_id) 
{
    // 獲得 weak 指向的地址，即對象內(nèi)存地址
    objc_object *referent = (objc_object *)referent_id; 
    // 找到管理 referent 的 entry 容器
    weak_entry_t *entry = weak_entry_for_referent(weak_table, referent); 
    // 如果 entry == nil，表示沒有弱引用需要置為 nil，直接返回
    if (entry == nil) { 
        /// XXX shouldn't happen, but does with mismatched CF/objc
        //printf("XXX no entry for clear deallocating %p\n", referent);
        return;
    }
    // zero out references
    weak_referrer_t *referrers;
    size_t count;
    if (entry->out_of_line()) { 
        // referrers 是一個數(shù)組，存儲所有指向 referent_id 的弱引用
        referrers = entry->referrers; 
        // 弱引用數(shù)組長度
        count = TABLE_SIZE(entry);    
    } 
    else {
        referrers = entry->inline_referrers;
        count = WEAK_INLINE_COUNT;
    }
    // 遍歷弱引用數(shù)組，將所有指向 referent_id 的弱引用全部置為 nil
    for (size_t i = 0; i < count; ++i) {
        objc_object **referrer = referrers[i];
        if (referrer) {
            if (*referrer == referent) {
                *referrer = nil;
            }
            else if (*referrer) {
                _objc_inform("__weak variable at %p holds %p instead of %p. "
                             "This is probably incorrect use of "
                             "objc_storeWeak() and objc_loadWeak(). "
                             "Break on objc_weak_error to debug.\n", 
                             referrer, (void*)*referrer, (void*)referent);
                objc_weak_error();
            }
        }
    }
    // 從 weak_table 中移除對應(yīng)的弱引用的管理容器
    weak_entry_remove(weak_table, entry);
}

id obj = [[NSObject alloc] init];
id __weak obj1 = obj;
/*----- 編譯 -----*/
id obj1;
objc_initWeak(&obj1,obj);
objc_destroyWeak(&obj1);

// NSObject.mm
1. objc_initWeak
2. storeWeak
// objc-weak.mm
3. weak_register_no_lock
4. weak_unregister_no_lock

if (!atomic) {
    oldValue = *slot;
    *slot = newValue;
} else {
    spinlock_t& slotlock = PropertyLocks[slot];
    slotlock.lock();
    oldValue = *slot;
    *slot = newValue;        
    slotlock.unlock();
}

    if (offset == 0) {
        return object_getClass(self);
    }
    // Retain release world
    id *slot = (id*) ((char*)self + offset);
    if (!atomic) return *slot;
    // Atomic retain release world
    spinlock_t& slotlock = PropertyLocks[slot];
    slotlock.lock();
    id value = objc_retain(*slot);
    slotlock.unlock();
    // for performance, we (safely) issue the autorelease OUTSIDE of the spinlock.
    return objc_autoreleaseReturnValue(value);