今天无意这中遇到一个奇怪的崩溃，先上引起崩溃的代码：

- (void)dealloc
{__weak __typeof(self)weak_self = self;NSLog(@"%@", weak_self);
}

当执行到dealloc的时候，程序就crash 掉了。
崩溃信息如下：

objc[4572]: Cannot form weak reference to instance (0x160f6f890) of class MFChatRoomBoardController. It is possible that this object was over-released, or is in the process of deallocation.
(lldb)
error: empty command
(lldb) bt
* thread #1: tid = 0x35914d, 0x0000000182307aac libobjc.A.dylib`_objc_trap(), queue = 'com.apple.main-thread', stop reason = EXC_BREAKPOINT (code=1, subcode=0x182307aac)* frame #0: 0x0000000182307aac libobjc.A.dylib`_objc_trap()frame #1: 0x0000000182307b24 libobjc.A.dylib`_objc_fatal(char const*, ...) + 88frame #2: 0x0000000182319890 libobjc.A.dylib`weak_register_no_lock + 316frame #3: 0x0000000182320688 libobjc.A.dylib`objc_initWeak + 224frame #4: 0x000000010022bf8c MakeFriends`-[MFChatRoomBoardController dealloc](self=0x0000000160f6f890, _cmd="dealloc") + 36 at MFChatRoomBoardController.m:31

其中，可以在控制台明确看到这样一段描述:

objc[4572]: Cannot form weak reference to instance (0x160f6f890) of class MFChatRoomBoardController. It is possible that this object was over-released, or is in the process of deallocation.

说明不允许在 dealloc 的时候取 weak self.

查看了一下 weak_register_no_lock 的函数代码，找到问题所在。

id
weak_register_no_lock(weak_table_t *weak_table, id referent_id, id *referrer_id)
{objc_object *referent = (objc_object *)referent_id;objc_object **referrer = (objc_object **)referrer_id;if (!referent  ||  referent->isTaggedPointer()) return referent_id;// ensure that the referenced object is viablebool deallocating;if (!referent->ISA()->hasCustomRR()) {deallocating = referent->rootIsDeallocating();}else {BOOL (*allowsWeakReference)(objc_object *, SEL) = (BOOL(*)(objc_object *, SEL))object_getMethodImplementation((id)referent, SEL_allowsWeakReference);if ((IMP)allowsWeakReference == _objc_msgForward) {return nil;}deallocating =! (*allowsWeakReference)(referent, SEL_allowsWeakReference);}if (deallocating) {_objc_fatal("Cannot form weak reference to instance (%p) of ""class %s. It is possible that this object was ""over-released, or is in the process of deallocation.",(void*)referent, object_getClassName((id)referent));}// now remember it and where it is being storedweak_entry_t *entry;if ((entry = weak_entry_for_referent(weak_table, referent))) {append_referrer(entry, referrer);} else {weak_entry_t new_entry;new_entry.referent = referent;new_entry.out_of_line = 0;new_entry.inline_referrers[0] = referrer;for (size_t i = 1; i < WEAK_INLINE_COUNT; i++) {new_entry.inline_referrers[i] = nil;}weak_grow_maybe(weak_table);weak_entry_insert(weak_table, &new_entry);}// Do not set *referrer. objc_storeWeak() requires that the // value not change.return referent_id;
}

可以看出，runtime 是通过检查引用计数的个数来判断对象是否在 deallocting， 然后通过

    if (deallocating) {_objc_fatal("Cannot form weak reference to instance (%p) of ""class %s. It is possible that this object was ""over-released, or is in the process of deallocation.",(void*)referent, object_getClassName((id)referent));}

这段代码让程序crash。

再看一下 _objc_fatal 这个函数

void _objc_fatal(const char *fmt, ...)
{va_list ap; char *buf1;char *buf2;va_start(ap,fmt); vasprintf(&buf1, fmt, ap);va_end (ap);asprintf(&buf2, "objc[%d]: %s\n", getpid(), buf1);_objc_syslog(buf2);_objc_crashlog(buf2);_objc_trap();
}

可以看到这个函数实际会在控制台输出一段信息，然后调用 _objc_trap() 引起 crash. 而最后一个函数调用刚好也对上我们之前的崩溃堆栈。