Understanding the dealloc Process of Objective‑C Objects in MRC and ARC

In the previous article we introduced the creation process of OC objects; this article explains the deallocation (dealloc) process.

In manual reference counting (MRC) you must call release; example:

-(void)dealloc
{
    /* release ivars, remove observers, stop timers, remove notifications, nil delegates, etc. */
    [super dealloc];
}

Under ARC, most objects are handled automatically; only non‑Cocoa objects need manual handling:

-(void)dealloc
{
    /* non‑OC objects such as CF objects */
}

Important notes when implementing dealloc:

Do not use __weak inside dealloc ; it can cause a crash because the object is already being deallocated.

Prefer directly setting instance variables to nil rather than using properties.

Avoid calling other methods, especially GCD or thread‑related APIs, from dealloc .

The article then dives into the runtime implementation of dealloc in NSObject. The flow is:

Call _objc_rootDealloc(self) .

_objc_rootDealloc forwards to objc_object::rootDealloc() .

rootDealloc either frees the object directly or calls object_dispose(obj) depending on isa flags.

object_dispose invokes objc_destructInstance(obj) and then frees the memory.

objc_destructInstance runs C++ destructors, removes associated objects, and clears ARC ivar bookkeeping.

Key runtime functions are shown below:

Void _objc_rootDealloc(id obj)
{
    assert(obj);
    obj->rootDealloc();
}

inline void objc_object::rootDealloc()
{
    if (isTaggedPointer()) return;
    if (fastpath(isa.nonpointer && !isa.weakly_referenced && !isa.has_assoc && !isa.has_cxx_dtor && !isa.has_sidetable_rc))
    {
        assert(!sidetable_present());
        free(this);
    }
    else
    {
        object_dispose((id)this);
    }
}

id object_dispose(id obj)
{
    if (!obj) return nil;
    objc_destructInstance(obj);
    free(obj);
    return nil;
}

void *objc_destructInstance(id obj)
{
    if (obj) {
        bool cxx = obj->hasCxxDtor();
        bool assoc = obj->hasAssociatedObjects();
        if (cxx) object_cxxDestruct(obj);
        if (assoc) _object_remove_assocations(obj);
        obj->clearDeallocating();
    }
    return obj;
}

Further details of object_cxxDestruct , _object_remove_assocations , and clearDeallocating are provided, showing how C++ destructors are called, how associated objects are removed, and how weak references and side‑table reference counts are cleared.

The article concludes with a flowchart and a summary of the main steps: call C++ destructors, remove associations, clear deallocating state, and finally free the memory.