Is TInterfacedObject autofree thread safe in Delphi?

Question

Is TInterfacedObject._Release and TInterfacedObject._AddRef thread safe? I can see FRefCount is thread safe, but Destroy is not protected by locks or something else.

function TInterfacedObject._Release: Integer;
begin
  Result := InterlockedDecrement(FRefCount);
  if Result = 0 then
    Destroy;
end;

Could different thread execute Destroy at the same time causing a race condition?

The whole point is that you don't call destroy because you work with the interface rather than the object. — David Heffernan, Aug 23 '23 at 14:51
I know that I cannot call Destroy directly. If one thread checking Result is 0 and try to free, while another thread add ref? Could it happen? — qgi, Aug 23 '23 at 14:54
If result reaches zero then nothing can have a reference any more — David Heffernan, Aug 23 '23 at 15:02
@qgi yes, even in multi-thread. If the refcount falls to 0, it means there are NO REFERENCES WHATSOEVER to the object. If a thread is trying to increment the refcount, there must have been an existing reference to the object to begin with, so the refcount should not fall to 0 while the thread is incrementing the refcount. Unless you are doing something unusual, like passing around references without refcounting them — Remy Lebeau, Aug 23 '23 at 16:40

score 4 · Accepted Answer · answered Aug 23 '23 at 18:00

Implementation of reference counting methods in TInterfacedObject is thread safe as reference count is incremented and decremented atomically.

When reference count of object instance reaches zero in _Release method, that means there is no longer any other strong reference to that object and destroying the object will be thread-safe.

Similarly when you look at _AddRef method, when you already have a strong reference to an object, taking another strong reference from it will also be thread-safe.

However, that is not the whole story and there are other things that need to be taken in consideration when handling reference counted instances across multiple threads.

reference assignment is not thread-safe - you cannot have a variable and write to it from multiple threads in thread-safe manner without using additional access protection mechanisms that will ensure only single thread has access to that reference at a time, like locks.
taking strong reference from weak reference is not thread-safe - if thread only has a weak reference to some reference counted instance then you cannot assign that weak reference to a strong one in thread-safe manner

Is TInterfacedObject autofree thread safe in Delphi?

1 Answers1