Re: Do I need to call Release on smart pointer

"Carl Daniel [VC++ MVP]" <>
Wed, 31 May 2006 07:01:35 -0700
Jeff Partch [MVP] wrote:

"Igor Tandetnik" <> wrote in message

With apologies and hesitation... Doesn't it depend on whether the
OtherFunc1 and/or OtherFunc2 AddRef the interface pointers they

Well, I assumed OtherFunc1 and OtherFunc2 are correctly written. They
should return either a non-AddRef'ed raw pointer, or a smart pointer.

More apologies, but the reason I asked is that I routinely encounter
wizard generated wrappers which are keen to expose methods like:
LPDISPATCH GetHtmlDocument() -- that do return AddRef'd pointers, so
in a function like...

VOID CSomeClass::SomeFunc(VOID)
   ISmartPointer p = m_someWrapper.GetSomeDispatch();
} can expect the ref count to go up, up and up.

I'm not sure anymore - but some part of my brain says that the COM
guidelines suggested that functions that return an interface to a newly
allocated object should AddRef the pointer before returning it. This is the
convention for CoCreateInstace, for example. Such pointers are frequently
returned through T** or void** parameters instead of the function return
value. If you're using plain pointers, such conventions would seem to make
the code more clear and robust, but using smart pointers, it's a bit of a
conflict of interest. Fortunately, all of the (common) smart pointer
classes also provide a way to set the value of the contained pointer without
AddRef'ing it to accomodate such situations:

VOID CSomeClass::SomeFunc(VOID)
   ISmartPointer p;

In general, there's no way to tell which construct you need - other than
reading the documentation for GetSomeDispatch, which should clearly indicate
whether the returned interface has been AddRef'd.

In the COM smart pointer class that I always used, I added a couple more
member functions (inspired by a Don Box article, IIRC):

template<class T> class SmartPtr
  // ...
    T** AsPPTArg();

    void** AsPPVArg();


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