Avoiding double-destruction when an object is released

Raymond Chen

Raymond



As we saw last time,
trying to do too much in one’s destructor can lead to an object
being destroyed twice
.
The standard way to work around this problem is to set an artificial
reference count during destruction.



class MyObject : public IUnknown
{

ULONG Release()
{
LONG cRef = InterlockedDecrement(&m_cRef);
if (cRef == 0) {
m_cRef = DESTRUCTOR_REFCOUNT;
delete this;
}
return cRef;
}

private:
}
enum { DESTRUCTOR_REFCOUNT = 42 };
~MyObject()
{
if (m_fNeedSave) Save();
assert(m_cRef == DESTRUCTOR_REFCOUNT);
}
};


If you have a common implementation of IUnknown,
you can set the reference count to DESTRUCTOR_REFCOUNT
in your implementation of IUnknown::Release like we
did here, and assert that the value is correct in your implementation’s
destructor.
Since C++ runs base class destructors after derived class destructors,
your base class destructor will check the reference count after the
derived class has done its cleanup.



By setting the reference count to an artificial non-zero value,
any AddRef() and
Release() calls that occur will not trigger
a duplicate destruction (assuming of course that nobody in
the destructor path has a bug that causes them to over-release).
The assertion at the end ensures that no new references to the
object have been created during destruction.



This is really more of a workaround than a rock-solid solution,
because it assumes that no functions called during the destruction
sequence retain a reference to the object beyond the function’s
return.
This is in general not something you can assume about COM.
In general, a method is free to call
AddRef and hang onto a pointer to an object
in order to complete the requested operation later.
Some methods
(such as

the IPersistPropertyBag::Load method
)
explicitly forbid such behavior, but these types of methods are
more the exception rather than the rule.



Exercise:
Why is it safe to perform a simple assignment
m_cRef = DESTRUCTOR_REFCOUNT instead of
the more complicated
InterlockedExchangeAdd(&m_cRef, DESTRUCTOR_REFCOUNT)?

Raymond Chen
Raymond Chen

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