Fingerpointing at smart pointers (31 Dec 2005)

In an ATL COM client which uses #import to generate wrapper code for objects, I recently tracked down a subtle reference-counting issue down to this single line:

  IComponentArray *compArray = app->ILoadComponents();

This code calls a method ILoadComponents on an application object which returns an array of components. Innocent-looking as it is, this one-liner caused me quite a bit of grief. If you can already explain what the reference counting issue is, you shouldn't be wasting your time reading this blog. For the rest of us, I'll try to dissect the problem.

(And for those who don't want to rely on my explanation: After I had learnt enough about the problem so that I could intelligently feed Google with search terms, I discovered a Microsoft Knowledge Base article on this very topic. However, even after reading the article, some details were still unclear to me, especially since I don't live and breathe ATL all day.)

The #import statement automatically generates COM wrapper functions. For ILoadComponents, the wrapper looks like this:

inline IComponentArrayPtr IApplication::ILoadComponents () {
    struct IComponentArray * _result = 0;
    HRESULT _hr = raw_ILoadComponents(&_result);
    if (FAILED(_hr)) _com_issue_errorex(_hr, this, __uuidof(this));
    return IComponentArrayPtr(_result, false);
}

IComponentArrayPtr is a typedef-ed template instance of _com_ptr_t. The constructor used in the code snippet above will only call AddRef on the interface pointer if its second argument is true. In our case, however, the second arg is false, so AddRef will not be called. The IComponentArrayPtr destructor, however, always calls Release().

Feeling uneasy already? Yeah, me too. But let's follow the course of action a little bit longer. When returning from the wrapper function, the copy constructor of the class will be called, and intermediate IComponentArrayPtr objects will be created. As those intermediate objects are destroyed, Release() is called.

Now let us assume that the caller looks like above, i.e. we assign the return value of the wrapper function to a CComPtr<IComponentArray> type. The sequence of events is as follows:

• Wrapper function for ILoadComponents is called.
• Wrapper function calls into the COM server. The server returns an interface pointer for which AddRef() was called (at least) once inside the server. The reference count is 1.
• Wrapper function constructs an IComponentArrayPtr smart pointer object which simply copies the interface pointer value, but does not call AddRef(). The refcount is still 1.

Now we return from the wrapper function. In C++, temporary objects are destroyed at the end of the "full expression" which creates them. See also section 6.3.2 in Stroustrup's "Design and Evolution of C++". This means that the following assignment is safe:

  CComPtr<IComponentArray> components = app->ILoadComponents();

ILoadComponents returns an object of type IComponentArrayPtr. At this point, the reference count for the interface is 1 (see above). The The compiler casts IComponentArrayPtr to IComponentArray*, then calls the CComPtr assignment operator which copies the pointer and calls AddRef on it. The refcount is now 2. At the completion of the statement, the temporary IComponentArrayPtr is destroyed and calls Release on the interface. The refcount is 1. Just perfect.

Now back to the original client code:

  IComponentArray *compArray = app->ILoadComponents();

Here, we assign to a "raw" interface pointer, rather than to a CComPtr, When returning from the wrapper function, the refcount for the interface is 1. The compiler casts IComponentArrayPtr to IComponentArray* and directly assigns the pointer. At the end of the statement (i.e. the end of the "full expression"), the temporary IComponentArrayPtr is destroyed and calls Release, decrementing the refcount is 0. The object behind the interface pointer disappears, and subsequent method calls on compArray will fail miserably or crash!

So while ATL, in conjunction with the compiler's #import support, is doing its best to shield us from the perils of reference counting bugs, it won't help us if someone pulls the plug from the ATL force-field generator by incorrectly mixing smart and raw pointers.

This kind of reference counting bug would not have occurred if I had used raw interface pointers throughout; the mismatch in calls to AddRef and Release would be readily apparent in such code. However, those smart pointers are indeed really convenient in practice because they make C++ COM code so much simpler to read. However, they do not alleviate the programmer from learning about the intricacies of reference counting. You better learn your IUnknown before you do CComPtr.

This reminds me of Joel Spolsky's The Perils of JavaSchools, which is soooo 1990 (just like myself), but good fun to read.

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