Every so often I need to write some code that uses COM interfaces, but since I do so rather infrequently I often forget the COM reference counting rules. The rules are documented on MSDN but the documentation could really do with some basic examples, and finding such examples elsewhere can be rather difficult in my experience (perhaps they’re all in paper books). So this post is going to present one such example, primarily to help my future forgetful self.

Here’s a function that iterates through the objects an IObjectArray and invokes a callback function for each object:

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template <typename T>
void ForEachObject(IObjectArray* objectArrayPtr,
                   std::function<void(T*)> callback)
{
  UINT count;
  if (FAILED(objectArray->GetCount(&count)))
    return;
  T* objectPtr;
  for (UINT i = 0; i < count; ++i)
  {
    // IObjectArray::GetAt() will call AddRef() on the returned object.
    if (SUCCEEDED(objectArrayPtr->GetAt(i, IID_PPV_ARGS(&objectPtr))))
    {
      callback(objectPtr);
      // Call Release() when we're done with the object to decrement the
      // reference count that was incremented by IObjectArray::GetAt().
      objectPtr->Release();
    }
    // NOTE: If IObjectArray::GetAt() fails above we just keep on going,
    //       but the error handling strategy can of course differ on a
    //       case by case basis.
  }
}

The same function could be written using CComPtr, though there really isn’t much point in doing so in this particular case:

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template <typename T>
void ForEachObject(IObjectArray* objectArrayPtr,
                   std::function<void(T*)> callback)
{
  UINT count;
  if (FAILED(objectArray->GetCount(&count)))
    return;
  for (UINT i = 0; i < count; ++i)
  {
    // This CComPtr will be destroyed at the end of each iteration, and the
    // destructor will decrement the reference count that was incremented by
    // IObjectArray::GetAt().
    CComPtr<T> objectPtr;
    // IObjectArray::GetAt() will call AddRef() on the returned object.
    if (SUCCEEDED(objectArrayPtr->GetAt(i, IID_PPV_ARGS(&objectPtr))))
    {
      callback(objectPtr);
    }
  }
}

And here’s an example of using the function implemented above:

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int counter = 0;
ForEachObject<IShellItem>(
  objectArray,
  [&counter](IShellItem* item) { ++counter; }
);