PRB: ATL Server with Singleton Object Never Unloads from Memory

CAUSE

This problem can occur if your singleton class contains an inner object that increments the module lock count. Consider the following code, where CMySingletonComponent and CMyInnerObject are ATL "Simple Objects":

class CMySingletonComponent : public CComCoClass<...>, ...
   {
   public:

      DECLARE_CLASSFACTORY_SINGLETON(CMySingletonComponent)
      CComObject<CMyInnerObject>* m_pInner;

      HRESULT FinalConstruct ()
      {
         return CComObject<CMyInnerObject>::CreateInstance (&m_pInner);
      }

      void FinalRelease ()
      {
         delete m_pInner;
      }
      ...
   };
				

An ATL local server doesn't unload until the module lock count goes to zero, which in the case above never happens, because the CComObject<> contained inside the singleton class above increments the module lock count in its constructor but is never given a chance to decrement it in its destructor.

The contained CComObject<>'s destructor is called only if the singleton is destroyed. The singleton is destroyed only if the class factory is destroyed. The class factory in turn is destroyed only if the module lock count goes to zero, which can't happen because the CComObject<> in the singleton is never destroyed and doesn't decrement the module lock count.

class CMySingletonComponent : public CComCoClass<...>, ...
   {
   public:

      DECLARE_CLASSFACTORY_SINGLETON(CMySingletonComponent)
      CComObjectNoLock<CMyInnerObject>* m_pInner;

      HRESULT FinalConstruct ()
      {
         m_pInner = new CComObjectNoLock<CMyInnerObject>;
         ATLASSERT (m_pInner);
         return m_pInner->FinalConstruct();
      }

      void FinalRelease ()
      {
         delete m_pInner;
      }
      ...

   };