Step 3. Support QueryInterface

To expose the filter's new interfaces to clients, do the following: 

• Include the DECLARE_IUNKNOWN macro in the public declaration section of your filter:

  public:
      DECLARE_IUNKNOWN;
  

• Override CUnknown::NonDelegatingQueryInterface to check for the IIDs of the two interfaces:

  STDMETHODIMP CGrayFilter::NonDelegatingQueryInterface(REFIID riid, 
      void **ppv)
  {
      if (riid == IID_ISpecifyPropertyPages)
      {
          return GetInterface(static_cast<ISpecifyPropertyPages*>(this),
             ppv);
      }
      if (riid == IID_ISaturation)
      {
          return GetInterface(static_cast<IYuvGray*>(this), ppv);
      }
      return CBaseFilter::NonDelegatingQueryInterface(riid, ppv);
  }
  

See Also

• How to Implement IUnknown