IDataObject as a Standard for Object Data Transfer

As I mentioned, IDataObject is the standardized interface for data exchange. It makes sense for any object capable of performing any type of data transfer to do as much as it can through IDataObject instead of through custom interfaces. We can see this in practice with the Polyline component object we've used in earlier chapters. In Chapter 8, we eliminated the private members of the IPolyline5 interface that had to do with file input and output and replaced those members with the IPersistStorage and IPersistStreamInit interfaces. Now we can do the same thing in CHAP10\POLYLINE, replacing the data exchange members of IPolyline8, shown in the following, with IDataObject:

//From Chapter 8's Polyline
interface IPolyline8 : public IUnknown
    {
    [Other members]

    STDMETHOD(DataSet)      (THIS_ LPPOLYLINEDATA, BOOL, BOOL) PURE;
    STDMETHOD(DataGet)      (THIS_ LPPOLYLINEDATA) PURE;
    STDMETHOD(DataSetMem)   (THIS_ HGLOBAL, BOOL, BOOL, BOOL) PURE;
    STDMETHOD(DataGetMem)   (THIS_ HGLOBAL FAR *) PURE;
    STDMETHOD(RenderBitmap) (THIS_ HBITMAP FAR *) PURE;
    STDMETHOD(RenderMetafile) (THIS_ HMETAFILE FAR *) PURE;
    STDMETHOD(RenderMetafilePict) (THIS_ HGLOBAL FAR *) PURE;

    [Other members]
    }

In other words, we can eliminate from IPolyline8 all the separate functions for exchange of its native data in both get and set directions because FORMATETC allows us to specify different formats and different mediums through IDataObject calls directly. The new interface, IPolyline10, for this chapter's version of Polyline no longer has these specific member functions.

In addition, Polyline has been working with its own private notification interface, IPolylineAdviseSink8, with members such as OnDataChange, OnPointChange, and so on. With the addition of IDataObject to the Polyline object, we can remove the OnDataChange member from the updated IPolylineAdviseSink10 interface and instead use IAdviseSink for this notification. Any client can now use Polyline for data exchange and notification without having to be aware of the custom IPolyline10 and IPolylineAdviseSink10 interfaces.

Along with these changes to Polyline are the necessary changes to Component Cosmo (CHAP10\COCOSMO), as follows:

• Polyline has one additional file, IDATAOBJ.CPP, which implements the IDataObject interface for the CPolyline class.
• Polyline maintains a data advise holder object internally for implementing the advise members of IDataObject. Where Polyline used to call IPolylineAdviseSink8::OnDataChange, it now calls IDataAdviseHolder::SendOnDataChange.
• IPolyline8::DataSet and IPolyline8::DataGet are now private member functions of the C++ CPolyline class because they are used internally by the Polyline object in the rest of its implementation.
• Polyline now registers a private clipboard format ("Polyline Figure") to identify its native data.
• Polyline implements IDataObject::EnumFormatEtc using OleRegEnumFormatEtc and registry entries for its native Polyline Figure format, CF_METAFILEPICT, and for CF_BITMAP.
• CoCosmo adds the file IADVSINK.CPP, in which it receives OnDataChange notifications from Polyline. CoCosmo implements the IAdviseSink interface as part of its CCosmoDoc object, which now uses multiple inheritance from both its normal base class CDocument and IAdviseSink. Take a look at CCosmoDoc::QueryInterface to see why you must be careful to correctly typecast the pointers you assign to the void ** out-parameter. Failure to do this can cause all sorts of strange problems, such as premature calls to the document destructor, as we described in Chapter 2.
• CoCosmo implements a set of IUnknown members on its CCosmoDoc class to support the interface implementation of IAdviseSink.
• All of CoCosmo's clipboard handling in CCosmoDoc::RenderFormat and CCosmoDoc::Paste now work through Polyline's IDataObject, which CoCosmo queries for when needed.

None of the code to implement these features is much different from the code we've seen so far, so there's no need to show any of it here. However, with the addition of IDataObject along with IPersistStorage, the Polyline object is now very close to an embedded content object. What it lacks are the interfaces IViewObject2, which we'll see in the next chapter, and IOleObject, which we'll see in Chapter 19. Even when we make those changes, Component Cosmo can still use Polyline as before, totally ignoring any additional interfaces. This shows the flexibility of the multiple interface paradigm made possible by QueryInterface. In later chapters, we'll see how Polyline can support two different types of client (CoCosmo with inside knowledge and a container with no knowledge outside OLE Documents) merely by implementing a number of interfaces for both.