Object Extensibility The techniques presented so far allow clients to select and load binary components dynamically that can evolve their implementation layout over time without requiring client recompilation. This by itself is extremely useful for building dynamically composable systems. However, one aspect of the object that cannot evolve over time is its interface. This is because the client compiles object, uuid(DF12E156-A29A-lldO-8C2D-0080C73925BA)] interface ICatDog : ICat, IDog { II illegal, multiple bases HRESULT Meowbark(void); } if we reach this point, *ppv is non-null and must be AddRef'ed (guideline A2) reinterpret_cast (*ppv)->AddRef(); return S_OK; Because the object derives from the type used in the cast, the compiled versions of the cast statements simply add a fixed offset to the object's thi s pointer to find the beginning of the base class's layout. Note that when asked for the common base interface IExtensi bl eObject, the implementation statically casts itself to IFastStri ng. This is because the intuitive version of the statement When the client calls the function with no second parameter pfs = CallCreateFastString("Hi Bob!"); n = pfs->Find("ob"); the original FastStri ng DLL is loaded and the search is performed from left to right. If the client indicates that the string is in a spoken language that parses from right to left pfs = CallCreateFastString("Hi Bob!", false); n = pfs->Find("ob"); the alternative version of the DLL (FastStri ngRL. DLL) is loaded and the search will be performed starting at the rightmost position of the string. The key observation is that callers of Call CreateFastStri ng do not care which DLL is used to implement the object's methods. All that matters is that a pointer to an IFastStri ng-compatible vptr is returned by the function and that the vptr provides useful and semantically correct functionality. This form of runtime polymorphism is extremely useful for building a dynamically composed system from binary components.

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Sub TryToSnoreAndlgnore(obj as Object) On Error Resume Next ' ignore errors Di m pug as lPug Set pug = obj • VM calls QueryInterface If Not (pug is Nothing) Then pug.Snore End if Dim cat as lCat Set cat = obj • VM calls QueryInterface If Not (cat is Nothing) Then cat.lgnoreMaster End if End Sub object, uuid(DF12E152-A29A-l1dO-8C2D-0080C73925BA)] interface ICat : IAnimal { HRESULT 19noreHumans(void); } FastString::FastString(const char *psz) m_cch(strlen(psz)), m_psz(new char[m_cch + 1]) { strcpy(m_psz, psz); }class IFastString { public: virtual void Delete(void) = 0; virtual int Length(void) = 0; virtual int Find(const char *psz) COM does impose one limitation on interface inheritance: COM interfaces cannot derive directly from more than one interface. The following is not legal in COM: IFastString* CreateFastString(const char *psz) { IFastString *pfsResult = new FastString(psz); if (pfsResul t) pfsResult->DuplicatePointer(); return pfsResult; }

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eIP ISBN: 0-201-63446-5 Text printed on recycled and acid-free paper. 2 3 4 5 6 7 8 9 10-MA-01009998 Second Printing, February 1998. object, uuid(BDA4A270-A1BA-lldO-8C2C-0080C73925BA)] interface ICalculator : IBaselnterface { HRESULT Clear(void); HRESULT Add([in] long n); HRESULT Sum([out, retval] long *pn); }works to the C++ programmer's advantage, as C++ can produce code that is potentially more performant than languages that require a runtime layer to deal with COM. Unlike C++ programmers, Visual Basic and Java programmers never see Querylnterface, AddRef, or Release. For these two languages, the details of IUnknown are hidden deep below each language's supporting virtual machine. In Java, Querylnterface simply maps to a type-cast: Interfaces and IDL Method definitions in IDL are simply annotated C function prototypes. Interface definitions in IDL require an extension to C, as C has no intrinsic support for the concept. The IDL interface keyword is used to begin the definition of an interface. The interface definition has four components: the interface name, the base interface name, the interface body, and the interface attributes. The interface body is simply a collection of method definitions and supporting type definition statements: