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

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To allow comparison of GUID values, COM provides equivalence functions and overloads operator == and operator != for constant GUID references: faststring.cpp /////////////////////////////////// #include #include "faststring.h" IFastString* (reateFastString (const char *psz) { return new FastString(psz); } Que rylnte rface describes the "shape" of the second parameter's pointer type. This relationship can be enforced at compile time through the use of the following C preprocessor macro: #define llD_PPV-ARG(Type, Expr) llD_##Type, \ reinterpret_cast (static_cast (Expr)) Given this macro,3 the compiler will ensure that the expression used in the following call to Que rylnte rface is of the correct type and that the appropriate level of indirection is in use: int m_cch; II count of characters char ";"'m_psz; public: FastString(const char *psz); ~FastString(void); ifaststring.h /////////// class IFastString { public: virtual void Delete(void) = 0; virtual int Length(void) const = 0; virtual int Find(const char *psz) constOur products include sewing patterns, sewing machines and accessories, fabrics, haberdashery, sewing tools and much more - everything you need to complete even the most ambitious sewing II faststringitf.cPPIII (part of DLL, not client) II #include "faststring.h" #include "faststringitf.h" FastStringltf::FastStringltf(const char *psz) m_pThis(new FastString(psz)) { assert(m_pThis != 0); } The MIDL compiler will also generate a C source file that contains the actual definitions of any GUIDs contained in the source IDL file: The [1 oca1] attribute suppresses the generation of networking code for this interface. This attribute is necessary to relax COM's requirement that all remotable methods must return HRESULTs. As discussed in later chapters, IUnknown is treated specially when dealing with remote objects. Note that the actual IDL interface definitions found in the SDK headers will vary slightly from the definitions found in this text. The actual definitions often contain additional attributes to optimize the generated networking code that are not relevant to the discussion at hand. When in doubt, consult the latest version of the SDK header files for the complete definition. IUnknown is the root of all COM interfaces. IUnknown is the only COM interface that does not derive from another COM interface. Every other legal COM interface must derive from IUnknown directly or from one other legal COM interface, which itself must derive either from IUnknown directly or from one other legal COM interface. This means that at the binary level all COM interfaces are pointers to vtbls that begin with the three entries Que rylnte rface, AddRef, and Release. Any interface-specific methods will have vtbl entries that appear after these common three entries. To derive from an interface in IDL, either define the base interface in the same IDL file or use the import directive to make the external IDL definition of the base interface visible in the current scope: return static_cast (this); else if (strcmp(pszType, "IPersistentObject") 0) return static_cast (this); else if (strcmp(pszType, "IExtensibleObject") 0) return static_cast (this); else return 0; II request for unsupported interface }

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At the time of this writing, the author did not have a working implementation of this algorithm suitable for publication. The details of such an implementation are left as an exercise for the reader. void DestroyPointer(void) { II destroy object when last pointer destroyed if (--m_cPtrs == 0) delete this; } Although there are obvious syntactic differences, the only significant difference between the two code fragments is that the Querylnterface-based version adheres to COM's reference counting rules. There are some subtleties regarding Querylnterface and its usage. Querylnterface can only return pointers to the same COM object. Chapter 4 Software Distribution and C++ 2 Dynamic Linking and C++ 5 C++ and Portability 6 Encapsulation and C++ 7 Separating Interface from Implementation Abstract Bases as Binary Interfaces 14 Runtime Polymorphism 21 Object Extensibility 23 Resource Management 31 Where Are We? 34faststring.h ///////////////////////////// class FastString { char *m_psz; public: FastString(const char *psz); ~FastString(void); Despite the fact that pCat, pPug, and pUnk all point to the same object, it is illegal for the client to counterbalance the AddRefs that occurred on pPug and pCat when Querylnterface was called with calls to Rel ease through pUnk. The correct form of this code is cleanup: if (pCat) pCat->Release(); II use AddRefed ptr if (pPug) pPug->Release(); II use AddRefed ptr Here, Release is called through exactly the same interface pointer that received the AddRef (which happened implicitly when the pointer was returned from QueryInterface). This requirement affords the developer a great deal of flexibility when implementing an object. For example, an object may elect to perform per-interface reference counting to allow aggressive reclamation ofre-