object, uuid(DF12E152-A29A-l1dO-8C2D-0080C73925BA)] interface ICat : IAnimal { HRESULT 19noreHumans(void); } Traditionally, C++ libraries have been distributed in source code form. The users of a library would be expected to add the implementation source files to their make system and recompile the library sources locally using their C++ compiler. Assuming that the library adhered to a commonly supported subset of the C++ programming language, this was a perfectly workable approach. The net effect of this procedure was that the executable code of the library would be bundled as part of the overall client application. Assume that for the FastStri ng class just shown, the generated machine code for the four methods occupied 16MB worth of space in the target executable image (remember, to perform 0(1) search, a lot of code might be needed given the standard time versus space trade-off that binds most algorithms). As shown in Figure 1.1, if three applications use the FastStri ng library, each of the three executables will contain the 16MB worth of code. This means that if an end-user installs all three client applications, the FastSt ri ng implementation occupies 48MB worth of disk space. Worse yet, if the end-user runs the three client applications simultaneously, the FastStri ng code occupies 48MB worth of virtual memory, as the operating system cannot detect the duplicate code that is present in each executable image. One additional problem with this scenario is that once the library vendor finds a defect in the FastStri ng class, there is no way to field-replace the implementation. Once the FastStri ng code is linked into the client application, one can no longer simply replace the FastStri ng code directly at the enduser's machine. Instead, the library vendor must broadcast source code updates to the developer of each client application and hope that they will rebuild their applications to take advantage of the repairs that were made. Clearly, the modularity of the FastStri ng component is lost once the client runs the linker and produces the final executable.

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IFastString* CreateFastString(const char *psz) { IFastString *pfsResult = new FastString(psz); if (pfsResul t) pfsResult->DuplicatePointer(); return pfsResult; } include "ifaststring.h" i nt f(voi d) { int n = -1; IFastString *pfs = CreateFastString("Hi Bob!"); if (pfs) { n = pfs->Find(" ob"); pfs->Delete(); } class IExtensibleObject { public: virtual void *Dynamic_Cast(const char* pszType) virtual void DuplicatePointer(void) = 0; virtual void DestroyPointer(void) = 0; object, uuid(DF12E153-A29A-lldO-8C2D-0080C73925BA)] interface IOog : IAnimal { HRESULT Bark(void); }Dynamic versus Static Invocation 350 Bidirectional Interface Contracts 356 Aliasing in IDL 370 Asynchronous Methods 375 Where Are We? 375

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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 }To Judith S., who helped me master the one thing more daunting than COM and made this book possible, and Barbara, who stayed long enough to see how it all turned out. import "unknwn.idl"; [object, uuid(DF12E151-A29A-l1dO-8C2D-00BOC73925BA)] interface IAnimal : IUnknown { HRESULT Eat(void); } Methods and Results Method results are one aspect of COM where the logical and physical worlds diverge. Virtually all COM methods physically return an error number of type HRESULT. The use of a uniform result type allows COM's remoting architecture to overload the result of a method and also indicate communications errors simply by reserving a range of values for RPC errors. HRESULTs are 32-bit integers that provide information to the caller's runtime environment about what type of error may have occurred (e.g., network errors, server failures). For many COM-compatible implementation languages (e.g., Visual Basic, Java), these HRESULTs are intercepted by a supporting runtime or virtual machine and mapped to programmatic exceptions.

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These 32 hexadecimal digits represent the 128-bit value of the GUID. Naming interfaces and implementations using GUIDs is important for avoiding name collisions across multiple components. To create a new GUID, COM exposes an API function that uses a decentralized uniqueness algorithm to produce a new 128-bit value that will never again occur in nature: HRESULT CoCreateGuid(GUID *pguid);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- Using COM Interface Pointers c++ programmers must use the methods of IUnknown explicitly because the c++ language mapping of COM does not provide a runtime layer between the client's code and the object's code. To this end, IUnknown is simply a set of promises that all COM programmers make to one another. In general, this