Essential COM

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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 S_OK - successful normal operation S_FALSE - used to return logical false successfully E_FAIL - generic failure E_NOTIMPL - method not implemented E_UNEXPECTED - method called at incorrect time Resource Management and IUnknown As was the case with Dupl i catePoi nter and DestroyPoi nter from the previous chapter, IUnknown's AddRef and Re 1ease methods have a very simple protocol that all interface pointer users must adhere to. These rules free the client from managing the lifetime of an object when multiple interface pointers may or may not point to the same object. Clients need only follow the simple AddRef/Rel ease rules uniformly for every interface pointer that they encounter and the object will manage its own lifetime. The Component Object Model Specification contains very precise definitions of the reference counting rules of COM. Understanding the motivation behind these definitions is critical for mastering COM programming in C++. COM's reference counting rules can be distilled down to the following three simple axioms: 1. When a non-null interface pointer is copied from one memory location to another, AddRef should be called to notify the object of the additional reference. 2. Re 1 ease must be called prior to overwriting a memory location that contains a non-null interface pointer to notify the object that the reference is being destroyed. Figure 2.4 shows the standard notation for the CatDog class. Note that the only inference one can make from this diagram is that, barring catastrophic failure, CatDog objects will expose the four interfaces ICat, 1D09, IAnimal, and IUnknown. extern "C" const lID IID_IUnknown; interface IUnknown { virtual HRESULT STDMETHODCALLTYPE Querylnterface(REFIID riid, void **ppv) = 0; virtual ULONG STDMETHODCALLTYPE AddRef(void) = 0; virtual ULONG STDMETHODCALLTYPE Release(void) = 0; } ;

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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? 34 Visual Basic does not require clients to type-cast. Instead, when an interface pointer is assigned to a type-incompatible variable, the Visual Basic VM silently calls Querylnterface on behalf of the client:

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pfs = CreateFastString("Feed BOB"); if (pfs) { ppo = (IPersistentObject *) pfs->Dynamic_Cast("IPersistentObject"); if (ppo) { ppo->Save("C:\\autoexec.bat"); ppo->DestroyPointer(); } c++ has been with us for some time now. The community of C++ programmers is quite large, and much is known about the traps and pitfalls of the language. From its inception, C++ has benefited from a highly literate team of developers who, while working at Bell Laboratories, not only produced the first C++ development product (CFRONT) but also published many of the seminal works on C++. Most of the C++ canon was published in the late 1980s and early 1990s. During this era, many C++ developers (including the authors of virtually every important C++ book) worked on UNIX workstations and built fairly monolithic applications using the compiler and linker technology of the day. The environment in which this generation of developers worked has understandably Recent issues of C++ Report are available at http://www.develop.comJdbox/cxx/lnterfacePtr.htm and http://www.develop.comJdbox/cxx/SmartPtr.htm.

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place was to call del ete thi s, it is legal to optimize away the reference count for non-heap-based objects:

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lCat ''(pCat = 0; hr = pUnk->Querylnterface(llD_lCat, (void**)&pCat); if (SUCCEEDED(hr)) {II the object is Cat-compatible pCat->lgnoreMaster(); pCat->Release(); II R2 } } 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 } long arg2 = 20, arg3 = 30; p->Methodl(lO, &arg2, &arg3); the object cannot count on receiving the actual value of20 via parg2. If the object is running in the same execution context as the caller and both parties are implemented in C++, then ,o:parg2 will in fact contain the value of 20 on method entry. However, if the object is accessed from a different execution context or one party is implemented in a language that optimizes away the initialization of the out-only parameter, then the caller's initialization will be lost. attributel, attribute2, ... ] interface IThislnterface : IBaselnterface { typedef1; typedef2; method1; method2; }

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Figure 1.7 shows the runtime layout of FastStri ng. To use the FastStri ng data type, clients simply need to include the interface definition file and call CreateFastStri ng to begin working: When programming against the physical name of an interface in C or C++, the lID of a given interface is simply the interface's logical name prepended with the IID_ suffix. For example, the interface ICal cul ator would have an lID that could be manipulated programmatically using the IDL-generated constant IID_ICalculator. C++ namespaces can be used to deal with symbolic name collisions between interfaces. Because few C++ compilers can support 128-bit integers, COM defines a C structure to represent the 128-bit value of a GUID and provides aliases for the types lID and CLSID using typedefs: 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);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; } define END_INTERFACE_TABLE() \ { 0, 0, 0 } }; return table; } All that is required is a routine tha 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 addition of new techniques for finding interfaces beyond the normal offset calculation used when simply casting to a base class. The source code that accompanies this book includes a header file, i nttabl e. h, that defines interface table entries as follows: