In 2004 a national award winning seamstress was struggling to find the haberdashery and sewing supplies she needed to enjoy her life long hobby. As a result, she worked together with her son to establish Sew Essential and solve this 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 With these two modifications in place, the corresponding client code now becomes much more uniform and unambiguous: To allow comparison of GUID values, COM provides equivalence functions and overloads operator == and operator != for constant GUID references:

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object, uuid(DF12E153-A29A-lldO-8C2D-0080C73925BA)] interface IOog : IAnimal { HRESULT Bark(void); } Library of Congress-in-Publication Data Box, Don, 1962Essential COM / Don Box. p. cm. Includes bibliographical references and index. ISBN 0-201-63446-5 (alk. paper) 1. Object-oriented programming (Computer science) 2. C++ (Computer program language) 1. Title. OA76.64.8693 1998 005.1-dc21lCat cat; try { cat = (ICat)obj; II VM calls Querylnterface cat.lgnoreMaster(); } catch (Throwable ex) { II ignore method or QI failures } } This technique works because the compiler has enough knowledge about the layout and type hierarchy of the implementation class to examine an object at runtime to determine whether it in fact derives from IPersistentObject. Therein lies the problem. RTTI is a very compiler-dependent feature. Again, the DWP mandates the syntax and semantics for RTTI, but each compiler vendor's implementation of RTTI is unique and proprietary. This effectively destroys the compiler independence that has been achieved by using abstract base classes as interfaces. This is unacceptable for a vendor-neutral component architecture. One very tractable solution to the problem is to leverage the semantics of dynami c_cast without using the actual compiler-dependent language feature. Exposing a well-known method explicitly from each interface that will perform the semantic equivalent of dynami c_cast can achieve the desired effect without requiring all entities to use the same C++ compiler: Software Distribution and C++ To understand the problems related to using C++ as a component substrate, it helps to examine how C++ libraries were distributed in the late 1980s. Consider a library vendor who has developed an algorithm that can perform substring searches on 0(1) time (i.e., the search time will be constant and not

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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. other versions may be present on the system. Unfortunately, over time, the number of versioned DLLs present on the end-user's system could conceivably exceed the number of actual client applications due to poor software configuration practices. Simply examining the system directory of any computer that has been in use for more than six months would reinforce this. Ultimately, this versioning problem is rooted in the compilation model of C++, which was not designed to support independent binary components. By requiring client knowledge of object layout, C++ introduces a tight binary coupling between the client and object executables. Normally, binary coupling works to C++'s favor, as it allows compilers to produce extremely efficient code. Unfortunately, this tight binary coupling prevents class implementations from being replaced without client recompilation. Because of this coupling and the compiler and linker incompatibilities mentioned in the previous section, simply exporting C++ class definitions from DLLs does not provide a reasonable binary component architecture. faststring.h /////////////////////////////////// #include "ifaststring.h" class FastString : public IFastString { const int m_cch; // count of characters char ''''m_psz; public: FastString(const char *psz); ~FastString(void);public void TryToSnoreAndlgnore(Object obj) { lPug pug; try { pug = (IPug)obj; II VM calls Querylnterface pug.Snore(); } catch (Throwable ex) { II ignore method or QI failures } With this type hierarchy in place, the client is able to query an object dynamically for a given interface using a compiler-independent construct: The term execution context is used by the COM specification to describe what has since been renamed an apartment. An apartment is neither a thread nor a process; however, it shares common attributes with both. Apartments are fully described in Chapter 5.

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class IFastString { public: virtual void *Dynamic_Cast(const char *pszType) =0; virtual void Delete(void) = 0; virtual int Length(void) = 0; virtual int Find(const char *psz) 0; } ; II calculator.idl [object, uuid(BDA4A270-AIBA-lldO-BC2C-OOBOC73925BA)] interface ICalculator : IUnknown { import "unknwn.idl"; II bring in def. of IUnknown HRESULT Clear(void); HRESULT Add([in] long n); HRESULT Sum([out, retval] long *pn); } short sum = 10; short s = itf.Method2(20); sum += s; If the HRESULT returned by the method indicates an abnormal result, the Java Virtual Machine will map the HRESULT to a Java exception. The C++ code fragment needs to manually inspect the HRESULT returned by the method and handle the abnormal result accordingly.const HRESULT CALC_E_IAMHOSED MAKE_HRESULT(SEVERITY_ERROR, FACILITY_ITF, Ox200 + 15); The convention for user-defined HRESULTs is to select information code values above Ox200 to avoid reusing values already used by system HRESULTs. Although not crucial, this avoids overloading values that already have meaning for standard interfaces. For example, most HRESULTs have text-based human-readable descriptions that can be resolved at runtime using the FormatMessage API function. By selecting HRESULTs that do not collide with system-defined values, there is no chance that an erroneous error message may be found. To allow methods to return a logical result that is not related to the method's physical HRESULT, COM IDL supports the retval parameter attribute. The retva1 attribute indicates that the associated physical method parameter is actually the logical result of the operation and, in environments that support this, the parameter should be mapped as the result of the operation. Given the following IDL method definition: IUnknown The COM interface IUnknown serves the same purpose as the IExtensi bl eObject interface defined in the previous chapter. The following is the final version of IExtensi bl eObject that appeared at the end of the chapter: class PugCat : public lPug, public lCat { LONG m_cRef; protected: virtual -PugCat(void); public: PugCat(void); II lUnknown methods STDMETHODIMP Query1nterface(REFI1D riid, void **ppv); STDMETHODlMP_(ULONG) AddRef(void); STDMETHODlMP_(ULONG) Release(void); II lAnimal methods STDMETHODlMP Eat(void); II 1Dog methods STDMETHOD1MP Bark(void);