Essential COM

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Recent issues of C++ Report are available at http://www.develop.comJdbox/cxx/lnterfacePtr.htm and http://www.develop.comJdbox/cxx/SmartPtr.htm. We offer a wide range of aids for the elderly, including crutches, wheelchair accessories, bathroom aids, toilet aids, kitchen aids anddressingaids. 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: lPug *pPug = 0; hr = punk->Querylnterface(llD_PPV-ARG(lPug, &pPug)); This macro closes the hole introduced by the vo; d ** parameter without imposing any runtime costs. severity>_ For example, the HRESULT STG_S_CONVERTED indicates that the facility code is FACILITY_STORAGE (which means that this result is related to Structured Storage or Persistence); the severity bit is SEVERITY_SUCCESS (which means that the call was able to perform the operation successfully) and that, in this case, the operation converted the underlying file to support structured storage. HRESULTs that are universal and are not tied to a particular technology use FACILITY_NULL and their symbolic name does not contain the facility code prefix. Some common FACILITY_NULL HRESULTs are

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class IFastString { public: virtual void Delete(void) = 0; virtual int Length(void) = 0; virtual int Find(const char *psz) int FindlOthBobCIFastString *pfs) { IFastString2 *pfs2=dynamic_cast (pfs); if (pfs2) II the object derives from IFastString2 return pfs2->FindN("Bob", 10); else { II object doesn't derive from IFastString2 error("Cannot find 10th occurrence of Bob"); return -1; } }

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Simply modifying the interface class to have additional virtual function declarations after the existing method declarations would result in a binary vtbl format that is a superset of the original version, as any new vtbl entries would appear after those that correspond to the original methods. Object implementations that compile against the new interface definition would have any additional method entries appended to the original vtbllayout: class IExtensibleObject { public: virtual void *Dynamic_Cast(const char* pszType) =0; virtual void Delete(void) = 0; } ; A client that wants to treat an object as a CatlDog simply uses Que rylnte rface to bind both types of pointers to the object. If one of the Querylnterface calls fails, then the object in question is not a Cat/Dog and the client can cope with this as it chooses. Because implementations can expose multiple interfaces, there is little loss of semantic or type information in prohibiting interfaces from having more than one base inheritance. COM supports a notation technique that expresses which interfaces are available from an object. This technique adheres to the COM philosophy of separation of interface from implementation and does not betray any implementation details of the object other than the list of interfaces it exposes. 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: 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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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); Interface and Implementation 95 Class Objects 97 Activation 100 Using the SCM 103 Classes and Servers 107 Generalizations 115 Optimizations 118 Interface and Implementation Again Monikers and Composition 131 Monikers and Persistence 134 Server Lifetime 138 Classes and IDL 141 Class Emulation 145 Competent Categories 147 Where Are We? 153 class IPersistentObject : public IExtensibleObject { public: virtual bool Load(const char *pszFileName) 0; virtual bool Save(const char *pszFileName) 0; } ; lCat cat; try { cat = (ICat)obj; II VM calls Querylnterface cat.lgnoreMaster(); } catch (Throwable ex) { II ignore method or QI failures } }

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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.

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generated type library as documentation for the enumeration. If one ignores the attributes in an IDL file, the syntax is simply that of C. IDL supports structures, unions, arrays, enumerations, and typedefs with a syntax identical to that of their C counterparts. When defining COM methods in IDL, one needs to indicate explicitly whether the caller or the callee will be writing or reading each method parameter. This is accomplished using the parameter attributes [i n] and [out]: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.

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class IPersistentObject { public: virtual void *Dynamic_Cast(const char *pszType) =0; virtual void Delete(void) = 0; virtual bool Load(const char *pszFileName) 0; virtual bool Save(const char *pszFileName) 0; } ; The Dynami c_Cast method was used for runtime type discovery and was analogous to C++'s dynami c_cast operator. The Dupl i catePoi nter method was used to notify the object that an interface pointer had been duplicated. The DestroyPoi nter method was used to notify the object that an interface pointer had been destroyed and any resources it held could be released. The following is the C++ definition of IUnknown: bool SaveString(IFastString *pfs, canst char *pszFN){ bool bResult = false; IPersistentObject *ppo = dynamic_cast (pfs); if (ppo) bResult = ppo->Save(pszFN); return bResult; }

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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: 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-dc21 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 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: