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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 } } As previously described, clients call Querylnterface providing the object with a pointer to an interface pointer as the second parameter together with an lID that identifies the type of interface pointer that is expected: IPug *pPug = 0; hr = punk->QueryInterface(IID_IPug, (void**)&pPug); Unfortunately, the following looks equally correct to the C++ compiler: IPug *pPug = 0; hr = punk->QueryInterface(IID_ICat, (void**)&pPug); This more subtle variation also compiles correctly: IPug *pPug = 0; hr = punk->QueryInterface(IID_IPug, (void**)pPug); Given that the rules of inheritance do not apply to pointers, this alternative definition of QueryInterface does not alleviate the problem: HRESULT QueryInterface(REFIID riid, IUnknown** ppv); as implicit upcasting applies only to instances and pointers to instances, not pointers to pointers to instances: IDe ri ved *~'rppd; IBase **ppb = ppd; II illegal This same limitation applies to references to pointers as well. The following alternative definition is arguably more convenient for clients to use: HRESULT QueryInterface(const IID& riid, void* ppv); as it allows clients to forgo the cast. Unfortunately, this solution does not reduce the number of errors (both of the preceding errors are still possible) and, by eliminating the need for a cast, removes a visual indicator that C++ type safety might be in jeopardy. Given the desired semantics of QueryInterface, the argument types Microsoft chose are reasonable, if not type safe or elegant. The simplest way to avoid QueryInterface-related errors is to always be certain that the lID matches the type of the interface pointer that is passed as the second parameter to QueryInterface. In effect, the first parameter to they continue to work properly. The problem arises when new clients that expect IFastStri ng to have four methods happen to use older objects that do not implement this method. When the client calls the Fi ndN method on an object compiled against the original interface definition, the results are very well defined. The program crashes. The problem with this technique is that it breaks the encapsulation of the object by modifying the public interface. Just as changing the public interface to a C++ class can cause compile-time errors when the client code is rebuilt, changing the binary interface definition will cause runtime errors when the client code is reexecuted. This implies that interfaces must be immutable and cannot change once published. The solution to this problem is to allow an implementation class to expose more than one interface. This can be achieved either by designing an interface to derive from another related interface or by allowing an implementation class to inherit from several unrelated interface classes. In either case, the client could use C++'s Runtime Type Identification (RTTI) feature to interrogate the object at runtime to ensure that the requested functionality is indeed supported by the object currently in use. Consider the simple case of an interface extending another interface. To add a Fi ndN operation to IFastStri ng that allows finding the nth occurrence of a substring, one would derive a second interface from IFastStr.i ng and add the new method definition there:

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ride each of the pure virtual functions with meaningful implementations. This inheritance relationship will result in objects that have an object layout that is a binary superset of the layout of the interface class (which ultimately is just a vptr/vtbl). This is because the "is-a" relationship between derived and base class applies at the binary level in C++ just as it applies at the modeling level in object-oriented design: This solution almost works. Clients compiled against the original version of the interface are blissfully ignorant of any vtbl entries beyond the first three. When older clients get newer objects that have the vtbl entry for Fi ndN, The class's implementation of AddRef notes that the caller has duplicated an interface pointer by incrementing the reference count. The updated reference count is returned for diagnostic purposes:the interface class, as the position of the virtual destructor in the vtbl can vary from compiler to compiler. One workable solution to this problem is to add an explicit Delete method to the interface as another pure virtual function and have the derived class delete itself in its implementation of this method. This results in the correct destructor being executed. The updated version of the interface header file looks like this: 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. Note that the only modification is the addition of a private data member. To initialize this member properly, the constructor would need to be modified as follows: Our rings are sized using the UK sizing system. To convert your EU or US size to UK, use our conversion chart which can be found here.

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Assuming the client usage just shown, the required semantics and mechanism for type discovery are in place, but each implementation class must implement this functionality by hand: 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: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; } ;

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import "unknwn.idl"; [object, uuid(DF12E151-A29A-l1dO-8C2D-00BOC73925BA)] interface IAnimal : IUnknown { HRESULT Eat(void); }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 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 faststring.cpp /////////////////////////////////// #include #include "faststring.h" IFastString* (reateFastString (const char *psz) { return new FastString(psz); } To allow comparison of GUID values, COM provides equivalence functions and overloads operator == and operator != for constant GUID references: