Essential COM

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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 return static_cast (this); is ambiguous because IFastStri ng and IPersi stentObject both derive from IExtensibleObject. If IExtensibleObject was a virtual base class of both IFastString and IPers;stentObject, then this cast would not be ambiguous and the statement would compile. However, introducing virtual base classes adds needless runtime complexity to the resultant object and also introduces compiler dependences. This is because virtual bases are yet another C++ language feature that has several proprietary implementations. With the class definition in place, the vendor would implement the member functions in a separate source file:

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all compilers on a given platform must produce equivalent machine code sequences for the following client code fragment: 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: 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: 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: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. Because each pointer is now treated as an autonomous entity with respect to lifetime, the client does not need to be concerned with which pointer refers to which object. Instead, the client simply follows the two simple rules and allows the object to manage its own lifetime. If desired, the idiom of calling Dupl i catePoi nter and DestroyPo; nter cOllld easily be hidden behind a C++ smart pointer. Using this reference counting scheme allows an object to expose multiple interfaces in an extremely uniform manner. The capability to expose multiple interfaces from a single implementation class allows a data type to participate in a variety of contexts. For example, a new persistence subsystem could define its own custom interface for telling objects to load and save themselves to some customized storage medium. The FastStri ng class could add support for this functionality simply by deriving from the subsystem's persistence interface. Adding this support does not in any way affect the installed base of clients that may be using the prior persistence interface to save and load a string to disk. Having a mechanism for runtime interface negotiation can act as a cornerstone for building a dynamic system from components that can evolve over time. if we reach this point, *ppv is non-null and must be AddRef'ed (guideline A2) reinterpret_cast (*ppv)->AddRef(); return S_OK; late to object layout. Because all member functions in COM interface definitions are pure virtual, the derived class must provide an implementation of every method that is present in any of its interfaces. Methods that are common to two or more interfaces (e.g., Querylnterface, AddRef, etc.) need to be implemented only once, 3 Which was highly inspired by a discussion between the author and Tye McQueen during a COM seminar. int m_cch; II count of characters char ";"'m_psz; public: FastString(const char *psz); ~FastString(void);

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Type Coercion and IUnknown The previous chapter described the motivations for needing runtime type discovery in a dynamically composed system. The C++ language provides a reasonable mechanism for dynamic type discovery through its dynami c_cast operator. Although this language feature has a proprietary implementation for each compiler, the previous chapter was able to leverage the concept by adding an explicit method to each interface that performs the semantic equivalent of dynami c_cast. COM's IUnknown interface also has such a method and it is called Querylnterface. The following is the IDL description of Que rylnte rface: HRESULT Querylnterface([in] REFIID riid, [out] void **ppv); 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. To allow comparison of GUID values, COM provides equivalence functions and overloads operator == and operator != for constant GUID references:

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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 include "smartif.h" void TryToSnoreAndIgnoreC/* [in] *1 IUnknown *pUnk) { II copy constructor calls Querylnterface Smartlnterface pPug = pUnk; FastString::FastString(const char *psz) m_cch(strlen(psz)), m_psz(new char[m_cch + 1]) { strcpy(m_psz, psz); } 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; } ; 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:

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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: define SUCCEEDED(hr) (long(hr) >= 0) #define FAILED(hr) (long(hr) < 0) These two macros take advantage of the fact that the severity bit is also the sign bit when treating an HRESULT as a signed integer. The SDK headers contain the definitions of all standard HRESULTs. These HRESULTs have symbolic names that correspond to the three components of an HRESULT using the following format: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 first parameter (ri i d) is the physical name of the interface being requested. The second parameter (ppv) points to an interface pointer variable that on successful completion will contain the requested interface pointer. In response to a Querylnterface request, if the object does not support the requested interface type, it must return E_NOINTERFACE after setting *ppv to null. If the object does support the requested interface, it must overwrite *ppv with a pointer of the requested type and return the HRESULT S_OK. Because ppv is an [out] parameter, the implementation of Querylnterface must AddRef the resultant pointer prior to returning control to the caller (see guideline A2 from earlier in this chapter). This AddRef call must be matched by a Re1ease call from the client. The following shows runtime type discovery using C++'s dynami c_cast operator based on the Dog/Cat type hierarchy described earlier in this chapter: Note that all but one of the entry points into the FastSt ri ng DLL are virtual functions. The virtual functions of the interface class are always called indirectly via a function pointer stored in a vtbl, without requiring the client to link against their symbolic names at development time. This means that the interface methods are immune to symbolic mangling differences between compilers. The only entry point that is explicitly linked against by name is CreateFastStri ng, the global function that bootstraps the client into the world of FastStri ng. Note, however, that this function was exported using extern "C", which suppresses symbolic mangling. This implies that all C++ compilers expect the import library and DLL to export the same symbol. The net result of using this technique is that one can safely expose a class from a DLL using one C++ environment and access the class from any other C++ development environment. This capability is critical for building a vendorneutral substrate for reusable components. Implementing IUnknown Given the client-side usage patterns that have been described, implementing the methods of lUnknown is fairly straightforward. Assume the Dog/Cat type hierarchy previously described. To define a C++ class that implements the lPug and lCat interfaces, one simply needs to add the most derived versions of each interface to the list of base classes: