Essential COM

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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; } ; short sum = 10; short s; HRESULT hr = pltf->Method2(20, &s); if (FAILED(hr)) throw hr; sum += s; is roughly equivalent to the following Java code: class IFastString { public: virtual void Delete(void) = 0; virtual int Length(void) = 0; virtual int Find(const char *psz)

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Component-Oriented Programming = Polymorphism + (Really) Late Binding + (Real, Enforced) Encapsulation + Interface Inheritance + Binary Reuse In any case, for me, the whole debate is kind of amusing. The 00 purists who live on comp.object and comp.object.corba get completely bent out of shape while pointing their fingers at COM saying "but it's not really object oriented." You can argue against this in two ways: 1. "It is too! It's just that your definition of 00 is wrong."shaped much of the mindset of the C++ community at large. One of the principal goals for C++ was to allow programmers to build user-defined types (UDTs) that could be reused outside their original implementation context. This principle underpins the idea of class libraries or frameworks as we know them today. Since the introduction of C++, a marketplace for C++ class libraries emerged, albeit rather slowly. One reason that this marketplace has not grown as fast as one might expect is related to the With the class definition in place, the vendor would implement the member functions in a separate source file:

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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: Virtually every production-quality C++ compiler currently in use uses the basic concepts of vptrs and vtbls. There are two basic techniques for laying out vtbls: the CFRONT technique and the adjustor thunk technique. Each of these two techniques has its own way of dealing with the subtleties of multiple inheritance. Fortunately, on any given platform, one technique tends to dominate (Win32 compilers use adjustor thunks, Solaris compilers use CFRONT-style vtbls). Fortunately, neither vtbl format affects the C++ source code that one must write but rather is an artifact of the generated code. Consult Stan Lippman's most excellent Inside the C++ Object Model for a great discussion of both techniques. Based on the assumptions made so far, it is now possible to solve the problem of compiler dependence. Assuming that all compilers on a given platform implement the virtual function call mechanism in the same manner, the C++ interface class can be defined to expose the public operations of the data type as virtual functions, confident that all compilers will generate equivalent machine code for client-side method invocations. This assumption of uniformity requires that no interface class can have data members and no interface class can derive directly from more than one other interface class. Because there are no data members in the interface class, one cannot implement these methods in any sensible manner. To reinforce this, it is useful to define the interface members as pure virtual functions, indicating that the interface class defines only the potential to call the methods, not their actual implementations: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. 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 } Interfaces and IDL Method definitions in IDL are simply annotated C function prototypes. Interface definitions in IDL require an extension to C, as C has no intrinsic support for the concept. The IDL interface keyword is used to begin the definition of an interface. The interface definition has four components: the interface name, the base interface name, the interface body, and the interface attributes. The interface body is simply a collection of method definitions and supporting type definition statements:

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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: Figure 2.2 HRESULTs As shown in Figure 2.2, HRESULTs are partitioned into three bit-fields: the severity bit, the facility code, and the information code. The severity bit indicates whether the operation succeeded or not, the facility code indicates what technology the HRESULT corresponds to, and the information code is the precise result within the specified technology and severity. The SDK headers define two macros that simplify coding with .HRESULTs: proportional to the length of the target string). This is a nontrivial task, admittedly. To make the algorithm as simple to use as possible, the vendor would create a string class based on the algorithm that would represent fast text strings in any client program. To do this, the vendor would prepare a header file that contains a class definition: II faststringitf.cPPIII (part of DLL, not client) II #include "faststring.h" #include "faststringitf.h" FastStringltf::FastStringltf(const char *psz) m_pThis(new FastString(psz)) { assert(m_pThis != 0); } object, uuid(BDA4A270-A1BA-lldO-8C2C-0080C73925BA)] interface ICalculator : IBaselnterface { HRESULT Clear(void); HRESULT Add([in] long n); HRESULT Sum([out, retval] long *pn); }

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Simplicity Leads to Malleability mal·le·a·ble (mal'e-e-bel) adjective 1. Capable of being shaped or formed, as by hammering or pressure: a malleable metal. 2. Easily controlled or influenced; tractable. 3. Able to adjust to changing circumstances; adaptable: the malleable mind of the pragmatist. 3 as the compiler and linker will initialize each vtbl to point to the single method implementation. This is a natural side effect of using multiple inheritance in C++. The following is a class definition that produces objects that support the IPug and lCat interfaces: Despite the fact that pCat, pPug, and pUnk all point to the same object, it is illegal for the client to counterbalance the AddRefs that occurred on pPug and pCat when Querylnterface was called with calls to Rel ease through pUnk. The correct form of this code is cleanup: if (pCat) pCat->Release(); II use AddRefed ptr if (pPug) pPug->Release(); II use AddRefed ptr Here, Release is called through exactly the same interface pointer that received the AddRef (which happened implicitly when the pointer was returned from QueryInterface). This requirement affords the developer a great deal of flexibility when implementing an object. For example, an object may elect to perform per-interface reference counting to allow aggressive reclamation ofre-