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: We've also grown our team but have always maintained the ethos the business was founded on - for sewers by sewers. Our passionate staff set us apart by offering excellent before and after sales service because they love and use our class IExtensibleObject { public: virtual void *Dynamic_Cast(const char* pszType) =0; virtual void DuplicatePointer(void) = 0; virtual void DestroyPointer(void) = 0; };

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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); The [1 oca1] attribute suppresses the generation of networking code for this interface. This attribute is necessary to relax COM's requirement that all remotable methods must return HRESULTs. As discussed in later chapters, IUnknown is treated specially when dealing with remote objects. Note that the actual IDL interface definitions found in the SDK headers will vary slightly from the definitions found in this text. The actual definitions often contain additional attributes to optimize the generated networking code that are not relevant to the discussion at hand. When in doubt, consult the latest version of the SDK header files for the complete definition. IUnknown is the root of all COM interfaces. IUnknown is the only COM interface that does not derive from another COM interface. Every other legal COM interface must derive from IUnknown directly or from one other legal COM interface, which itself must derive either from IUnknown directly or from one other legal COM interface. This means that at the binary level all COM interfaces are pointers to vtbls that begin with the three entries Que rylnte rface, AddRef, and Release. Any interface-specific methods will have vtbl entries that appear after these common three entries. To derive from an interface in IDL, either define the base interface in the same IDL file or use the import directive to make the external IDL definition of the base interface visible in the current scope:With these two modifications in place, the corresponding client code now becomes much more uniform and unambiguous: II calculator_i.c - generated by MIDL const lID IID_ICalculator = { OxBDA4A270, OxAIBA, OxlldO, {OxBC, Ox2C, OxOO, OxBO, OxC7, Ox39, Ox25, OxBA }; Every project that will use this interface must either add cal cul ator_i. c to its makefile or include cal cul ator_i. c in one C or C++ source file using the C preprocessor. If this is not done, then the symbol IID_ICalculator will not have storage allocated for its 128-bit value and the project will not link due to unresolved external symbols. COM imposes no limitations on the depth of an interface hierarchy, provided that the terminal. base interface is IUnknown. The following IDL is completely legal and reasonable COM:

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Clients can interrogate the object reliably at runtime to discover whether or not the object is IFastStri ng2 compatible by using C++'s dynami c_cast operator: 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. 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 theFunction Method2(argl as Integer) As Integer Because C++ uses no supporting runtime to access COM interfaces, the Microsoft C++ mapping of this method looks like this: 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]: ifaststring.h /////////// class IFastString { public: virtual void Delete(void) = 0; virtual int Length(void) const = 0; virtual int Find(const char *psz) const Using COM Interface Pointers c++ programmers must use the methods of IUnknown explicitly because the c++ language mapping of COM does not provide a runtime layer between the client's code and the object's code. To this end, IUnknown is simply a set of promises that all COM programmers make to one another. In general, this faststring.h ///////////////////////////// class FastString { char *m_psz; public: FastString(const char *psz); ~FastString(void);