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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:

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place was to call del ete thi s, it is legal to optimize away the reference count for non-heap-based objects:

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class IFastString2 : public IFastString { public: II real version 2.0 virtual int FindN(const char *psz, int n) if we reach this point, *ppv is non-null and must be AddRef'ed (guideline A2) reinterpret_cast (*ppv)->AddRef(); return S_OK; Optimizing Querylnterface The de facto implementation of Querylnterface shown earlier in this chapter is fairly straightforward and easily maintained by anyone who has a basic understanding of COM and C++. However, many production environments and frameworks favor a data-driven implementation to achieve greater extensibility and better performance due to code size reduction. Such implementations assume that each COM-compliant class provides a table that maps each supported lID onto some aspect of the object using fixed offsets or some other technique. In essence, the implementation of Querylnterface shown earlier in this chapter builds a table based on the compiled machine code for each sequential if statement and the fixed offsets are calculated using the stati c_cast operator (stat; c_cast simply adds the offset of the base class to find the typecompatible vptr). To implement a table-driven Querylnterface, one first needs to define what the table will contain. At a minimum, each table entry will need to contain a pointer to an lID and some additional state that allows the implementation to find the object's vptr for the requested inte

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set pUnk2 to point to first object if (pUnk2) pUnk2->Release(); if (pUnk2 = pUnkl) pUnk2->AddRef(); requires all persistent objects to support the Length and Find operations as well. For some very small subset of objects, this might make sense. However, to make the IPers; stentObject interface as generic as possible, it should really be its own interface and not derive from IFastStri ng: class IPersistentObject { public: virtual void Delete(void) = 0; virtual bool Load(const char *pszFileName) virtual bool Save(const char *pszFileName) pfs = CreateFastString("Feed BOB"); if (pfs) { ppo = (IPersistentObject *) pfs->Dynamic_Cast("IPersistentObject"); if (ppo) { ppo->Save("C:\\autoexec.bat"); ppo->DestroyPointer(); } object, uuid(DF12E156-A29A-lldO-8C2D-0080C73925BA)] interface ICatDog : ICat, IDog { II illegal, multiple bases HRESULT Meowbark(void); } S_OK - successful normal operation S_FALSE - used to return logical false successfully E_FAIL - generic failure E_NOTIMPL - method not implemented E_UNEXPECTED - method called at incorrect timeenum COLOR { RED, GREEN, BLUE }; the vl_enum attribute applies to the enumeration definition COLOR. This particular attribute informs the IDL compiler that the network representation for COLOR should be 32 bits, not 16 bits, which is the default. The hel pstri ng attribute also applies to COLOR and injects the string "This is a color!" into the II calculator.idl [object, uuid(BDA4A270-AIBA-lldO-BC2C-OOBOC73925BA)] interface ICalculator : IUnknown { import "unknwn.idl"; II bring in def. of IUnknown HRESULT Clear(void); HRESULT Add([in] long n); HRESULT Sum([out, retval] long *pn); } FastString::FastString(const char *psz) m_cch(strlen(psz)), m_psz(new char[m_cch + 1]) { strcpy(m_psz, psz); } short sum = 10; short s = itf.Method2(20); sum += s; If the HRESULT returned by the method indicates an abnormal result, the Java Virtual Machine will map the HRESULT to a Java exception. The C++ code fragment needs to manually inspect the HRESULT returned by the method and handle the abnormal result accordingly. 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; } ;

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if (riid == IID_IUnknown) *ppv = static_cast (this); As was shown in the case of FastSt ri ng and IExtensi b1eObj ect from the previous chapter, this code will not compile because the cast is ambiguous and could be satisfied by using more than one base class. Instead, the implementation needs to select a more precise type to use in the cast:

class IFastString { public: virtual void Delete(void) = 0; virtual int Length(void) = 0; virtual int Find(const char *psz) Recent issues of C++ Report are available at http://www.develop.comJdbox/cxx/lnterfacePtr.htm and http://www.develop.comJdbox/cxx/SmartPtr.htm.