怎么在DLL使用类.doc

IntroductionI have seen quite a lot of code explaining how to use classes exported from a DLL in an application. However, all these describe the usage of the exported classes by linking implicitly to the DLL. Refreshing our DLL concepts, there are two ways for an application to use a function written in a DLL. The first way is to have your applications source code simply reference symbols contained in the DLL. This causes the loader to implicitly load (and link) the required DLL when the application is invoked. This is known as implicit linking.The second way is for the application to explicitly load the required DLL (using a LoadLibrary() call) and explicitly link to the desired exported symbol while the application is running. In other words, if the application decides that it wants to call a function in a DLL, it can explicitly load the DLL into the process address space, get the virtual memory address of the function contained within the DLL, and then call the function using this memory address. The beauty of this technique is that everything is done while the application is running and the application can unload the DLL from its process address space when it has finished its work with the DLL. As you might have guessed, this technique is known as explicit linking.BackgroundSo far, I spoke of using functions, but hey, what about using classes exported from a DLL? Well, in the case of implicitly linked DLLs, there is no difference at all. But what about loading DLLs explicitly and using the exported classes? Well, under normal circumstances, it cannot be done, but I wrote this article not to explain to you why it cannot be done, but to give you an idea as to how you can do it. Thats right! Using exported classes by loading a DLL using a LoadLibrary() call.But before proceeding further, I warn you that the method given below is sort of a hack, and if for any reason you plan to use it in your project, please take the prior approval of your boss . (if by any chance you do manage to get his/her approval on this technique!). However, this column is basically for your understanding and also for extreme cases when you just cant do without this hack.Using the codeIf you look at the sample code, you can see that I have created a Calculator DLL called Calc.DLL and I am using the calculating powers present in the DLL in my console application called UserOfCalc (I couldnt think of a better name!).Collapse/ Calc.DLL contains an exported class/ called CCalc that contains 3 methods called Add,Sub and GetLastFunc (). It is as follows:/ CALC.H - declares the CCalc class/ that is exported from the DLL/ and is imported in the EXE#include #ifdef CALC_EXPORTS#define CALC_API _declspec (dllexport)#else#define CALC_API _declspec (dllimport)#endif#define SOME_INSTN_BUF 260class CALC_API CCalcprivate:TCHAR m_szLastUsedFuncSOME_INSTN_BUF;public: CCalc (); int Add (int i, int j); int Sub (int i, int j); TCHAR* GetLastUsedFunc ();The implementation of this DLL is as shown in the file Calc.cpp:Collapse#include Calc.h#include BOOL APIENTRY DllMain (HANDLE, DWORD, LPVOID) return TRUE;/ Ctor, initializes the m_szLastFuncCalled arrayCCalc:CCalc () memset (m_szLastUsedFunc, 0, sizeof (m_szLastUsedFunc); strcpy (m_szLastUsedFunc, No function used yet);int CCalc:Add (int i, int j) strcpy (m_szLastUsedFunc, Add used); return (i + j);int CCalc:Sub (int i, int j) strcpy (m_szLastUsedFunc, Sub used); return (i - j);Now, how do we use the functions present in this Calc class by explicitly loading the DLL? The steps are as follows:1. The first step is to load the Calc.DLL library in your application using LoadLibrary. CollapseHMODULE hMod = LoadLibrary (Calc.dll);if (NULL = hMod) printf (LoadLibrary failedn); return 1;2. Since you have the header file of Calc.DLL, the next step is to allocate a block of memory that matches the class layout, and call your constructor code. CollapseCCalc *pCCalc = (CCalc *) malloc (sizeof (CCalc);if (NULL = pCCalc) printf (memory allocation failedn); return 1;But why in the C+ world are we using malloc instead of new! Because the new operator calls CCalcs constructor for which we dont have any access. Remember, we have to load the DLL dynamically and hence there is no definition of CCalcs constructor available to us at build time.Hence we just obtain an uninitialized block of memory whose size equals the CCalc class size.3. If you look up the exported functions in Dumpbin.exe (thats located under your Microsoft Visual StudioVC98Bin directory), and type dumpbin /exports, you will see a list of functions exported by the DLL. (By the way, I have used a DEF file to unmangle the mangled function names.) It is as shown in the figure. The list contains the virtual memory address of the functions Add, Sub, GetLastUsedFunc and the constructor.Since we obtained the block of memory, we have to call the constructor to initialize the block of memory. So, we get the relative virtual address of the constructor in the DLL.CollapsePCTOR pCtor = (PCTOR) GetProcAddress (hMod, CCalc);if (NULL = pCtor) printf (GetProcAddress failedn); return 1;PCTOR is a function pointer and is present at the top of UserOfCalc.cpp. It is defined as follows:Collapsetypedef void (WINAPI * PCTOR) ();4. Since we have the address of the constructor, we have to explicitly call it to initialize the block of memory obtained by malloc. Yes, but how do we associate an object for the constructor? If you remember, when any member function is called, including the constructor, the address of the object gets quietly passed to the called function and this address is stored in the stack. On an Intel based machine, this address of the object is pushed onto the stack via the ECX register. So, if you create a class and call its member function, the ECX register contains the this pointer. This screen shot should make things clearer.If you observe the disassembly window, just after the execution of the line: CollapseLEA ECX, EBP-4you will see that the contents of ECX and &bmw are the same. On a machine having a different processor architecture, it could be another register instead of ECX. We just have to figure that out.5. Coming back to our Calc.dll, since we already have the address of a block of memory (that will in the future be an object), we move this address into the ECX register by using the Visual C+ inline assembler syntax: Collapse_asm MOV ECX, pCCalc 6. Since we have already obtained the address of the constructor, we just say: CollapsepCtor ();7. When your function pointer pCtor() returns from the DLL, it would have initialized the object of the class contained in the DLL. Voila! 8. To call any other member function of the Calc class, once again move pCalc to ECX and obtain the proc address of the exported function and simply