Importing into an Application Using __declspec(dllimport)
A program that uses public symbols defined by a DLL is said to import them. When you create header files for applications that use your DLLs to build with, use __declspec(dllimport) on the declarations of the public symbols. The keyword __declspec(dllimport) works whether you export with .def files or with the __declspec(dllexport) keyword.
To make your code more readable, define a macro for __declspec(dllimport) and then use the macro to declare each imported symbol:
#define DllImport __declspec( dllimport )
DllImport int j;
DllImport void func();
Using __declspec(dllimport) is optional on function declarations, but the compiler produces more efficient code if you use this keyword. However, you must use __declspec(dllimport) for the importing executable to access the DLL's public data symbols and objects. Note that the users of your DLL still need to link with an import library.
You can use the same header file for both the DLL and the client application. To do this, use a special preprocessor symbol that indicates whether you are building the DLL or building the client application. For example:
#ifdef _EXPORTING
#define CLASS_DECLSPEC __declspec(dllexport)
#else
#define CLASS_DECLSPEC __declspec(dllimport)
#endif
class CLASS_DECLSPEC CExampleA : public CObject
{ ... class definition ... };