You receive a C2564 error or a C2197 error when you port code that uses GetProcAddress from C to C++ (117428)

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In a traditional C application, use GetProcAddress() to obtain the address of a function to be called. Declare a variable of type FARPROC, initialize the pointer with the value returned from GetProcAddress(), and then call the function through a pointer as shown:

void func1(void)
{
   HINSTANCE       hLib;
   FARPROC         lpfnDLLProc;
   UINT            param1 = 1;
   int             param2;

   hLib = LoadLibrary ("dll1.dll");
   if (hLib)
   {
      lpfnDLLProc = GetProcAddress (hLib, "DLLProc");
      (*lpfnDLLProc) (param1, (LPINT)&param2);
      FreeLibrary (hLib);
   }
}
				

When not compiling with STRICT, FARPROC is defined in the WINDOWS.H file as follows:

typedef int (CALLBACK* FARPROC)();
				

When the sample code above is converted to C++, a type-mismatch error occurs because C and C++ have a fundamental difference in the way they interpret empty parentheses in function declarations. In C, a function declared as follows:

int (*funcptr)();
				

declares a function that accepts an unknown number of arguments. In C++, the same declaration represents a function that accepts no arguments. In other words, in C++, the statement is equivalent to:

int (*funcptr)(void);
				

Because of this difference, when a pointer of type FARPROC is used to call a function with parameters in C, no error occurs. In C++, when the function being passed to GetProcAddress() has parameters, the formal/actual-parameter-mismatch error occurs because the function of type FARPROC is defined as a function that has void parameters rather than as a function that accepts parameters.

To eliminate the error, define the function pointer as a pointer to a function with the correct number of parameters and then typecast the return value from GetProcAddress() to the appropriate type:

typedef void (CALLBACK *ULPRET)(UINT,LPINT);

void func1(void)
{
   HINSTANCE       hLib;
   ULPRET          lpfnDLLProc;
   UINT            param1 = 1;
   int             param2;

   hLib = LoadLibrary ("dll1.dll");
   if (hLib)
   {
      lpfnDLLProc = (ULPRET) GetProcAddress (hLib,"DLLProc");
      (*lpfnDLLProc) (param1, (LPINT)&param2);
      FreeLibrary (hLib);
   }
}