Delphi调用VC++6.0编写的Dll

用VC++6.0编写了一个简单的dll，里面包含一个减法函数subtract(int a,int b)，Dll命名为ff.Dll
代码如下：
1.ff.cpp:
// ff.cpp : Defines the entry point for the DLL application.
//
#include "StdAfx.h"
#include "ff.h"
BOOL APIENTRY DllMain( HANDLE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
      )
{
    switch (ul_reason_for_call)
 {
  case DLL_PROCESS_ATTACH:
  case DLL_THREAD_ATTACH:
  case DLL_THREAD_DETACH:
  case DLL_PROCESS_DETACH:
   break;
    }
    return TRUE;
}
// This is an example of an exported variable
FF_API int nFf=0;
// This is an example of an exported function.
FF_API int fnFf(void)
{
 return 42;
}
// This is the constructor of a class that has been exported.
// see ff.h for the class definition
CFf::CFf()
{
 return;
}
FF_API int subtract(int a,int b)
{
 return (a-b);
}
2.StdAfx.cpp:
// stdafx.cpp : source file that includes just the standard includes
// ff.pch will be the pre-compiled header
// stdafx.obj will contain the pre-compiled type information
#include "stdafx.h"
#ifdef FF_EXPORTS
#define FF_API __declspec(dllexport)
#else
#define FF_API __declspec(dllimport)
#endif
// This class is exported from the ff.dll
class FF_API CFf {
public:
 CFf(void);
 // TODO: add your methods here.
};
extern FF_API int nFf;
FF_API int fnFf(void);
extern "C" FF_API int subtract(int a,int b);
// TODO: reference any additional headers you need in STDAFX.H
// and not in this file
3.ff.h:
// The following ifdef block is the standard way of creating macros which make ex