直接上程式碼：

#include <fstream>
#include <iostream>
#include <Windows.h>
#include <string>
using namespace std;

string UTF8ToGB(const char* str)
{
     string result;
     WCHAR *strSrc;
     LPSTR szRes;

     //獲得臨時變數的大小
     int i = MultiByteToWideChar(CP_UTF8, 0, str, -1, NULL, 0);
     strSrc = new WCHAR[i+1];
     MultiByteToWideChar(CP_UTF8, 0, str, -1, strSrc, i);

     //獲得臨時變數的大小
     i = WideCharToMultiByte(CP_ACP, 0, strSrc, -1, NULL, 0, NULL, NULL);
     szRes = new CHAR[i+1];
     WideCharToMultiByte(CP_ACP, 0, strSrc, -1, szRes, i, NULL, NULL);

     result = szRes;
     delete []strSrc;
     delete []szRes;

     return result;
}