通过判断高字节的第1位从而知道是ASCII或者汉字编码。

//类声明
class CChineseCode
{
  public:
      static void UTF_8ToUnicode(wchar_t* pOut,char *pText);  			 //把UTF-8转换成Unicode
      static void UnicodeToUTF_8(char* pOut,wchar_t* pText);  			 //Unicode 转换成UTF-8
      static void UnicodeToGB2312(char* pOut,wchar_t uData);  			 //把Unicode 转换成 GB2312
      static void Gb2312ToUnicode(wchar_t* pOut,char *gbBuffer);		 //GB2312 转换成　Unicode
      static void GB2312ToUTF_8(string& pOut,char *pText, int pLen); //GB2312 转为 UTF-8
      static void UTF_8ToGB2312(string &pOut, char *pText, int pLen);//UTF-8 转为 GB2312
};

//类实现
void CChineseCode::UTF_8ToUnicode(wchar_t* pOut,char *pText)
{
   char* uchar = (char *)pOut;

   uchar[1] = ((pText[0] & 0x0F) << 4) + ((pText[1] >> 2) & 0x0F);
   uchar[0] = ((pText[1] & 0x03) << 6) + (pText[2] & 0x3F);

   return;
}


void CChineseCode::UnicodeToUTF_8(char* pOut,wchar_t* pText)
{
   // 注意 WCHAR高低字的顺序,低字节在前，高字节在后
   char* pchar = (char *)pText;

   pOut[0] = (0xE0 | ((pchar[1] & 0xF0) >> 4));
   pOut[1] = (0x80 | ((pchar[1] & 0x0F) << 2)) + ((pchar[0] & 0xC0) >> 6);
   pOut[2] = (0x80 | (pchar[0] & 0x3F));

   return;
}

void CChineseCode::UnicodeToGB2312(char* pOut,wchar_t uData)
{
   WideCharToMultiByte(CP_ACP,NULL,&uData,1,pOut,sizeof(wchar_t),NULL,NULL);
   return;
}

void CChineseCode::Gb2312ToUnicode(wchar_t* pOut,char *gbBuffer)
{
   ::MultiByteToWideChar(CP_ACP,MB_PRECOMPOSED,gbBuffer,2,pOut,1);
   return ;
}

void CChineseCode::GB2312ToUTF_8(string& pOut,char *pText, int pLen)
{
   char buf[4];
   int nLength = pLen* 3;
   char* rst = new char[nLength];

   memset(buf,0,4);
   memset(rst,0,nLength);

   int i = 0;
   int j = 0;
   while(i < pLen)
   {
           //如果是英文直接复制就可以
           if( *(pText + i) >= 0)
           {
                   rst[j++] = pText[i++];
           }
           else
           {
                   wchar_t pbuffer;
                   Gb2312ToUnicode(&pbuffer,pText+i);

                   UnicodeToUTF_8(buf,&pbuffer);

                   unsigned short int tmp = 0;
                   tmp = rst[j] = buf[0];
                   tmp = rst[j+1] = buf[1];
                   tmp = rst[j+2] = buf[2];

                   j += 3;
                   i += 2;
           }
   }
   rst[j] = '';

   //返回结果
   pOut = rst;
   delete []rst;

   return;
}

void CChineseCode::UTF_8ToGB2312(string &pOut, char *pText, int pLen)
{
   char * newBuf = new char[pLen];
   char Ctemp[4];
   memset(Ctemp,0,4);

   int i =0;
   int j = 0;

   while(i < pLen)
   {
       if(pText > 0)
       {
               newBuf[j++] = pText[i++];
       }
       else
       {
               WCHAR Wtemp;
               UTF_8ToUnicode(&Wtemp,pText + i);
               UnicodeToGB2312(Ctemp,Wtemp);
               newBuf[j] = Ctemp[0];
               newBuf[j + 1] = Ctemp[1];
               i += 3;
               j += 2;
       }
   }
   newBuf[j] = '';
   pOut = newBuf;
   delete []newBuf;
   return;
}

C++ UTF8编码转换 CChineseCode