win32 wstring <-> string

static std::string w2c(std::wstring str)
  {
    int nlength = str.length();
    int nbytes = WideCharToMultiByte(0,0,str.c_str(),nlength,NULL,0,NULL,NULL);
    if(nbytes == 0) return "";

    char*buff = new char[nbytes+1];
    WideCharToMultiByte(0,0,str.c_str(),nlength,buff,nbytes,NULL,NULL);
    buff[nbytes] = '\0';
    std::string ret_str = std::string(buff);
    delete [] buff;
    return ret_str;   
  }

static std::wstring c2w(std::string str)
  {
    if(str.length() == 0) return std::wstring();

    int nu = str.length();
    size_t n =(size_t)MultiByteToWideChar(CP_ACP,0,str.c_str(),nu,NULL,0);
    wchar_t*wbuff = new wchar_t[n+1];
    MultiByteToWideChar(CP_ACP,0,str.c_str(),(int)nu,wbuff,(int)n);
    wbuff[n] = 0;

    std::wstring wstr_ret = std::wstring(wbuff);
    delete []wbuff;
    return wstr_ret;
  }