Here, I set m_Cnn to NULL because at this point there is no established connection.

bool Database::Open(char* UserName, char* Pwd, char* CnnStr)
{

   HRESULT hr;
   try
   {
      if(m_Cnn==NULL)
         return 0;
      hr    = m_Cnn.CreateInstance(
         __uuidof( ADODB::Connection ) );
      m_Cnn->Open(CnnStr, UserName, Pwd, NULL);
   }
   catch(_com_error &e)
   {
      //   Handle errors
      return 0;
   }
   return 1;
}

Here, __uuidof( ADODB::Connection ) returns the interface id that is used to create an instance of a Connection object. If a connection object is created successfully, the Open() method tries to establish a physical connection to the data source. Open() takes three parameters: connection string, user name, and password. If a physical connection is established, this function returns 1. I used a catch handler to catch the exception thrown by the methods of Connection object.

RecPtr Database::Execute(char* CmdStr)
{
   try
   {
      if(m_Cnn==NULL)
         return NULL;
      return m_Cnn->Execute(CmdStr,NULL,1);
   }
   catch(_com_error &e)
   {
      //   Handle errors
      return NULL;
   }
}

The Execute() method takes a valid SQL query and returns a smart pointer to the Recordset object.

bool Database::Close()
{
   //is there any established connection?
   //if no, return 0
   if (m_Con==NULL)
      return 0;
   //if any, close that connection
   try
   {
      m_Con->Close();
      m_Con=NULL;
   }
   catch(_com_error &e)
   {
      //   Handle errors
      return 0;
   }
   return hr==S_OK;
}


Database::~Database()
{
   try
   {
   //close connection, if not yet closed
      if (m_Con)
      {
         m_Con->Close();
         m_Con=NULL;
      }
   }
   catch(_com_error &e)
   {
      //   Handle errors
   }
}

Notice that you must initialize COM before using this class. You can do this by adding the following code before using this class.

   if(FAILED(::CoInitialize(NULL)))
      return;

It is good programming practice if you uninitialize COM. You can do this by using ::CoUninitialize() at the end of program.