Re: Is there a better way to access the data of a template class?

From:

"Scot T Brennecke" <ScotB@MVPs.spamhater.org>

Newsgroups:

microsoft.public.vc.mfc

Date:

Fri, 15 May 2009 23:37:15 -0500

Message-ID:

<u9lo#$d1JHA.1424@TK2MSFTNGP02.phx.gbl>

You chose to use a templated class to generalize for various data types of a column. By their very
nature, templated classes lead to uglier, harder to read syntax, despite their otherwise elegance.
Other implementations of similar things have chosen to use a discriminated union called a variant.
There are a number of classes that encapsulate the variant types:
_variant_t Class:
http://msdn.microsoft.com/en-us/library/x295h94e.aspx
CComVariant Class:
http://msdn.microsoft.com/en-us/library/ac97df2h.aspx
COleVariant Class:
http://msdn.microsoft.com/en-us/library/4xd0wkhw.aspx

"Walter Eicher" <walter.eicher@nospam.microcrystal.ch> wrote in message
news:5inq051cubr73i7skb47u5hfrqink4n8pu@4ax.com...

On Fri, 15 May 2009 14:01:11 +0200, Walter Eicher
<walter.eicher@nospam.microcrystal.ch> wrote:

a wrong version of class TSimpleDataBase

the correct one is here:

class TSimpleDataBase
{
public:

   void AddRecord (const VRecord& rec);
};

using namespace std;
void TSimpleDataBase::AddRecord (const VRecord& rec)
{
   LPCTSTR ctx = _T("TSimpleDB::AddRecord");

   if (mDB == NULL)
     throw TException (_T("DB not open!"), ctx);

   TFormat sci;
   sci.Scientific ();
   sci.Precision (16);

   wstring s = _T("INSERT INTO ") + mTableName + _T(" VALUES (");

   DBRecord dbr = rec.Record ();
   for (UInt i = 0; i < dbr .size (); ++i)
   {
     Tstringstream ss; // typedef basic_stringstream<TCHAR>

     VDBColumn* col = rec[i];

     switch (col->ColType ())
     {
     case eCol_DATE:
       {
         TTimeDate val = *static_cast<TTimeDate*>(col->DataPtr ());
         ss << val.GetTime ();
       }
       break;

     case eCol_INT:
       {
         Int val = *static_cast<Int*>(col->DataPtr ());
         ss << val;
       }
       break;

     case eCol_DOUBLE:
       {
         Double val = *static_cast<Double*>(col->DataPtr ());
         ss << sci(val);
       }
       break;

     case eCol_TEXT:
       {
         Tstring val = *static_cast<Tstring*> (col->DataPtr ());
         ss << _T("'") << val << _T("'");
       }
       break;

     default:
       throw TException (_T("create table, illegal column type!"),
                      ctx);
     }

     s += ss.str ();

     if (i < mRecordDef.size () - 1)
       s += _T(", ");
   }
   s += _T(")");

   sqlite3_stmt* stmt;

   int rc = sqlite3_prepare16_v2 (mDB, s.c_str (), -1, &stmt, NULL);
   if (rc != SQLITE_OK)
     throw TException (_T("prepare add record failed!"), ctx);

   sqlite3_step (stmt);
   if (sqlite3_finalize (stmt) != SQLITE_OK)
     throw TException (_T("add record failed!"), ctx);
}

Sorry
Walter