Why do I need an instance of Odbc reader, but now Odbc connection?

On the class level, I have declared:

System::Data::Odbc::OdbcConnection 开发者_运维百科conn;
System::Data::Odbc::OdbcDataReader datareader; //doesnt work
System::Data::Odbc::OdbcDataReader^  datareader; //works

However, the dataReader has to be declared as OdbcDataReader^. I dont understand why.

With OdbcConnection conn; you are instantiating a connection object directly (on the stack). Not sure if that is a good idea but it is possible.

You cannot do that with a OdbcDataReader datareader; because that class only has hidden (private/internal) constructors. That is by design, you are not supposed to create DataReaders directly but instead get them from a call to ExecuteReader(). And ExecuteReader returns OdbcDataReader^ . See MSDN.

I'm not sure if this answers your question but here goes:

Connection and DataReader classes come in pairs on the .NET Framework, depending on the database technology used. So you have OdbcConnection and OdbcDataReader, also SqlConnection and SqlDataReader, etc. You have always to use them in pairs. Note anyway that all of these implement the common interfaces IDataConnection and IDataReader.

EDIT. Fine, I completely misunderstood the question. :-/

I usually use C#, not C++, but I think that it is because you can directly create a new instance of OdbcConnection, but as for the OdbcDataReader, you need to obtain an instance by executing the ExecuteReader method on the corresponding OdbcCommand. ExecuteReader returns a pointer to a new OdbcDataReader object.