Using Smart Pointers with ADOCE

< BACK  NEXT >

[oR]

As discussed in Chapter 14 (COM and ActiveX), COM components are accessed most easily through smart pointer class templates. In Chapter 14 smart pointers were created using _COM_SMARTPTR_TYPEDEF after header (.h) files were used that included the interface definitions. An alternate, and sometimes easier, approach is to import a type library (TLB) file that includes all the interface definitions. The #import will generate smart pointer class templates for each interface contained in the TLB file.

You can create a TLB from the IDL files with the Microsoft Interface Definition Language compiler (MIDL.EXE). This is distributed with Microsoft Visual Studio and eMbedded Visual C++. You will need two TLB files for accessing databases ADOCE31.TLB and ADOXCE31.TLB. These TLB files may not be distributed with your development tool, so for your convenience copies have been placed on the CDROM in the directory \examples. If you need to build them from IDL files, enter the command "midl.exe adoce31.idl" in the folder where the IDL files are located (typically the relevant "include" folder). You will need to ensure that the following IDL files are available for inclusion: oaidl.idl, objidl.idl, ocidl.idl, oleidl.idl, unknwn.idl, and wtypes.idl.

Two #import statements are required, one for each of the TLB files. Names in these TLB files may well clash with names used elsewhere, such as in other header files. Therefore, the namespace for each #import is changed using the rename_namespace attribute. In the code for this chapter, the namespace for ADOCE is "AdoNS" and for ADOXCE is "AdoXNS". Even when using these namespaces, the EOF name used in ADO still clashes, and so this is renamed to "A_EOF". This means that the ADOCE method called "GetEOF" now needs to be referred to as "GetA_EOF". You will also need to do the following:

• Include "comdef.h" for all the usual declarations.

• Use _COM_SMARTPTR_TYPEDEF to create a smart pointer class for IUnknown.

• Supply a default implementation of _com_issue_errorex, since this is not provided in the run times for Windows CE.

Here, then, is the code you will need to include before using smart pointer classes for ADOCE:

 #include <comdef.h> _COM_SMARTPTR_TYPEDEF(IUnknown, __uuidof(IUnknown)); #import "adoce31.tlb" rename ("EOF", "A_EOF")     rename_namespace ("AdoNS") #import "adoxce31.tlb" rename_namespace ("AdoXNS") void _com_issue_errorex(HRESULT hr, IUnknown* pUnkn, REFIID riid) {   cout  _T("COM Error: ")   hr   endl; } 

The #import statements will result in .tlh (header) and .tli (implementation) files being created for ADOCE and ADOXCE. These files will be placed in the output directory for the build (for example, "sh3dbg"). It is well worth looking at these files to determine the precise implementation of the methods and properties, since smart pointers change the return types and parameters for methods and properties.

The _com_issue_errorex function is called by the smart pointer functions that wrap the interface functions when an HRESULT returns a failure. Many of the smart pointer functions that would usually return an HRESULT return a value that would normally be returned as an out parameter. This makes them easier to call, but means that the HRESULT cannot be tested. Therefore, you should have _com_issue_errorex raise an exception or report the error in some way. In the sample above the value in the offending HRESULT is just displayed.

Table 16.1 shows the smart pointer interface classes created by these imports together with the namespace for each of the classes.

Table 16.1. Smart pointer classes for ADOCE and ADOXCE

Component	Class	Purpose
ADOCE	AdoNS ::_RecordsetPtr	Returns a virtual database table whose fields contain data returned from the database
ADOCE	AdoNS ::_CollectionPtr	Generic collection class used, for example, by field collections
ADOCE	AdoNS ::FieldsPtr	Collection of fields, generally in a Recordset
ADOCE	AdoNS ::FieldPtr	Interface representing a single field
ADOCE	AdoNS ::PropertiesPtr	A collection of properties generally used to describe a connection
ADOCE	AdoNS ::PropertyPtr	Interface representing a single property
ADOCE	AdoNS ::_ ConnectionPtr	Interface representing a connection to a data source
ADOCE	AdoNS ::ErrorsPtr	Collection of errors from a data source provider returned when an error occurs
ADOCE	AdoNS ::ErrorPtr	Interface representing a single error
ADOXCE	AdoXNS ::_CatalogPtr	Interface representing a catalog (database)
ADOXCE	AdoXNS ::TablesPtr	Collection of tables in a catalog
ADOXCE	AdoXNS ::_TablePtr	Interface representing a single table
ADOXCE	AdoXNS ::ColumnsPtr	Collection of columns in a table
ADOXCE	AdoXNS ::_ColumnPtr	Interface representing a single column
ADOXCE	AdoXNS ::ADOXCEPropertiesPtr	Properties collection, such as properties for a column in a table
ADOXCE	AdoXNS ::ADOXCEPropertyPtr	Interface representing a single property
ADOXCE	AdoXNS ::IndexesPtr	Collection of indexes for a table
ADOXCE	AdoXNS ::KeysPtr	Collection of keys (primary, foreign, or unique) for a table
ADOXCE	AdoXNS ::_KeyPtr	Interface representing a single key for a table

< BACK  NEXT >