INFO: Replacing #import's Exception Raising Mechanism for ADO

ID: Q177425

The information in this article applies to:

Microsoft Visual C++, 32-bit Editions, version 5.0

SUMMARY

Using the #import statement to create your client application introduces exception handling through the _com_error exception class when a wrapper for an object's method encounters a failed HRESULT. You might have valid reasons to replace this mechanism with your own implementation.

MORE INFORMATION

There are two ways to use #import and not have it raise exceptions for failed HRESULTS. The first is simply to use the raw_interfaces_only clause with the #import statement. However, this negates some of the advantages of the wrapper classes that #import provides.

The second technique is by providing your own implementation for _com_raise_error, which has the following prototype and default implementation:


   void __stdcall _com_raise_error(HRESULT hr, IErrorInfo* perrinfo = 0)
   throw(_com_error);

   void __stdcall
   _com_raise_error(HRESULT hr, IErrorInfo* perrinfo = 0) throw(_com_error)
   {
       throw _com_error(hr, perrinfo);
   }

This function is declared but not implemented in the Comdef.h file. If you provide your own implementation in an .OBJ file, the linker uses that implementation as opposed to bringing it in from the Comsupp.lib file. _com_raise_error exists in its own object in the Comsupp.lib file so it can be easily replaced by your code.

Following is a sample implementation of the #import's exception raising function.

NOTE: Currently the compiler ignores a function exception-specification and generates the following warning:

warning C4290: C++ Exception Specification ignored.

At this time, the implementation details of exception specification have not been standardized, and are accepted but not implemented in Microsoft Visual C++. Code compiled with ignored exception specifications may need to be recompiled and linked to be reused in future versions supporting exception specifications. You can avoid this warning by using the warning pragma:


#pragma warning( disable : 4290 )

Sample Code


   void __stdcall
   _com_raise_error(HRESULT hr, IErrorInfo* perrinfo = 0) throw(_com_error)
       {
           //This message box is for demonstration purpose only.
           AfxMessageBox( "_com_raise_error (HRESULT, IErrorInfo*)" );
           //Your own error handling code or just an abort.
       }

    #import <msado15.dll>
        ...


     _bstr_t     bstrEmpty(L"");
     _ConnectionPtr  Conn1 = NULL;
     Conn1.CreateInstance( __uuidof( Connection ) );
     Conn1->Open( bstrEmpty, bstrEmpty, bstrEmpty );

This code attempts to open an ActiveX Data Objects (ADO) connection object without providing any valid connection information. Replacing _com_raise_error prevented the _com_error from being raised.

Additional query words: kbdse kbgrpvcdb kbgrpmdac kbdsupport kbdatabase kbado

Keywords : kbCompiler
Version : winnt:5.0
Platform : winnt
Issue type : kbinfo