Q175784: INFO: Replacing #import’s Exception Raising Mechanism

Article: Q175784
Product(s): Microsoft C Compiler
Version(s): 5.0,6.0
Operating System(s): 
Keyword(s): kbATM kbDatabase kbMDAC kbGrpDSVCDB kbGrpDSMDAC kbDSupport
Last Modified: 12-JUN-2002

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
- Microsoft Visual C++.NET (2002) 
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NOTE: Microsoft Visual C++ NET (2002) supported both the managed code model that is provided by the .NET Framework and the unmanaged native Windows code model. The information in this article applies to unmanaged Visual C++ code only.

SUMMARY
=======

Using #import 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 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 ) 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, the linker uses that as opposed to
bringing it in from COMSUPP.LIB. _com_raise_error exists in its own object in
COMSUPP.LIB just so it can be easily replaced by your code.

Below is an example of implementation of #import's exception raising function:

  Note: Currently our compiler ignores a function exception-specification and
  generates the warning:

  warning C4290: C++ Exception Specification ignored.

  According to C++ WP if any declaration of a function has an exception-
  specification, all declarations, including the definition, of that function
  shall have an exception-specification with the same set of type- ids.

  void __stdcall
     _com_raise_error(HRESULT hr, IErrorInfo* perrinfo ) 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 "C:\Program Files\Common Files\System\ado\msado15.dll" no_namespace rename("EOF", "adoEOF" )

         _bstr_t     bstrEmpty(L"");
         _ConnectionPtr  Conn1 = NULL;
         Conn1.CreateInstance( __uuidof( Connection ) );
         Conn1->Open( bstrEmpty, bstrEmpty, bstrEmpty,0 ); 

This code attempts to open an ADO connection object without providing any valid
connection information. By replacing _com_raise_error, you prevented the
_com_error from being raised.

However, just because you have replaced this function, you may still need to trap
for exceptions. Consider the code snippet below.

  #import "C:\Program Files\Common Files\System\ado\msado15.dll" no_namespace rename("EOF", "adoEOF" )
      _ConnectionPtr  Conn1 = NULL;
      // Conn1.CreateInstance( __uuidof( Connection ) );
      Conn1->Open( bstrEmpty, bstrEmpty, bstrEmpty ,0); 

In this case, Conn1 is not a valid object and the interface pointer to this
non-existent object is NULL, resulting in _com_raise_erro being called. However,
the overloaded -> operator method will return a null interface, on which the
compiler then attempts to invoke the Open() method, resulting in a Win32
exception. Testing Conn1 for NULL first before calling Open() would prevent this
exception.

Additional query words:

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Keywords          : kbATM kbDatabase kbMDAC kbGrpDSVCDB kbGrpDSMDAC kbDSupport 
Technology        : kbVCsearch kbAudDeveloper kbVC500 kbVC600 kbVC32bitSearch kbVCNET kbVC500Search
Version           : :5.0,6.0
Issue type        : kbinfo

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