Learning DCOM

Using the _com_error Class

The Visual C++ compiler-level support for COM includes a class called _com_error. In order to use this class, you must include comdef.h. Like the FormatMessage technique, this class allows you to display the error string of an associated HRESULT. Moreover, this class allows you to deal with COM exceptions via the IErrorInfo interface. An abbreviated class definition of _com_error follows:

class _com_error {
public:
   // Constructors
   _com_error(HRESULT hr, IErrorInfo* perrinfo = NULL) throw();

   // Accessors
   HRESULT Error() const throw();

   // IErrorInfo method accessors
   _bstr_t Description() const throw(_com_error);

   // FormatMessage accessors
   const TCHAR * ErrorMessage() const throw();

   . . .
};

Using this class, you can simply display the error message associated with the bad HRESULT by calling the _com_error::ErrorMessage method, as shown here:

HRESULT hr = pOcr->OcrImage(. . .);
if (FAILED(hr)) {
   _com_error error(hr);
   MessageBox(NULL, error.ErrorMessage(), L"Error", MB_OK);
}

The _com_error class is better than the FormatMessage technique because it can deal with COM exceptions via IErrorInfo. For example, the following code snippet is similar to the previous code. However, it is expanded to check for COM exception support that is provided by the object. Notice that if the object doesn’t support COM exceptions, you pass only the HRESULT into the constructor of _com_error, but if the object supports COM exceptions, you pass both the HRESULT and the IErrorInfo ...