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Note Microsoft Visual C++ .NET 2002 and Microsoft Visual C++ .NET 2003 support both the managed code model that is provided by the Microsoft .NET Framework and the unmanaged native Microsoft Windows code model. The information in this article applies only to unmanaged Visual C++ code. Microsoft Visual C++ 2005 supports both the managed code model that is provided by the Microsoft .NET Framework and the unmanaged native Microsoft Windows code model.
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.
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:
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: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.
According to C++ whitepaper 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.
warning C4290: C++ Exception Specification ignored.
However, just because you have replaced this function, you may still need to trap for exceptions. Consider the code snippet below.
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.
Article ID: 175784 - Last Review: February 28, 2014 - Revision: 5.0