FAQ 34.17 What mechanism does COM define for error handling?

COM sends us right back to the Dark Ages, since COM functions return error codes for doing error handling. Or, stated in a positive way, COM defines a mechanism for handling errors that is language and location transparent and is consistent with its binary standard.

The return value of most COM methods (AddRef and Release are two exceptions) is an HRESULT (a "handle to a result object"). An HRESULT is a 32-bit value that is divided into three fields. The first field (1 bit) can be tested to determine whether or not a function succeeded. The second field (15 bits) indicates what subsystem returned the error code. The third field (16 bits) is the return code. The biggest drawback of using this mechanism is that it requires the caller to explicitly test the value of the HRESULT after each COM method call.

Just because COM uses HRESULT does not prevent programmers from using C++ exception handling in the C++ code within the implementation of a method of a COM class, provided the exceptions are caught and handled by the COM method. In any event, C++ exceptions cannot be used for transmitting the error to the caller or to another COM object.