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++ 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.
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.