Without:
How can I use FormatMessage()
to get the error text for a HRESULT
?
HRESULT hresult = application.CreateInstance("Excel.Application");
if (FAILED(hresult))
{
// what should i put here to obtain a human-readable
// description of the error?
exit (hresult);
}
Here's the proper way to get an error message back from the system for an HRESULT
(named hresult in this case, or you can replace it with GetLastError()
):
LPTSTR errorText = NULL;
FormatMessage(
// use system message tables to retrieve error text
FORMAT_MESSAGE_FROM_SYSTEM
// allocate buffer on local heap for error text
|FORMAT_MESSAGE_ALLOCATE_BUFFER
// Important! will fail otherwise, since we're not
// (and CANNOT) pass insertion parameters
|FORMAT_MESSAGE_IGNORE_INSERTS,
NULL, // unused with FORMAT_MESSAGE_FROM_SYSTEM
hresult,
MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
(LPTSTR)&errorText, // output
0, // minimum size for output buffer
NULL); // arguments - see note
if ( NULL != errorText )
{
// ... do something with the string `errorText` - log it, display it to the user, etc.
// release memory allocated by FormatMessage()
LocalFree(errorText);
errorText = NULL;
}
The key difference between this and David Hanak's answer is the use of the FORMAT_MESSAGE_IGNORE_INSERTS
flag. MSDN is a bit unclear on how insertions should be used, but Raymond Chen notes that you should never use them when retrieving a system message, as you've no way of knowing which insertions the system expects.
FWIW, if you're using Visual C++ you can make your life a bit easier by using the _com_error
class:
{
_com_error error(hresult);
LPCTSTR errorText = error.ErrorMessage();
// do something with the error...
//automatic cleanup when error goes out of scope
}
Not part of MFC or ATL directly as far as I'm aware.