How can I (is there a way to) convert an HRESULT into a system specific error message?

Question

According to this , there's no way to convert a HRESULT error code into a Win32 error code. Therefore (at least to my understanding), my use of FormatMessage in order to generate error messages (ie

std::wstring Exception::GetWideMessage() const
{
    using std::tr1::shared_ptr;
    shared_ptr<void> buff;
    LPWSTR buffPtr;
    DWORD bufferLength = FormatMessageW(
        FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_IGNORE_INSERTS,
        NULL,
        GetErrorCode(),
        0,
        reinterpret_cast<LPWSTR>(&buffPtr),
        0,
        NULL);
    buff.reset(buffPtr, LocalFreeHelper());
    return std::wstring(buffPtr, bufferLength);
}

) does not work for HRESULTs.

How do I generate these kinds of system-specific error strings for HRESULTs?

Answer 1

This answer incorporates Raymond Chen's ideas, and correctly discerns the incoming HRESULT, and returns an error string using the correct facility to obtain the error message:

/////////////////////////////
// ComException

CString FormatMessage(HRESULT result)
{
    CString strMessage;
    WORD facility = HRESULT_FACILITY(result);
    CComPtr<IErrorInfo> iei;
    if (S_OK == GetErrorInfo(0, &iei) && iei)
    {
        // get the error description from the IErrorInfo 
        BSTR bstr = NULL;
        if (SUCCEEDED(iei->GetDescription(&bstr)))
        {
            // append the description to our label
            strMessage.Append(bstr);

            // done with BSTR, do manual cleanup
            SysFreeString(bstr);
        }
    }
    else if (facility == FACILITY_ITF)
    {
        // interface specific - no standard mapping available
        strMessage.Append(_T("FACILITY_ITF - This error is interface specific.  No further information is available."));
    }
    else
    {
        // attempt to treat as a standard, system error, and ask FormatMessage to explain it
        CString error;
        CErrorMessage::FormatMessage(error, result); // <- This is just a wrapper for ::FormatMessage, left to reader as an exercise :)
        if (!error.IsEmpty())
            strMessage.Append(error);
    }
    return strMessage;
}