ArcGIS Engine Developer Guide

VISUAL C++
.ocx files. While it is possible to engineer a header file from a type library, it is a
tedious process. The #import command automates the creation of the necessary
files required by the compiler. Since the command was developed to support
DTC, when using it to import ArcGIS type libraries, there are a number of
parameters that must be passed so the correct import takes place. For further
information on this process, see the later section ‘Importing ArcGIS type libraries’.
Handling errors in ATL
It is possible to just return an E_FAIL HRESULT code to indicate the failure
within a method; however, this does not give the caller any indication of the
nature of the failure. There are a number of Windows-standard HRESULTs
available, for example, E_INVALIDARG (one or more arguments are invalid)
and E_POINTER (invalid pointer). These error codes are listed in the window
header file winerror.h. Not all development environments have comprehensive
support for HRESULT; Visual Basic clients often see error results as “Automation
Error—Unspecified Error”. ATL provides a simple mechanism for working with
the COM error information object that can provide an error string description, as
well as an error code.
When creating an ATL object, the Object wizard has an option to support
ISupportErrorInfo. If you toggle the option on, when the wizard completes, your
object will implement the interface ISupportErrorInfo, and a method will be added
that looks something like this:
STDMETHODIMP MyClass::InterfaceSupportsErrorInfo(REFIID riid)
{
static const IID* arr[] =
{
&IID_IMyClass,
};
for (int i = 0; i < sizeof(arr) / sizeof(arr[0]); i++)
{
if (InlineIsEqualGUID(*arr[i], riid))
return S_OK;
}
Although Visual C++ does support an exception
mechanism (try ... catch), it is not recommended
to mix this with COM code. If an exception
unwinds out of a COM interface, there is no
guarantee the client will be able to catch this,
and the most likely result is a crash.
}
return S_FALSE;
It is now possible to return rich error messages by calling one of the ATL error
functions. These functions even work with resource files to ensure easy internationalization
of the message strings.
// Return a simple string.
AtlReportError(CLSID_MyClass, _T("No connection to Database."),
IID_IMyClass, E_FAIL);
// Get the Error Text from a resource string
AtlReportError(CLSID_MyClass, IDS_DBERROR, IID_IMyClass, E_FAIL,
_Module.m_hInstResource);
To extract an error string from a failed method, use the Windows function
GetErrorInfo. This is used to retrieve the last IErrorInfo object on the current
thread and clears the current error state.
Chapter 4 • Developer environments • 101