ArcGIS Engine Developer Guide

VISUAL C++
IFooPtr ipFoo;
HRESULT hr = ipFoo.CreateInstance(CLSID_Foo);
if (FAILED(hr))
return hr; // Return object creation failure code to caller.
Active Template Library smart types
ATL defines various smart types, as seen in the list below. You are free to combine
both the ATL and DTC smart types in your code. However, it is typical to
use the DTC for smart pointers, as they are easily generated by importing type
libraries. For BSTR and VARIANT types, the ATL versions for CComBSTR and
CComVariant are typically used.
ATL smart types include:
• CComPtr—encapsulates a COM interface pointer by wrapping the AddRef and
Release methods of the IUnknown interface
• CComQIPtr—encapsulates a COM interface and supports all three methods of
the IUnknown interface: QueryInterface, AddRef, and Release
• CComBSTR—encapsulates the BSTR data type
• CComVariant—encapsulates the VARIANT data type
• CRegKey—provides methods for manipulating Windows registry entries
• CComDispatchDriver—provides methods for getting and setting properties and
calling methods through an object’s IDispatch interface
• CSecurityDescriptor—provides methods for setting up and working with the
Discretionary Access Control List (DACL)
This section examines the first four smart types and their uses. The example code
below, written with ATL smart pointers, looks like the following:
// Get a CLSID GUID constant.
extern "C" const GUID __declspec(selectany) CLSID_Foo = \
{0x2f3b470c,0xb01f,0x11d3,{0x83,0x8e,0x00,0x00,0x00,0x00,0x00,0x00}};
STDMETHODIMP SomeClass::Do ()
{
// Create Instance of Foo class and QI for IFoo interface.
CComPtr ipFoo;
HRESULT hr = CoCreateInstance(CLSID_Foo, NULL, CLSCTX_INPROC_SERVER,
IID_IFoo, (void **)&ipFoo);
if (FAILED(hr)) return hr;
The equality operator (“==”) may have different
implementations when used during smart
pointer comparisons. The COM specification
states object identification is performed by
comparing the pointer values of IUnknown. The
DTC smart pointers will perform necessary QI
and comparison when using the “==” operator.
However, the ATL smart pointers will not do
this, so you must use the ATL IsEqualObject
method.
// Call method on IFoo to get IBar.
CComPtr ipBar;
HRESULT hr = ipFoo->get_Bar(&ipBar);
if (FAILED(hr)) return hr;
// IBar interface for IGak interface
CComQIPtr ipGak(ipBar);
// Call method on IGak.
hr = ipGak->DoSomething();
112 • ArcGIS Engine Developer Guide