ArcGIS Engine Developer Guide

VISUAL C++
extern "C" const GUID __declspec(selectany) CLSID_Foo = \
{0x2f3b470c,0xb01f,0x11d3,{0x83,0x8e,0x00,0x00,0x00,0x00,0x00,0x00}};
// Declare Smart Pointers for IFoo, IBar, and IGak interfaces.
_COM_SMARTPTR_TYPEDEF(IFoo, __uuidof(IFoo));
_COM_SMARTPTR_TYPEDEF(IBar, __uuidof(IBar));
_COM_SMARTPTR_TYPEDEF(IGak, __uuidof(IGak));
STDMETHODIMP SomeClass::Do()
{
// Create Instance of Foo class and QueryInterface (QI) for IFoo interface.
IFooPtr ipFoo;
HRESULT hr = ipFoo.CreateInstance(CLSID_Foo);
if (FAILED(hr)) return hr;
// Call method on IFoo to get IBar.
IBarPtr ipBar;
hr = ipFoo->get_Bar(&ipBar);
if (FAILED(hr)) return hr;
// QI IBar interface for IGak interface.
IGakPtr ipGak(ipBar);
// Call method on IGak.
hr = ipGak->DoSomething();
if (FAILED(hr)) return hr;
// Explicitly call Release().
ipGak = 0;
ipBar = 0;
// Let destructor call IFoo’s Release.
return S_OK;
}
One of the main advantages of using the DTC smart pointers is that they are
automatically generated from the #import compiler statement for all interface
and coclass definitions in a type library. For more details on this functionality, see
the later section ‘Importing ArcGIS type libraries’.
It is possible to create an object implicitly in a DTC smart pointer’s constructor,
for example:
IFooPtr ipFoo(CLSID_Foo)
However, this will raise a C++ exception if there is an error during object creation—for
example, if the DLL containing the object implementation was
accidentally deleted. This exception will typically be unhandled and cause a crash.
A more robust approach is to avoid exceptions in COM, call CreateInstance
explicitly, and handle the failure code, for example:
Chapter 4 • Developer environments • 111