ArcGIS Engine Developer Guide

VISUAL C++
expected. It is better to pass unambiguous types to constructors, that is, types
that are not themselves smart types with overloaded cast operators.
// Perform QI of IUnknown.
IUnknownPtr ipUnk = ipSmartPointer;
// Ensure IUnknown* constructor of CComVariant is used.
CComVariant myVar2(ipUnk.GetInterfacePtr());
A common practice with smart pointers is to use Detach to return an object from
a method call. When returning an interface pointer, the COM standard is to
increment reference count of the [out] parameter inside the method implementation.
It is the caller’s responsibility to call Release when the pointer is no longer
required. Consequently, care must be taken to avoid calling Detach directly on a
member variable. A typical pattern is shown below:
STDMETHODIMP CFoo::get_Bar(IBar **pVal)
{
if (pVal==0) return E_POINTER;
// Constructing a local smart pointer using another smart pointer
// results in an AddRef (if pointer is not 0).
IBarPtr ipBar(m_ipBar);
// Detach will clear the local smart pointer, and the
// interface is written into the output parameter.
*pVal = ipBar.Detach();
// This can be combined into one line:
// *pVal = IBarPtr(m_ipBar).Detach();
return S_OK;
}
The above pattern has the same result as the following code. Note that a conditional
test for a zero pointer is required before AddRef can be called. Calling
AddRef (or any method) on a zero pointer will result in an access violation
exception and typically crash the application:
STDMETHODIMP CFoo::get_Bar(IBar **pVal)
{
if (pVal==0) return E_POINTER;
// Copy the interface pointer (no AddRef) into the output parameter.
*pVal = m_ipBar;
// Make sure interface pointer is nonzero before calling AddRef.
if (*pVal)
*pVal->AddRef();
return S_OK;
}
When using a smart pointer to receive an object from an [out] parameter on a
114 • ArcGIS Engine Developer Guide