ArcGIS Engine Developer Guide

VISUAL C++
::SetWindowText(m_hWnd, OLE2CT(bstrTitle));
}
return S_OK;
Implementing noncreatable classes
Noncreatable classes are COM objects that cannot be created by CoCreateInstance.
Instead, the object is created within a method call of a different object, and an
interface pointer to the noncreatable class is returned. This type of object is
found in abundance in the geodatabase model. For example, FeatureClass is
noncreatable and can only be obtained by calling one of a number of methods;
one example is the IFeatureWorkspace::OpenFeatureClass method.
One advantage of a noncreatable class is that it can be initialized with private
data using method calls that are not exposed in a COM API. Below is a simplified
example of returning a noncreatable object:
// Foo is a cocreatable object.
IFooPtr ipFoo;
HRESULT hr = ipFoo.CreateInstance(CLSID_Foo);
// Bar is a noncreatable object; cannot use ipBar.CreateInstance(CLSID_Bar).
IBarPtr ipBar;
// Use a method on Foo to create a new Bar object.
hr = ipFoo->CreateBar(&ipBar);
ipBar->DoSomething();
The steps required to change a cocreatable ATL class into a noncreatable class are
shown below:
1. Add “noncreatable” to the .idl file’s coclass attributes.
[
uuid(DCB87952-0716-4873-852B-F56AE8F9BC42),
noncreatable
]
coclass Bar
{
[default] interface IUnknown;
interface IBar;
};
2. Change the class factory implementation to fail any cocreate instances of the
noncreatable class. This happens via ATL’s object map in the main DLL
module.
BEGIN_OBJECT_MAP(ObjectMap)
OBJECT_ENTRY(CLSID_Foo, CFoo)
// Creatable object
OBJECT_ENTRY_NON_CREATEABLE(CLSID_Bar, CBar) // Noncreatable object
END_OBJECT_MAP()
3. Optionally, the registry entries can be removed. First, remove the registry
script for the object from the resources (Bar.rgs in this example). Then change
the class definition DECLARE_REGISTRY_RESOURCEID(IDR_BAR) to
DECLARE_NO_REGISTRY().
Chapter 4 • Developer environments • 107