template< class Base >
class CComTearOffObject : public Base
Parameters
Base
Your tear-off class, derived from CComTearOffObjectBase and the interfaces you want your tear-off object to support.
ATL implements its tear-off interfaces in two phases — the CComTearOffObjectBase methods handle the reference count and QueryInterface, while CComTearOffObject implements IUnknown.
CComTearOffObject implements a tear-off interface as a separate object that is instantiated only when that interface is queried for. The tear-off is deleted when its reference count becomes zero. Typically, you build a tear-off interface for an interface that is rarely used, since using a tear-off saves a vtable pointer in all the instances of your main object.
You should derive the class implementing the tear-off from CComTearOffObjectBase and from whichever interfaces you want your tear-off object to support. CComTearOffObjectBase is templatized on the owner class and the thread model. The owner class is the class of the object for which a tear-off is being implemented. If you do not specify a thread model, the default thread model is used.
You should create a COM map for your tear-off class. When ATL instantiates the tear-off, it will create CComTearOffObject<CYourTearOffClass> or CComCachedTearOffObject<CYourTearOffClass>.
For example, in the BEEPER sample, the CBeeper2 class is the tear-off class and the CBeeper class is the owner class:
class CBeeper2 : public ISupportErrorInfo,
public CComTearOffObjectBase<CBeeper>
{
public:
CBeeper2() {}
STDMETHOD(InterfaceSupportsErrorInfo)(REFIID riid)
{
return (InlineIsEqualGUID(IID_IBeeper,riid)) ?
S_OK : S_FALSE;
}
BEGIN_COM_MAP(CBeeper2)
COM_INTERFACE_ENTRY(ISupportErrorInfo)
END_COM_MAP()
};
class CBeeper :
public IDispatchImpl<IBeeper, &IID_IBeeper,
&LIBID_BeeperLib>,
public CComObjectRoot,
public CComCoClass<CBeeper, &CLSID_Beeper>
{
public:
CBeeper();
BEGIN_COM_MAP(CBeeper)
COM_INTERFACE_ENTRY(IDispatch)
COM_INTERFACE_ENTRY(IBeeper)
COM_INTERFACE_ENTRY_TEAR_OFF(IID_ISupportErrorInfo,
CBeeper2)
END_COM_MAP()
...
};
#include <atlcom.h>
See Also CComCachedTearOffObject