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Important This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here.

CComTearOffObject Class 

This class implements a tear-off interface.


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()
...
};

Header: atlcom.h

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