CComTearOffObject Class

 

For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

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.

Public Constructors

NameDescription
CComTearOffObject::CComTearOffObjectThe constructor.
CComTearOffObject::~CComTearOffObjectThe destructor.

Public Methods

NameDescription
CComTearOffObject::AddRefIncrements the reference count for a CComTearOffObject object.
CComTearOffObject::QueryInterfaceReturns a pointer to the requested interface on either your tear-off class or the owner class.
CComTearOffObject::ReleaseDecrements the reference count for a CComTearOffObject object and destroys it.

CComTearOffObjectBase Methods

CComTearOffObjectBaseConstructor.

CComTearOffObjectBase Data Members

m_pOwnerA pointer to a CComObject derived from the owner class.

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 ATL_NO_VTABLE CBeeper :
   public CComObjectRootEx<CComSingleThreadModel>,
   public CComCoClass<CBeeper, &CLSID_Beeper>,
   public IDispatchImpl<IBeeper, &IID_IBeeper, &LIBID_NVC_ATL_COMLib, /*wMajor =*/ 1, /*wMinor =*/ 0>
{
public:
   CBeeper()
   {
   }

DECLARE_REGISTRY_RESOURCEID(IDR_BEEPER)

DECLARE_NOT_AGGREGATABLE(CBeeper)

BEGIN_COM_MAP(CBeeper)
   COM_INTERFACE_ENTRY(IBeeper)
   COM_INTERFACE_ENTRY(IDispatch)
   COM_INTERFACE_ENTRY_TEAR_OFF(IID_ISupportErrorInfo, CBeeper2)
END_COM_MAP()

// ISupportsErrorInfo
   STDMETHOD(InterfaceSupportsErrorInfo)(REFIID riid);


   DECLARE_PROTECT_FINAL_CONSTRUCT()

   HRESULT FinalConstruct()
   {
      return S_OK;
   }

   void FinalRelease()
   {
   }

public:

};

Base

CComTearOffObject

Header: atlcom.h

Increments the reference count of the CComTearOffObject object by one.

STDMETHOD_(ULONG, AddRef)();

Return Value

A value that may be useful for diagnostics and testing.

The constructor.

CComTearOffObject(void* pv);

Parameters

pv
[in] Pointer that will be converted to a pointer to a CComObject<Owner> object.

Remarks

Increments the owner's reference count by one.

The destructor.

~CComTearOffObject();

Remarks

Frees all allocated resources, calls FinalRelease, and decrements the module lock count.

The constructor.

CComTearOffObjectBase();

Remarks

Initializes the m_pOwner member to NULL.

A pointer to a CComObject object derived from Owner.

CComObject<Owner>* m_pOwner;

Parameters

Owner
[in] The class for which a tear-off is being implemented.

Remarks

The pointer is initialized to NULL during construction.

Retrieves a pointer to the requested interface.

STDMETHOD(QueryInterface)(REFIID iid, void** ppvObject);

Parameters

iid
[in] The IID of the interface being requested.

ppvObject
[out] A pointer to the interface pointer identified by iid, or NULL if the interface is not found.

Return Value

A standard HRESULT value.

Remarks

Queries first for interfaces on your tear-off class. If the interface is not there, queries for the interface on the owner object. If the requested interface is IUnknown, returns the IUnknown of the owner.

Decrements the reference count by one and, if the reference count is zero, deletes the CComTearOffObject.

STDMETHOD_ULONG Release();

Return Value

In non-debug builds, always returns zero. In debug builds, returns a value that may be useful for diagnostics or testing.

CComCachedTearOffObject Class
Class Overview

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