CComTearOffObject Class
The new home for Visual Studio documentation is Visual Studio 2017 Documentation on docs.microsoft.com.
The latest version of this topic can be found at CComTearOffObject Class.
This class implements a tear-off interface.
Syntax
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.
Members
Public Constructors
| Name | Description |
|---|---|
| CComTearOffObject::CComTearOffObject | The constructor. |
| CComTearOffObject::~CComTearOffObject | The destructor. |
Public Methods
| Name | Description |
|---|---|
| CComTearOffObject::AddRef | Increments the reference count for a CComTearOffObject object. |
| CComTearOffObject::QueryInterface | Returns a pointer to the requested interface on either your tear-off class or the owner class. |
| CComTearOffObject::Release | Decrements the reference count for a CComTearOffObject object and destroys it. |
CComTearOffObjectBase Methods
| CComTearOffObjectBase | Constructor. |
CComTearOffObjectBase Data Members
| m_pOwner | A pointer to a CComObject derived from the owner class. |
Remarks
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:
};
Inheritance Hierarchy
Base
CComTearOffObject
Requirements
Header: atlcom.h
CComTearOffObject::AddRef
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.
CComTearOffObject::CComTearOffObject
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.
CComTearOffObject::~CComTearOffObject
The destructor.
~CComTearOffObject();
Remarks
Frees all allocated resources, calls FinalRelease, and decrements the module lock count.
CComTearOffObject::CComTearOffObjectBase
The constructor.
CComTearOffObjectBase();
Remarks
Initializes the m_pOwner member to NULL.
CComTearOffObject::m_pOwner
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.
CComTearOffObject::QueryInterface
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.
CComTearOffObject::Release
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.