CComPolyObject Class

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This class implements IUnknown for an aggregated or nonaggregated object.

Syntax

template<class contained>  
class CComPolyObject : public IUnknown,
      public CComObjectRootEx<contained::_ThreadModel::ThreadModelNoCS>

Parameters

contained
Your class, derived from CComObjectRoot or CComObjectRootEx, as well as from any other interfaces you want to support on the object.

Members

Public Constructors

Public Methods

Public Data Members

Remarks

CComPolyObject implements IUnknown for an aggregated or nonaggregated object.

When an instance of CComPolyObject is created, the value of the outer unknown is checked. If it is NULL, IUnknown is implemented for a nonaggregated object. If the outer unknown is not NULL, IUnknown is implemented for an aggregated object.

The advantage of using CComPolyObject is that you avoid having both CComAggObject and CComObject in your module to handle the aggregated and nonaggregated cases. A single CComPolyObject object handles both cases. This means only one copy of the vtable and one copy of the functions exist in your module. If your vtable is large, this can substantially decrease your module size. However, if your vtable is small, using CComPolyObject can result in a slightly larger module size because it is not optimized for an aggregated or nonaggregated object, as are CComAggObject and CComObject.

If the DECLARE_POLY_AGGREGATABLE macro is specified in your object's class definition, CComPolyObject will be used to create your object. DECLARE_POLY_AGGREGATABLE will automatically be declared if you use the ATL Project Wizard to create a full control or Internet Explorer control.

If aggregated, the CComPolyObject object has its own IUnknown, separate from the outer object's IUnknown, and maintains its own reference count. CComPolyObject uses CComContainedObject to delegate to the outer unknown.

For more information about aggregation, see the article Fundamentals of ATL COM Objects.

Inheritance Hierarchy

CComObjectRootBase

CComObjectRootEx

IUnknown

CComPolyObject

Requirements

Header: atlcom.h

CComPolyObject::AddRef

Increments the reference count on the object.

STDMETHOD_(ULONG, AddRef)();

Return Value

A value that may be useful for diagnostics or testing.

CComPolyObject::CComPolyObject

The constructor.

CComPolyObject(void* pv);

Parameters

pv
[in] A pointer to the outer unknown if the object is to be aggregated, or NULL if the object if the object is not aggregated.

Remarks

Initializes the CComContainedObject data member, m_contained, and increments the module lock count.

The destructor decrements the module lock count.

CComPolyObject::~CComPolyObject

The destructor.

~CComPolyObject();

Remarks

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

CComPolyObject::CreateInstance

Allows you to create a new CComPolyObject<contained > object without the overhead of CoCreateInstance.

static HRESULT WINAPI CreateInstance(  
    LPUNKNOWN pUnkOuter, 
    CComPolyObject<contained>** pp);

Parameters

pp
[out] A pointer to a CComPolyObject< contained> pointer. If CreateInstance is unsuccessful, pp is set to NULL.

Return Value

A standard HRESULT value.

Remarks

The object returned has a reference count of zero, so call AddRef immediately, then use Release to free the reference on the object pointer when you're done.

If you don't need direct access to the object, but still want to create a new object without the overhead of CoCreateInstance, use CComCoClass::CreateInstance instead.

CComPolyObject::FinalConstruct

Called during the final stages of object construction, this method performs any final initialization on the m_contained data member.

HRESULT FinalConstruct();

Return Value

A standard HRESULT value.

CComPolyObject::FinalRelease

Called during object destruction, this method frees the m_contained data member.

void FinalRelease();

CComPolyObject::m_contained

A CComContainedObject object derived from your class.

CComContainedObject<contained> m_contained;

Parameters

contained
[in] Your class, derived from CComObjectRoot or CComObjectRootEx, as well as from any other interfaces you want to support on the object.

Remarks

IUnknown calls through m_contained are delegated to the outer unknown if the object is aggregated, or to the IUnknown of this object if the object is not aggregated.

CComPolyObject::QueryInterface

Retrieves a pointer to the requested interface.

STDMETHOD(QueryInterface)(REFIID iid, void** ppvObject);
template <class Q>  
HRESULT QueryInterface(Q** pp);

Parameters

Q
The COM interface.

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

ppvObject
[out] A pointer to the interface pointer identified by iid. If the object does not support this interface, ppvObject is set to NULL.

pp
[out] A pointer to the interface identified by __uuidof(Q).

Return Value

A standard HRESULT value.

Remarks

For an aggregated object, if the requested interface is IUnknown, QueryInterface returns a pointer to the aggregated object's own IUnknown and increments the reference count. Otherwise, this method queries for the interface through the CComContainedObject data member, m_contained.

CComPolyObject::Release

Decrements the reference count on the object.

STDMETHOD_(ULONG, Release)();

Return Value

In debug builds, Release returns a value that may be useful for diagnostics or testing. In nondebug builds, Release always returns 0.

See Also

CComObjectRootEx Class
DECLARE_POLY_AGGREGATABLE
Class Overview