Q201321: HOWTO: Alternative Implementation of ATL Singleton
Article: Q201321
Product(s): Microsoft C Compiler
Version(s): WINDOWS:3.0
Operating System(s):
Keyword(s): kbActiveX kbCOMt kbVC600 kbATL300 kbGrpDSMFCATL kbArchitecture
Last Modified: 28-JUN-2001
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The information in this article applies to:
- The Microsoft Active Template Library (ATL) 3.0, included with:
- Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0
- Microsoft Visual C++, 32-bit Professional Edition, version 6.0
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0
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SUMMARY
=======
This article shows an implementation of singletons that differs from the default
ATL implementation in the following ways:
- The lifetime of an ATL singleton is tied to the class factory; the singleton
gets destroyed only when the class factory is destroyed, which is when the
EXE/DLL unloads [CComModule::RevokeClassObject() and CComModule::Term()]. The
singleton implementation in this article maintains a refcount and deletes
itself when it goes to zero.
- Implementing singletons in a DLL can be problematic. One scenario is if your
singleton object marked "Apartment", is in a DLL. The client creates two STA
threads, each one creates your singleton object. Both STA threads have the
same raw pointer to your singleton, allowing them to simultaneously call into
the singleton. You are probably not synchronizing access to instance data in
your singleton object, because according to COM rules, you don't need to
synchronize access to instance data for objects marked "Apartment". It's only
a matter of time before data is corrupted.
The singleton implementation in this article works around this problem by
returning a marshaled pointer in IClassFactory::CreateInstance(). But
marshaling the pointer introduces another potential problem. All calls are
now marshaled to the first Apartment that calls
IClassFactory::CreateInstance. If the first Apartment goes away, calls to the
singleton from other Apartments will fail. If you use the singleton
implementation in this article in a DLL marked "Apartment", you must ensure
that the first Apartment that uses the component stays alive. Because of all
these potential problems, you should only use singletons in a DLL when
absolutely necessary. If you just want to share data in the DLL within the
same process, an alternative is to create global/static variables to hold the
data. You just need to synchronize access to this data.
MORE INFORMATION
================
To use this version of a singleton, copy and paste the following into a header
file in your project:
/********** Start of singleton code **********/
template <class Base>
class CMyComObject: public CComObject<Base>
{
public:
// Variable that lets class factory know COM object is deleted.
CMyComObject** m_ppObj;
STDMETHOD_(ULONG, Release)()
{
ULONG l = InternalRelease();
if (l == 0)
{
// Set to NULL so class factory knows to recreate us.
*m_ppObj = NULL;
delete this;
}
return l;
}
static HRESULT WINAPI CreateInstance(CMyComObject<Base>** pp)
{
ATLASSERT(pp != NULL);
HRESULT hRes = E_OUTOFMEMORY;
CMyComObject<Base>* p = NULL;
ATLTRY(p = new CMyComObject<Base>())
if (p != NULL)
{
p->SetVoid(NULL);
p->InternalFinalConstructAddRef();
hRes = p->FinalConstruct();
p->InternalFinalConstructRelease();
if (hRes != S_OK)
{
delete p;
p = NULL;
}
}
*pp = p;
// Initialize m_ppObj to CComObject<> pointer in class factory.
p->m_ppObj = pp;
return hRes;
}
};
template <class T>
class CMyComClassFactorySingleton : public CComClassFactory
{
public:
CMyComObject<T>* m_pObj; // actual COM object
CComAutoCriticalSection m_sec; // A critical section is needed
// because class factory is stored in CComObjectCached. All clients
// get the same raw pointer to the class factory. Any instance
// data in this class factory must be synchronized.
// IClassFactory
STDMETHOD(CreateInstance)(LPUNKNOWN pUnkOuter, REFIID riid,
void** ppvObj)
{
if (NULL == ppvObj)
return E_POINTER;
*ppvObj = NULL;
// Aggregation is not supported in Singletons.
ATLASSERT(pUnkOuter == NULL);
if (pUnkOuter != NULL)
return CLASS_E_NOAGGREGATION;
HRESULT hRes = E_FAIL;
// Lock critical section.
m_sec.Lock();
// Create COM object.
if (NULL == m_pObj)
{
hRes = CMyComObject<T>::CreateInstance(&m_pObj);
if (SUCCEEDED(hRes))
{
// Free marshaled interface and release stream.
if (m_pStream)
{
AtlFreeMarshalStream(m_pStream);
m_pStream = NULL;
}
// Marshal interface into a stream.
// Can't use AtlMarshalPtrInProc since we need to use
// MSHLFLAGS_TABLEWEAK.
hRes = CreateStreamOnHGlobal(NULL, TRUE, &m_pStream);
if (SUCCEEDED(hRes))
{
hRes = CoMarshalInterface(m_pStream, IID_IUnknown,
m_pObj, MSHCTX_INPROC, NULL, MSHLFLAGS_TABLEWEAK);
if (FAILED(hRes))
{
m_pStream->Release();
m_pStream = NULL;
}
}
}
if (FAILED(hRes))
{
// Unlock critical section.
m_sec.Unlock();
return hRes;
}
}
CComPtr<IUnknown> spUnk;
// Unmarshal interface from stream.
hRes = AtlUnmarshalPtr(m_pStream, IID_IUnknown, &spUnk);
// unlock critical section
m_sec.Unlock();
if (FAILED (hRes))
return hRes;
// Call QI on unmarshaled interface.
hRes = spUnk->QueryInterface(riid, ppvObj);
return hRes;
}
LPSTREAM m_pStream; // IStream* used for storing marshaled interface.
void FinalRelease()
{
// Free marshaled interface and release stream.
if (m_pStream)
AtlFreeMarshalStream (m_pStream);
}
CMyComClassFactorySingleton() : m_pObj(NULL), m_pStream(NULL)
{
}
};
#define DECLARE_CLASSFACTORY_MYSINGLETON(obj)
DECLARE_CLASSFACTORY_EX(CMyComClassFactorySingleton<obj>)
/********** End of singleton code **********/
Instead of DECLARE_CLASSFACTORY_SINGLETON, use the
DECLARE_CLASSFACTORY_MYSINGLETON() macro in your CComCoClass-derived class:
class CMyComponent : public CComCoClass<...>, ...
{
DECLARE_CLASSFACTORY_MYSINGLETON(CMyComponent)
...
}
REFERENCES
==========
(c) Microsoft Corporation 1999, All Rights Reserved. Contributions by Samson
Tanrena, Microsoft Corporation.
Additional query words: kbDSupport CComObjectGlobal CComObjectNoLock CComObject single instance CComClassFactorySingleton
======================================================================
Keywords : kbActiveX kbCOMt kbVC600 kbATL300 kbGrpDSMFCATL kbArchitecture
Technology : kbVCsearch kbAudDeveloper kbATLsearch
Version : WINDOWS:3.0
Issue type : kbhowto
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