Containment是一种比较简单的复用方法，如果Component B复用Component A，Component B实际上是Component A的一个客户，Component B向客户提供的Component A的功能实际上是Component B直接调用Component A完成的。当然Component B可以扩充Component A的功能。Component B可以直接使用已经存在的Component A，而不需要对Component A做任何改动。
Containment的例子实现略(潘爱民《COM原理与应用》第四章)
Aggregation则比较复杂，Component A必须能够适应被Aggregation下的特殊处理。其核心在于QueryInterface函数。Aggregation涉及到聚合对象和被聚合对象双方的协作，体现了真正意义上的COM复用，而Containment只是客户程序和Component的嵌套，这是Containment和Aggregation的本质区别。
Aggregation的实现(摘自潘爱民《COM原理与应用》)。
Component A的Code
class INondelegatingUnknown
{
public:
virtual HRESULT __stdcall NondelegationQueryInterface(const IID& iid, void **ppv) = 0 ;
virtual ULONG __stdcall NondelegatingAddRef() = 0;
virtual ULONG __stdcall NondelegationRelease() = 0;
};
class CA : public ISomeInterface, public INondelegatingUnknown
{
protected:
ULONG m_Ref;
public:
CA(IUnknown *pUnknownOuter);
~CA();
public :
// Delegating IUnknown
virtual HRESULT __stdcall QueryInterface(const IID& iid, void **ppv) ;
virtual ULONG __stdcall AddRef() ;
virtual ULONG __stdcall Release() ;
// Nondelegating IUnknown
virtual HRESULT __stdcall NondelegationQueryInterface(const IID& iid, void **ppv);
virtual ULONG __stdcall NondelegatingAddRef();
virtual ULONG __stdcall NondelegationRelease();
virtual HRESULT __stdcall SomeFunction( ) ;
private :
IUnknown *m_pUnknownOuter; // pointer to outer IUnknown
};
// Implemention of class CA
CA::CA (IUnknown *pUnknownOuter)
{
m_Ref = 0;
g_CompANumber ++ ;
m_pUnknownOuter = pUnknownOuter;
}
CA::~CA()
{}
ULONG CA::NondelegatingAddRef()
{
m_Ref ++;
return (ULONG) m_Ref;
}
ULONG CA::NondelegationRelease ()
{
m_Ref --;
if (m_Ref == 0 )
{
g_CompANumber -- ;
delete this;
return 0;
}
return (ULONG) m_Ref;
}
HRESULT CA::NondelegationQueryInterface(const IID& iid, void **ppv)
{
if ( iid == IID_IUnknown )
{
*ppv = (INondelegatingUnknown *) this ;
((IUnknown *)(*ppv))->AddRef() ;
} else if ( iid == IID_SomeInterface )
{
*ppv = (ISomeInterface *) this ;
((ISomeInterface *)(*ppv))->AddRef() ;
}
else
{
*ppv = NULL;
return E_NOINTERFACE ;
}
return S_OK;
}
ULONG CA::AddRef ()
{
if ( m_pUnknownOuter != NULL )
return m_pUnknownOuter->AddRef();
else
return NondelegatingAddRef();
}
ULONG CA::Release ()
{
if ( m_pUnknownOuter != NULL )
return m_pUnknownOuter->Release ();
else
return NondelegationRelease();
}
HRESULT CA::QueryInterface(const IID& iid, void **ppv)
{
if ( m_pUnknownOuter != NULL )
return m_pUnknownOuter->QueryInterface(iid, ppv);
else
return NondelegationQueryInterface(iid, ppv);
}
HRESULT CA::SomeFunction()
{
printf("This is CA::SomeFunction!\n");
return S_OK;
}
由上面的代码可以看出，被聚合的对象需要实现两个IUnknown接口，Delegation Unknown和NonDelegation Unknown接口，NonDelegation Unknown是按正常方式实现的IUnknown接口。Delegation Unknown在非Aggregation使用时候直接把所有调用传给NonDelegation Unknown接口；而在Aggregation下，它把调用传给外部对象的接口，而此时外部对象通过NonDelegation接口对内部对象进行控制。
Aggregation下CAFactory的CreateInstance实现：
HRESULT CAFactory::CreateInstance(IUnknown *pUnknownOuter, const IID& iid, void **ppv)
{
HRESULT hr;
// iid must be IID_IUnknown for aggregating
if ( ( pUnknownOuter != NULL ) && ( iid != IID_IUnknown ) )
{ return CLASS_E_NOAGGREGATION; }
*ppv=NULL;
hr=E_OUTOFMEMORY;
//Create the object passing function to notify on destruction.
CA *pObj=new CA (pUnknownOuter);
if (NULL==pObj) return hr;
//Obtain the first interface pointer (which does an AddRef)
hr = pObj->NondelegationQueryInterface(iid, ppv);
if (hr != S_OK) {
//Kill the object if initial creation or FInit failed.
g_CompANumber --; // Reference count g_CompANumber be added in constructor
delete pObj;
}
return hr;
}
MFC的COM Aggregation实现：COM使用了C++嵌套类来实现COM接口，并且使用接口映射表来简化编程工作，MFC对COM的支持是从类CCmdTarget开始。
#define DECLARE_INTERFACE_MAP() private: static const AFX_INTERFACEMAP_ENTRY _interfaceEntries[]; protected: static const AFX_INTERFACEMAP interfaceMap; static const AFX_INTERFACEMAP* PASCAL GetThisInterfaceMap(); virtual const AFX_INTERFACEMAP* GetInterfaceMap() const; struct AFX_INTERFACEMAP_ENTRY
{
const void* piid; // the interface id (IID) (NULL for aggregate)
size_t nOffset; // offset of the interface vtable from m_unknown
};
struct AFX_INTERFACEMAP
{
#ifdef _AFXDLL
const AFX_INTERFACEMAP* (PASCAL* pfnGetBaseMap)(); // NULL is root class
#else
const AFX_INTERFACEMAP* pBaseMap;
#endif
const AFX_INTERFACEMAP_ENTRY* pEntry; // map for this class
};
由此可以很明显看出，MFC继续使用Map表来实现COM接口。查看具体的Map表的增加和删除宏可以更详细的了解架构。
#define BEGIN_INTERFACE_MAP(theClass, theBase) const AFX_INTERFACEMAP* PASCAL theClass::GetThisInterfaceMap() { return &theClass::interfaceMap; } const AFX_INTERFACEMAP* theClass::GetInterfaceMap() const { return &theClass::interfaceMap; } AFX_COMDAT const AFX_INTERFACEMAP theClass::interfaceMap = { &theBase::GetThisInterfaceMap, &theClass::_interfaceEntries[0], }; AFX_COMDAT const AFX_INTERFACEMAP_ENTRY theClass::_interfaceEntries[] = { #define INTERFACE_PART(theClass, iid, localClass) { &iid, offsetof(theClass, m_x##localClass) }, #define INTERFACE_AGGREGATE(theClass, theAggr)
{ NULL, offsetof(theClass, theAggr) }, #define END_INTERFACE_MAP() { NULL, (size_t)-1 } }; 其中，offsetof宏可以给出成员变量与分类之间的偏移量，编译器在编译时候计算这个常数。
而接口部分定义则使用宏BEGIN_INTERFACE_PART、INIT_INTERFACE_PART、END_INTERFACE_PART进行定义。
#define BEGIN_INTERFACE_PART(localClass, baseClass) class X##localClass : public baseClass { public: STDMETHOD_(ULONG, AddRef)(); STDMETHOD_(ULONG, Release)(); STDMETHOD(QueryInterface)(REFIID iid, LPVOID* ppvObj); #define INIT_INTERFACE_PART(theClass, localClass) size_t m_nOffset; INIT_INTERFACE_PART_DERIVE(theClass, localClass) #define INIT_INTERFACE_PART_DERIVE(theClass, localClass) X##localClass() { m_nOffset = offsetof(theClass, m_x##localClass); } #define END_INTERFACE_PART(localClass) } m_x##localClass; friend class X##localClass; 复习二将重点涉及COM一些高级概念，从Marshal到Thread Model（Apartment和Free)。
