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COMOBJ - Containment and Aggregation in a DLL |
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This sample introduces COM objects.
An object is a software construct that encapsulates or packages some data and some public methods that operate on that data. The data is often said to be "hidden" while the methods are exposed as the chief means to access the data. At compile time, programming languages provide varying degrees of syntactic support to express objects. For example, the C++ language offers a Class syntactic construct to express an abstract type (or class) of object. However, this C++ Class construct does allow for public access to an object's data. An object class is an abstraction that refers to an open-ended set of potential objects of the same specific kind. Objects are real things that occupy memory with their data and code instantiated in live binary form. The 'object' term typically refers to the runtime behavior of the this real software thing.
A COM object is a kind of object that originated with Microsoft's Component Object Model (COM). COM objects completely hide their data and expose their methods through a construct called an interface. A COM interface is a grouping of related methods that is uniquely identified for all programs and all time (by an Interface ID). Interfaces are used to encapsulate COM object feature sets. The most fundamental feature set in COM gives COM objects their nature as COM objects. The IUnknown interface exposes this feature set in several methods (AddRef, Release, and QueryInterface) that determine the common behavior governing all COM object lifetimes and how interfaces on COM objects are properly acquired.
Outside users of a COM object can only use the object by acquiring one of its interfaces. This acquisition of an interface is achieved by obtaining a pointer to the COM object's implementation of the interface. COM objects "know" about the interfaces they expose to clients and can provide pointer references to the interface implementations. Likewise, COM objects know about how many interface references they have handed out to clients and can thus control their own lifetime. When no references remain, the object normally removes itself from memory and ceases to exist.
The COMOBJ DLL offers several car-related COM object classes. Because COMOBJ.DLL supports no object handlers, class factories, full in-process servers, or marshaling, it is not a full-blown COM Server. Rather, it is a primitive precursor to a COM in-process server.
This DLL exposes the following COM objects: COCar, COUtilityCar, and COCruiseCar. Appropriate create functions are exported from this DLL: CreateCar, CreateUtilityCar, and CreateCruiseCar.
In this tutorial, COMOBJ works with the COMUSER code sample to show how an EXE client (COMUSER.EXE) calls COM object creation services and manipulates the COM objects that are created.
For functional descriptions and a tutorial code tour of COMOBJ, see the Code Tour section in COMOBJ.HTM. For details on setting up the programmatic usage of COMOBJ, see the Usage section in COMOBJ.HTM. To read COMOBJ.HTM, run TUTORIAL.EXE in the main tutorial directory and click the COMOBJ lesson in the table of lessons. You can also achieve the same thing by clicking the COMOBJ.HTM file after locating the main tutorial directory in the Windows Explorer. See also COMUSER.HTM in the main tutorial directory for more details on the COMUSER client application and how it works with COMOBJ.DLL itself. You must build COMOBJ.DLL before building COMUSER. The makefile for COMOBJ copies the necessary COMOBJ.H, COMOBJ.LIB, and COMOBJ.DLL files to the appropriate sibling directories once the files are built. The .H goes to \INC, the .LIB goes to \LIB, and the .DLL goes to \COMUSER.
In general, to set up your system to build and test the code samples in this COM Tutorial series, see TUTORIAL.HTM for details. The supplied makefile is Microsoft NMAKE-compatible. To create a debug build, issue the NMAKE command at the command prompt.
COMOBJ is a DLL that you can access from .EXE modules either by performing an explicit LoadLibrary call or implicitly loading the DLL by linking to its associated import library (.LIB) file. In either case, you need to include COMOBJ.H to declare the functions that are defined as exported in the COMOBJ DLL. In this lesson, a representative COMUSER.EXE application is provided to illustrate the programmatic use of COMOBJ.DLL. COMUSER is built in the COMUSER lesson (in sibling directory COMUSER). See below for more details.
The client sample and other related samples must be compiled before you can run the client. For more details on building the samples, see Building the Code Samples.
If you have already built the appropriate samples, COMUSER.EXE is the client executable to run for this sample.
Files Description
COMOBJ.TXT Short sample description.
MAKEFILE The generic makefile for building the COMOBJ.DLL code
sample.
COMOBJ.H The include file for declaring as imported or defining
as exported the service functions in COMOBJ.DLL.
COMOBJ.CPP The main implementation file for COMOBJ.DLL. Has DllMain
and the COM Object creation functions.
CAR.H The include file for the COCar COM object class.
CAR.CPP The implementation file for the COCar COM object class.
UTILCAR.H The include file for the COUtilityCar COM object class.
UTILCAR.CPP The implementation file for the COUtilityCar COM object
class.
CRUCAR.H The include file for the COCruiseCar COM object class.
CRUCAR.CPP The implementation file for the COCruiseCar COM object
class.
COMOBJI.H The include file for the internal class declarations and
resource identifier definitions for resources stored
inside the COMOBJ.DLL.
COMOBJ.RC The DLL resource definition file.
COMOBJ.ICO The icon resource.
This code sample is based on the code in DLLSKEL. See the DLLSKEL code tour in the sibling DLLSKEL directory for more detail on the DLL skeleton.
COMOBJ makes use of many of the utility classes and services provided by APPUTIL. For more details on APPUTIL, study the source code in the sibling APPUTIL directory and tutorial file APPUTIL.HTM in the main tutorial directory.
In the context of this tutorial, the goal of COMOBJ is to use a C++ DLL framework to illustrate various techniques for constructing COM objects. The DLL acts as a primitive server for several simple COM objects. Later in this series, COM objects are shown housed as COM component objects in COM servers. COM components are COM object types that have associated class factories, are housed in COM servers, and are registered in the registry.
The COM objects that are used in both the COMUSER and COMOBJ code samples represent sport utility vehicles. We invent some basic feature sets that are available for modeling such car objects. These feature sets are implemented as custom interfaces in COM objects. Custom interfaces are created by application programmers and are declared in the application's header files. In contrast to custom interfaces, standard interfaces (e.g., IUnknown) are provided by COM and are pre-declared in COM's system header files. In this sample, three custom COM interfaces are created to roughly model some feature sets on real cars: ICar, IUtility, and ICruise.
The most basic interface, ICar, has the following methods: Shift, Clutch, Speed, and Steer. Through this interface, car objects can be commanded to:
The IUtility Interface can add off-road and 4-wheel drive utility systems to our car objects: Offroad and Winch. Through this interface, these car objects can:
The ICruise interface can add automatic cruise control methods to our car objects: Engage, Adjust. Through this interface these car objects can:
Using these interfaces, COMOBJ exposes three COM object classes: COCar, COUtilityCar, and COCruiseCar. COCar exposes the basic ICar interface. COUtilityCar exposes the ICar and IUtility interfaces. COCruiseCar exposes the ICar and ICruise interfaces. So COCars have basic Car behavior (ICar). UtilityCars have basic Car behavior (ICar) with offroad utility systems (IUtility). COCruiseCars have basic Car behavior (ICar) with a cruise control system (ICruise).
COCar is a COM object class that exposes a native implementation of the ICar interface. The multiple interfaces of COCar are implemented using the nested class technique.
COUtilityCar is a COM object class that exposes a native implementation of the IUtility interface and reuses the COCar class and its ICar interface by containment. The multiple interfaces of COUtilityCar are implemented using the nested class technique.
COCruiseCar is a COM object class that exposes a native implementation of the ICruise interface and reuses the COCar class and its ICar interface by aggregation. The multiple interfaces of COCruiseCar are implemented using the nested class technique.
COM objects created from these three classes can be used by client software only as in-process COM objects. No object handler or marshaling is provided, and simple creation functions are supplied instead of fully functioning class factories. The idea here is to keep it simple to illustrate COM object interface augmentation and the COM object reuse techniques of containment and aggregation.
COMOBJ.H is an important file not only for compiling COMOBJ.DLL, but also for outside users of the DLL. As with DLLSKEL.H in the DLLSKEL code sample, this include file serves double duty by defining certain functions as exported and also declaring those functions as imported, depending on how COMOBJ.H is included.
COMOBJ.H uses the #if !defined(RC_INCLUDE) block to exclude the contents of this file if someone includes it in an .RC file. RC_INCLUDE is defined on the RC compiler invocation command lines. See RCFLAGS in the makefiles. In general, COMOBJ.H is written to support either C or C++ programs.
In ICARS.H in the sibling \INC directory, the car-related interfaces are declared. CARGUIDS.H is in the same common directory and defines the GUIDs for the interfaces. CARGUIDS.H also contains the CLSIDs for the class factories of the main COM object classes. Class Factories are not used in this code sample but will be studied in future lessons. So these two files are common include files used by the current code sample as well as future ones.
If your project consists of a suite of executables that all manage a common set of COM objects and interfaces (as this code sample series does), it is a good practice to factor out those common interface and GUID definitions into a common include directory. Such factoring can help you ensure that there are no conflicts in the interface or GUID definitions between executables in the application suite.
First the interfaces. From ICARS.H, here is a representative declaration for ICar.
DECLARE_INTERFACE_(ICar, IUnknown)
{
// IUnknown methods.
STDMETHOD(QueryInterface) (THIS_ REFIID, PPVOID) PURE;
STDMETHOD_(ULONG,AddRef) (THIS) PURE;
STDMETHOD_(ULONG,Release) (THIS) PURE;
// ICar methods.
STDMETHOD(Shift) (THIS_ short) PURE;
STDMETHOD(Clutch) (THIS_ short) PURE;
STDMETHOD(Speed) (THIS_ short) PURE;
STDMETHOD(Steer) (THIS_ short) PURE;
};
The DECLAREINTERFACE_ macro is used to publicly derive the ICar interface class from the existing COM IUnknown interface abstract base class. The STDMETHOD macro declares the method with the standard HRESULT error return code. The parameter is the method name. The STDMETHOD_ macro is used when the method returns something other than HRESULT. For example, in AddRef above, a ULONG return type is specified. THIS is used when the method accepts the void parameter. THIS_ is used when parameter types are accepted and those types are specified in parameter order. The PURE macro (in C++ it reduces to '=0') designates these methods as pure virtual functions, as all methods in C++ interface declarations must be. In C, this macro reduces to nothing. With these macros, you should be able to use the services of COMOBJ.DLL from either C or C++ programs.
A standard part of every interface is an IUnknown implementation, which includes the three methods QueryInterface, AddRef, and Release. All COM interfaces are derived from IUnknown. Additional declarations are necessary for the methods that are unique to each interface (for example, the Shift, Clutch, Speed, and Steer methods of ICar).
For this interface to be used in a way fully consistent with COM interfaces, it needs a globally unique identifier, or GUID, as in CARGUIDS.H.
DEFINE_GUID(IID_ICar,
0x0002da00, 0x0000, 0x0000, 0xc0, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x46);
With this definition, the GUID of the interface can be referred to by the name IID_ICar.
This GUID is part of a Microsoft pre-allocated sequence in which the first value 0x0002DAxx has values of xx in the range 00 to FF. Do not use the GUIDs in this sequence or any GUIDs in this code sample series elsewhere in other applications. You can obtain your own unique GUIDs by running the UUIDGEN utility supplied with the Platform SDK. The following command in the command prompt window will produce 10 such GUIDs in file GUIDS.C.
UUIDGEN -s -n10 -oGUIDS.C
This utility uses random number generation techniques to ensure absolute GUID uniqueness. It produces the GUIDs as a C structure:
INTERFACENAME = { /* b124eec0-e7a9-11ce-9a5f-444553540000 */
0x3c0869c0,
0xd721,
0x11ce,
{0x9a, 0x5f, 0x44, 0x45, 0x53, 0x54, 0x00, 0x00}
};
This structure is easy to edit into the format required by the DEFINE_GUID macro as shown above.
COMOBJ.CPP provides the main exported creation functions for the DLL. It also has the DllMain function that you saw in DLLSKEL. Because COMOBJ.CPP brings together the functionality of the DLL, it includes the following #include statements:
#include <windows.h> #include <ole2.h> #include <initguid.h> #include <apputil.h> #include <icars.h> #include <carguids.h> #include "comobji.h" #define _DLLEXPORT_ #include "comobj.h" #include "car.h" #include "utilcar.h" #include "crucar.h"
We include OLE2.H because we will need to call the COM/OLE library functions. Of special note is INITGUID.H. This file must be included only once in each executable, just after the other COM/OLE header files are included. What this ensures is that any GUIDs defined in this application will be instantiated as named data in the data segment of this executable DLL. If you do not include this file, you will get unresolved external references to the IID_IMyInterface GUIDs during linking. We include ICARS.H for the common car-related interface specifications. We include CARGUIDS.H for the interface GUID definitions.
As with DLLSKEL, we confine internal resource identifier macros and utility classes to a separate file, COMOBJI.H. We define the _DLLEXPORT_ macro to force export definition of the creation functions in this file. We next include COMOBJ.H for the STDENTRY macro definitions, as well as the main interfaces already discussed. We include CAR.H, UTILCAR.H, and CRUCAR.H for the COM object class declarations of the three main objects exposed by this DLL: COCar, COUtilityCar, and COCruiseCar.
There is a global variable worthy of note:
// Here is a pointer for use by the global Debug Message logging // macros. // Set by the ComObjInitMsgLog function call from an outside EXE // user. CMsgLog* g_pMsgLog = NULL;
The trace message log facility is a simple one that is meant to be global and nonintrusive. One central set of LOGxx macros all assume the same-named global variable (ie, g_pMsgLog) regardless of the executable module. Because the MsgLog window lives in a host EXE which has a message loop, we define this global variable in the DLL. It is later assigned by a call from the EXE to the ComObjInitMsgLog function which is also defined in COMOBJ.
As a primitive server providing the three COM objects, this DLL exports three creation functions: CreateCar, CreateUtilityCar, and CreateCruiseCar. Explicit calls to delete the objects are not needed, because COM object lifetimes are controlled by their own internal reference counts. The creation functions provide a requested interface pointer on the new object. After creation, it is only through interfaces that the objects are accessible. The Car, UtilityCar, and CruiseCar objects are all exposed as COM objects that are aggregatable outside this DLL via the pUnkOuter parameter to their creation functions. Here is a representative CreateCruiseCar function:
STDENTRY CreateCruiseCar(
IUnknown* pUnkOuter,
REFIID riid,
PPVOID ppv)
{
HRESULT hr;
COCruiseCar* pCob;
LOGF1("D: CreateCruiseCar.pUnkOuter=0x%X.",pUnkOuter);
// If the creation call is requesting aggregation (pUnkOuter !=
// NULL), the COM rules state the IUnknown interface MUST also
// concomitantly be requested. If it is not so requested (riid !=
// IID_IUnknown), then an error must be returned indicating that
// no aggregate creation of the COCruiseCar COM Object can be
// performed.
if (NULL != pUnkOuter && riid != IID_IUnknown)
hr = CLASS_E_NOAGGREGATION;
else
{
// Instantiate a COCruiseCar COM Object.
pCob = new COCruiseCar(pUnkOuter);
if (NULL != pCob)
{
// If we have succeeded in instantiating the COCruiseCar
// object, we initialize it to set up any subordinate
// objects (ie, via containment or aggregation).
hr = pCob->Init();
if (SUCCEEDED(hr))
{
// We QueryInterface this new COM Object not only to
// deposit the requested interface pointer into the
// caller's pointer variable, but to also automatically
// bump the Reference Count on the new
// COM Object after handing out this reference to it.
hr = pCob->QueryInterface(riid, (PPVOID)ppv);
}
}
else
hr = E_OUTOFMEMORY;
}
if (SUCCEEDED(hr))
LOGF1("D: CreateCruiseCar Succeeded. *ppv=0x%X.",*ppv);
return hr;
}
Using the REFIID parameter, this function attempts to return the requested interface pointer after the COCruiseCar COM object is created. The QueryInterface method of the new object's IUnknown interface is used to get the interface pointer. The pUnkOuter parameter determines whether the object is being aggregated into another object. The CoCruiseCar's Init method is called to compartmentalize any creation of subordinate objects, which applies here because a subordinate COCar object is created using aggregation in this Init method.
The COCar COM object class is declared in CAR.H.
class COCar : public IUnknown
{
public:
// Main Object Constructor & Destructor.
COCar(IUnknown* pUnkOuter);
~COCar(void);
// IUnknown members. Main object, non-delegating.
STDMETHODIMP QueryInterface(REFIID, PPVOID);
STDMETHODIMP_(ULONG) AddRef(void);
STDMETHODIMP_(ULONG) Release(void);
private:
// We declare nested class interface implementations here.
class CImpICar : public ICar
{
public:
// Interface Implementation Constructor & Destructor.
CImpICar(COCar* pBackObj, IUnknown* pUnkOuter);
~CImpICar(void);
// IUnknown members.
STDMETHODIMP QueryInterface(REFIID, PPVOID);
STDMETHODIMP_(ULONG) AddRef(void);
STDMETHODIMP_(ULONG) Release(void);
// ICar members.
STDMETHODIMP Shift(short nGear);
STDMETHODIMP Clutch(short nEngaged);
STDMETHODIMP Speed(short nMph);
STDMETHODIMP Steer(short nAngle);
private:
// Data private to this interface implementation of ICar.
ULONG m_cRefI; // Interface Ref Count (for
// debugging)
COCar* m_pBackObj; // Parent Object back pointer
IUnknown* m_pUnkOuter; // Outer unknown for
// Delegation
};
// Make the otherwise private and nested ICar interface
// implementation a friend to COM object instantiations
// of this selfsame COCar COM object class.
friend CImpICar;
// Private data of COCar COM objects.
// Nested ICar implementation instantiation. This ICar
// interface is instantiated inside this COCar object
// as a native interface.
CImpICar m_ImpICar;
// Main Object reference count.
ULONG m_cRefs;
// Outer unknown (aggregation & delegation).
IUnknown* m_pUnkOuter;
};
The ICar interface is implemented as a nested class. The nested CImpICar is declared in the private section of the main COCar object class. An instance of the ICar interface is created in the private section with this code.
// Nested ICar implementation instantiation. This ICar interface // is instantiated inside this COCar object as a native interface. CImpICar m_ImpICar;
Because the ICar interface implementation is nested in the COCar object class, no explicit creation and deletion operations are needed. Its lifetime is tied to the lifetime of the parent COCar object, so it is automatically constructed and destroyed as the parent object is. To ensure the CImpICar methods can access the private data of the COCar class, CImpICar is made a friend ot COCar.
The constructor function of CImpICar contains two important parameters. The pBackObj pointer is used by CImpICar methods to access private data in COCar. The pUnkOuter parameter signals aggregation if it is non-NULL, in which case it is stored within the CImpICar object and used for later delegation of calls to its own interfaces' IUnknown methods. More on this later, when we tour CAR.CPP.
The m_pUnkOuter variable is used when this object is aggregated inside another COM object. The value then is a pointer to the outermost controlling IUnknown, which is passed to the COCar constructor. COCar supports both containment and aggregation.
CAR.CPP defines the methods of COCar. The constructor uses an interesting member initializer to pass the pBackObj and pUnkOuter parameters mentioned above to the CImpICar constructor. This inner class's constructor is thereby executed prior to the main COCar constructor.
COCar::COCar(IUnknown* pUnkOuter) : m_ImpICar(this, pUnkOuter)
{
// Zero the COM object's reference count.
m_cRefs = 0;
// No AddRef necessary if non-NULL, as we're nested.
m_pUnkOuter = pUnkOuter;
LOGF1("D: COCar Constructor. m_pUnkOuter=0x%X.", m_pUnkOuter);
return;
}
Here is that inner CImpICar::CImpICar constructor.
COCar::CImpICar::CImpICar(
COCar* pBackObj,
IUnknown* pUnkOuter)
{
// Init the Interface Ref Count (used for debugging only).
m_cRefI = 0;
// Init the Back Object Pointer to point to the parent object.
m_pBackObj = pBackObj;
// Init the CImpICar interface's delegating Unknown pointer. We
// use the Back Object pointer for IUnknown delegation here if
// we are not being aggregated. If we are being aggregated, we
// use the supplied pUnkOuter for IUnknown delegation. In either
// case, the pointer assignment requires no AddRef because the
// CImpICar lifetime is quaranteed by the lifetime of the parent
// object in which CImpICar is nested.
if (NULL == pUnkOuter)
{
m_pUnkOuter = pBackObj;
LOG("D: COCar::CImpICar Constructor.Non-Aggregating.");
}
else
{
m_pUnkOuter = pUnkOuter;
LOG("D: COCar::CImpICar Constructor. Aggregating.");
}
return;
}
Note the logic used to assign the delegating m_pUnkOuter pointer. If the COCar object is not being aggregated (parameter pUnkOUter == NULL), m_pUnkOuter is assigned the COCar object's 'this' pointer (passed in the pBackObj parameter of the member initializer pointed out above). In this case, the ICar interface's IUnknown methods will all be delegated to the COCar object. If COCar is being aggregated (pUnkOuter != NULL), m_pUnkOuter is assigned a pointer to the outermost controlling IUnknown (passed in the pUnkOuter parameter of the member initializer pointed out above). The pUnkOuter parameter is then passed to the constructor of COCar. If pUnkOuter is non-NULL, it is passed to any subordinate objects that are created. It makes the code for the CImpICar IUnknown methods simple as well--they all blindly delegate through the interface's assigned m_pUnkOuter pointer. Here, for example, is CImpICar::QueryInterface.
STDMETHODIMP COCar::CImpICar::QueryInterface(
REFIID riid,
PPVOID ppv)
{
LOG("D: COCar::CImpICar::QueryInterface.Delegating.");
// Delegate this call to the outer object's QueryInterface.
return m_pUnkOuter->QueryInterface(riid, ppv);
}
When the COCar object is aggregated, calls to this method are delegated to the outermost controlling IUnknown. At the outermost aggregating object, it appears that this ICar interface is on the outer object rather than on the inner COCar object.
Since this COCar COM object does not reuse any COM objects, its IUnknown methods are easy. Here's QueryInterface:
STDMETHODIMP COCar::QueryInterface(
REFIID riid,
PPVOID ppv)
{
HRESULT hr = E_NOINTERFACE;
*ppv = NULL;
if (IID_IUnknown == riid)
{
*ppv = this;
LOG("D: COCar::QueryInterface. 'this' pIUnknown returned.");
}
else if (IID_ICar == riid)
{
*ppv = &m_ImpICar;
LOG("D: COCar::QueryInterface. pICar returned.");
}
if (NULL != *ppv)
{
// We've handed out a pointer to the interface so obey the COM
// rules and AddRef the reference count.
((LPUNKNOWN)*ppv)->AddRef();
hr = NOERROR;
}
return (hr);
}
For IID_IUnknown, the 'this' pointer is used, because a pointer to the object itself is a pointer to the object's IUnknown. For IID_ICar, QueryInterface passes the address of the nested instance of CImpICar (*ppv = &m_ImpICar). After passing an interface pointer, QueryInterface calls AddRef to maintain the object's reference count.
The "if (IID_Unknown == riid)" comparisons take advantage of an overloaded C++ == operator. Such overloaded operators are provided in the COM/OLE headers for types GUID, CLSID, and IID. IIDs are actually the 128-bit (16 byte) integer entities that you saw above. In C++, these entities can simply be referred to using references (as with IID_Unknown above).
The actual ICar methods (like CImpICar::Shift) in this code sample are empty LOG announcements that the methods have been called. This is where the skeleton ends and real application code would be added.
UTILCAR.H declares the COUtilityCar COM object class.
class COUtilityCar : public IUnknown
{
public:
// Main Object Constructor & Destructor.
COUtilityCar(IUnknown* pUnkOuter);
~COUtilityCar(void);
// A general public method for initializing this newly created
// COUtilityCar object. Creates any subordinate arrays,
// structures, or objects. Not exposed as a method in an
// interface.
HRESULT Init(void);
// IUnknown members. Main object, non-delegating.
STDMETHODIMP QueryInterface(REFIID, PPVOID);
STDMETHODIMP_(ULONG) AddRef(void);
STDMETHODIMP_(ULONG) Release(void);
// We get this ICar interface pointer via containment reuse of
// the ICar interface in an instantiated COCar.
ICar* m_pICar;
private:
// We show nested interface class implementations here.
// We implement the basic ICar interface in this COUtilityCar
// COM object class.
class CImpICar : public ICar
{
public:
// Interface Implementation Constructor & Destructor.
CImpICar(COUtilityCar* pBackObj, IUnknown* pUnkOuter);
~CImpICar(void);
// IUnknown members.
STDMETHODIMP QueryInterface(REFIID, PPVOID);
STDMETHODIMP_(ULONG) AddRef(void);
STDMETHODIMP_(ULONG) Release(void);
// ICar members.
STDMETHODIMP Shift(short nGear);
STDMETHODIMP Clutch(short nEngaged);
STDMETHODIMP Speed(short nMph);
STDMETHODIMP Steer(short nAngle);
private:
// Data private to this interface implementation of ICar
ULONG m_cRefI; // Interface Ref Count (for
// debugging)
COUtilityCar* m_pBackObj; // Parent Object back pointer
IUnknown* m_pUnkOuter; // Outer unknown for
// Delegation
};
// We implement the IUtility interface (ofcourse) in this
// COUtilityCar COM object class. This is the interface
// that we are using as an augmentation to the existing
// COCar COM object class.
class CImpIUtility : public IUtility
{
public:
// Interface Implementation Constructor & Destructor.
CImpIUtility(COUtilityCar* pBackObj, IUnknown* pUnkOuter);
~CImpIUtility(void);
// IUnknown members.
STDMETHODIMP QueryInterface(REFIID, PPVOID);
STDMETHODIMP_(ULONG) AddRef(void);
STDMETHODIMP_(ULONG) Release(void);
// IUtility members.
STDMETHODIMP Offroad(short nGear);
STDMETHODIMP Winch(short nRpm);
private:
// Data private to this interface implementation of
// IUtility
ULONG m_cRefI; // Interface Ref Count
// (for debugging)
COUtilityCar* m_pBackObj; // Parent Object back pointer
IUnknown* m_pUnkOuter; // Outer unknown for
// Delegation
};
// Make the otherwise private and nested ICar interface
// implementation a friend to COM object instantiations
// of this selfsame COUtilityCar COM object class.
friend CImpICar;
friend CImpIUtility;
// Private data of COUtilityCar COM objects.
// Nested ICar implementation instantiation.
CImpICar m_ImpICar;
// Nested IUtility implementation instantiation.
CImpIUtility m_ImpIUtility;
// Main Object reference count.
ULONG m_cRefs;
// Outer unknown (aggregation & delegation).
IUnknown *m_pUnkOuter;
};
COM objects of the COUtilityCar class augment COCar COM objects (which expose ICar interface features) with a native implementation of IUtility interface features (Offroad and Winch). A COUtilityCar COM object class reuses the COCar COM object class by containment. The multiple interfaces on this COM object class are implemented as nested classes.
To supply both the ICar and IUtility interfaces, we implement them in this COM object. The IUtility interface is a native implementation in the COUtilityCar COM object. The COUtilityCar object contains a delegating implementation of the ICar interface, called CImpICar in the sample code. The whole purpose of CImpICar is to delegate ICar calls to the real ICar implementation, which is located in a contained COCar object.
For this implementation, the COUtilityCar object's constructor creates a COCar object and holds a pointer to the ICar interface of the COCar object. This pointer is used to delegate calls to ICar methods (implemented in the COUtilityCar object) to the corresponding method implemented in the contained object's ICar interface. We declare the delegation pointer like this:
// We get this ICar interface pointer via containment reuse of the // ICar interface in an instantiated COCar. ICar* m_pICar;
This delegating pointer is assigned when the COUtilityCar is initialized by the Init call in the CreateUtilityCar function in COMOBJ.CPP.
// Instantiate a COUtilityCar COM Object.
pCob = new COUtilityCar(pUnkOuter);
if (NULL != pCob)
{
// If we have succeeded in instantiating the COUtilityCar object
// we initialize it to set up any subordinate objects (ie, via
// containment or aggregation).
hr = pCob->Init();
if (SUCCEEDED(hr))
{
// We QueryInterface this new COM Object not only to deposit
// the requested interface pointer into the caller's pointer
// variable, but to also automatically bump the Reference
// Count on the new COM Object after handing out this
// reference to it.
hr = pCob->QueryInterface(riid, (PPVOID)ppv);
}
}
else
hr = E_OUTOFMEMORY;
After creating a COUtilityCar COM object, the object's Init method is called to create and initialize any inner objects. In this case, an inner COCar COM object must be created. Here is the Init method (from UTILCAR.CPP):
HRESULT COUtilityCar::Init(void)
{
HRESULT hr;
LOG("D: COUtilityCar::Init.");
// Create an instance of the COCar object and ask for its ICar
// interface directly. We pass NULL for the pUnkOuter because we
// are not aggregating. It is here that we are reusing the COCar
// COM Object through the Containment technique. We cache the
// pointer to the ICar interface for later delegation use. We
// don't need to AddRef it here because CreateCar does that for us.
hr = CreateCar(NULL, IID_ICar, (PPVOID) &m_pICar);
return (hr);
}
The call to the CreateCar function creates a COCar COM object. COUtilityCar objects reuse COCar objects by containment, not aggregation, so the pUnkOuter parameter is NULL. Since the COCar COM object is not aggregated, CreateCar can explicitly ask it for its IID_ICar interface. The output parameter passes the address of the m_pICar delegation pointer (&m_pICar).
In UTILCAR.CPP, the IUnknown methods of CImpIUtility all delegate as was done for the ICar implementation in COCar (through the interface's m_pUnkOuter). But the COUtilityCar object contains a COCar object, so the delegating ICar implementation delegates calls (through m_pICar) to COCar's ICar implementation. Here is an example, ICar::Speed.
STDMETHODIMP COUtilityCar::CImpICar::Speed(
short nMph)
{
LOGF1("D: COUtilityCar::CImpICar::Speed. Delegating.
nMph=%i.",nMph);
m_pBackObj->m_pICar->Speed(nMph);
return NOERROR;
}
We need the m_pBackObj pointer to access the COUtilityCar object's m_pICar pointer because this method is inside the delegating CImpICar class. Even though this CImpICar class is a delegating interface implementation for the one in the COCar object, we must still delegate calls to the IUnknown of CImpICar using the interface's m_pUnkOuter. Doing so helps make the COUtilityCar object itself an aggregatable COM object; just because it contains a COCar object (to expose the ICar interface) doesn't mean it cannot in turn be either contained or aggregated by some other COM object. As we did for COCar, we code COUtilityCar to support either containment or aggregation--a good general practice for all COM objects since it's easy once you understand what's going on.
Now we can look at the COUtilityCar controlling IUnknown's QueryInterface method:
STDMETHODIMP COUtilityCar::QueryInterface(
REFIID riid,
PPVOID ppv)
{
HRESULT hr = E_NOINTERFACE;
*ppv = NULL;
if (IID_IUnknown == riid)
{
*ppv = this;
LOG("D: COUtilityCar::QueryInterface. 'this' pIUnknown
returned");
}
else if (IID_ICar == riid)
{
*ppv = &m_ImpICar;
LOG("D: COUtilityCar::QueryInterface. pICar
returned");
}
else if (IID_IUtility == riid)
{
*ppv = &m_ImpIUtility;
LOG("D: COUtilityCar::QueryInterface. pIUtility
returned");
}
if (NULL != *ppv)
{
// We've handed out a pointer to the interface so obey
// the COM rules and AddRef the reference count.
((LPUNKNOWN)*ppv)->AddRef();
hr = NOERROR;
}
return (hr);
}
The IID_ICar case looks deceptively simple. QueryInterface just gives back the address of the delegating ICar interface implementation (&m_ImpICar). The IID_IUtility case is indeed simple, since we have implemented IUtility as a native interface on this COUtilityCar object. QueryInterface gives back the address of that IUtility interface (&m_ImpIUtility).
The last point to make about COUtilityCar concerns its constructor. As before, we use member initializers for the constructors of the nested interface implementation classes. These are required because we are using nested interface classes.
COUtilityCar::COUtilityCar(IUnknown* pUnkOuter) :
m_ImpICar(this, pUnkOuter),
m_ImpIUtility(this, pUnkOuter)
{
...
}
CRUCAR.H declares the COCruiseCar COM object class.
class COCruiseCar : public IUnknown
{
public:
// Main Object Constructor & Destructor.
COCruiseCar(IUnknown* pUnkOuter);
~COCruiseCar(void);
// A general public method for initializing this newly created
// COCruiseCar object. Creates any subordinate arrays,
// structures, or objects. Not exposed as a method in
// an interface.
HRESULT Init(void);
// IUnknown members. Main object, non-delegating.
STDMETHODIMP QueryInterface(REFIID, PPVOID);
STDMETHODIMP_(ULONG) AddRef(void);
STDMETHODIMP_(ULONG) Release(void);
private:
// We declare nested class interface implementations here.
// We implement the ICruise interface (ofcourse) in this
// COCruiseCar COM object class. This is the interface
// that we are using as an augmentation to the existing
// COCar COM object class.
class CImpICruise : public ICruise
{
public:
// Interface Implementation Constructor & Destructor.
CImpICruise(COCruiseCar* pBackObj, IUnknown* pUnkOuter);
~CImpICruise(void);
// IUnknown members.
STDMETHODIMP QueryInterface(REFIID, PPVOID);
STDMETHODIMP_(ULONG) AddRef(void);
STDMETHODIMP_(ULONG) Release(void);
// ICruise members.
STDMETHODIMP Engage(BOOL bOnOff);
STDMETHODIMP Adjust(BOOL bUpDown);
private:
// Data private to this interface implementation of
// ICruise
ULONG m_cRefI; // Interface Ref Count (for
// debugging)
COCruiseCar* m_pBackObj; // Parent Object back pointer
IUnknown* m_pUnkOuter; // Outer unknown for
// Delegation
};
// Make the otherwise private and nested ICar interface
// implementation
// a friend to COM object instantiations of this selfsame
// COCruiseCar
// COM object class.
friend CImpICruise;
// Private data of COCruiseCar COM objects.
// Nested ICruise implementation instantiation. This ICruise
// interface is implemented inside this COCruiseCar object
// as a native interface.
CImpICruise m_ImpICruise;
// Main Object reference count.
ULONG m_cRefs;
// Outer unknown (aggregation & delegation). Used when
// this COCruiseCar object is being aggregated. Otherwise,
// it is used for delegation if this object is reused via
// containment.
IUnknown* m_pUnkOuter;
// We need to save the IUnknown interface pointer on the COCar
// object that we aggregate. We use this when we need to
// delegate IUnknown calls to this aggregated inner object.
IUnknown* m_pUnkCar;
};
With the COCruiseCar COM object class, once more the native interface, ICruise, is implemented as a nested interface class. COCruiseCar exposes the ICar interface through an aggregated COCar object. This is similar to the way COUtilityCar exposes its own native IUtility interface and also exposes the ICar interface, but COUtilityCar objects reuse the ICar interface through containment, not aggregation, of a COCar object. In the COMUSER code sample, we will build a COUtilityCruiseCar COM object that aggregates this COCruiseCar object. We'll then have a COM object that aggregates a COM object that aggregates a COM object.
CImpICruise itself has a private data member, m_pUnkOuter, for supporting aggregation and delegation. This pointer is used to delegate calls to IUnknown methods of the ICruise interface. If the COCruiseCar object is not aggregated, calls are delegated to its IUnknown. If the COCruiseCar object is aggregated, calls are delegated to the outermost controlling IUnknown. The m_pBackObj is for use in any of the ICruise methods to reference members of the COCruiseCar object.
As in previous interface implementations, the CImpICruise embedded class needs to be made a friend class to COCruiseCar so that CImpICruise's methods can access the private data in COCruiseCar.
COCruiseCar is itself constructed by aggregating the COCar COM object with this object's native ICruise interface implementation. That is what the m_pUnkCar pointer in the COCruiseCar object's private data is for. It caches a pointer to the aggregated inner COCar object. This pointer is used to delegate certain IUnknown calls to the inner object's IUnknown interface. More on this later.
When COCruiseCar is aggregated in an outer composite COM object, it has an m_pUnkOuter delegation pointer in its private data area. If it is not aggregated, this pointer merely serves as a record of the NULL pUnkOuter when a COCruiseCar object is created.
We will skip the COCruiseCar constructor function, because it is similar to that of other COM functions we have already studied.
The CreateCruiseCar function discussed earlier in this tour calls the COCruiseCar::Init method. Here is COCruiseCar::Init as it is implemented in CRUCAR.CPP:
HRESULT COCruiseCar::Init(void)
{
HRESULT hr;
// Set up the right pIUnknown for delegation. If we are being
// aggregated, then we pass the pUnkOuter in turn to any COM
// objects that we are aggregating. m_pUnkOuter was set in the
// Constructor.
IUnknown* pUnkOuter = (NULL == m_pUnkOuter) ? this :
m_pUnkOuter;
LOG("D: COCruiseCar::Init.");
// We create an instance of the COCar object and do this via the
// Aggregation reuse technique. Note we pass pUnkOuter as the
// Aggregation pointer. It is the 'this' pointer to this present
// COCruiseCar object if we are not being aggregated; otherwise
// it is the pointer to the outermost object's controlling
// IUnknown. Following the rules of Aggregation we ask CreateCar
// to give us an IID_IUnknown interface. We cache this pointer
// to the IUnknown of the new COCar COM object for later use in
// delegating IUnknown calls.
hr = CreateCar(pUnkOuter, IID_IUnknown, (PPVOID)&m_pUnkCar);
return (hr);
}
The CreateCar function creates and aggregates a COCar object and asks that the controlling IUnknown of the new COCar object be placed in the m_pUnkCar pointer discussed above.
This Init functionality could be attempted in the constructor function, but aggregation requests in the constructor can cause problems that are difficult to debug. Depending on the order of events, such creation calls often cause QueryInterface calls that are delegated to the outermost aggregating object. At that point, the source of any errors encountered can be difficult to detect. Placing aggregation requests in a separate Init method can make it easier to avoid problems, or to detect them if they arise. It is also convenient to make the Init method a member of the COCruiseCar class (rather than putting it in the CreateCruiseCar function) because the method has access to the object's private data and to the 'this' pointer. For example, in Init's conditional assignment of pUnkOuter above, we use both m_pUnkOuter and the 'this' pointer.
IUnknown* pUnkOuter = (NULL == m_pUnkOuter) ? this : m_pUnkOuter;
This conditional assignment gives pUnkOuter the right value for the CreateCar aggregation creation call. It's part of what is needed to make the COCruiseCar COM object an aggregating object that is at the same time aggregatable itself.
If m_pUnkOuter is NULL, this COCruiseCar object itself is not being aggregated. If it is not being aggregated, the inner COCar object still needs to be created as an aggregated object. In this case, COCruiseCar's 'this' controlling IUnknown pointer is passed in as the outermost controlling IUnknown. By doing so, any aggregated inner objects are informed that this COCruiseCar object is the outermost controlling IUnknown.
If m_pUnkOuter is not NULL, the COCruiseCar object is itself being aggregated, and it passes to any aggregated inner objects the pointer it receives in m_pUnkOuter as the outermost controlling IUnknown.
We mentioned earlier that m_pUnkCar was used to delegate certain IUnknown calls to the aggregated inner object. One place to see this delegation in action is in COCruiseCar::QueryInterface.
STDMETHODIMP COCruiseCar::QueryInterface(
REFIID riid,
PPVOID ppv)
{
HRESULT hr = E_NOINTERFACE;
*ppv = NULL;
if (IID_IUnknown == riid)
{
*ppv = this;
LOG("D: COCruiseCar::QueryInterface. 'this' pIUnknown
returned.");
}
else if (IID_ICruise == riid)
{
// This ICruise interface is implemented in this COCruiseCar
// object and might be called a native interface of COCruiseCar.
*ppv = &m_ImpICruise;
LOG("D: COCruiseCar::QueryInterface. pICruise
returned.");
}
if (NULL != *ppv)
{
// We've handed out a pointer to an interface, so obey the COM
// rules and AddRef its reference count.
((LPUNKNOWN)*ppv)->AddRef();
hr = NOERROR;
}
else if (IID_ICar == riid)
{
LOG("D: COCruiseCar::QueryInterface. ICar
delegating.");
// We didn't implement the ICar interface in this COCruiseCar
// object. The aggregated inner object (COCar) is contributing
// the ICar interface to this present composite or aggregating
// COCruiseCar object. So, to satisfy a QI request for the
// ICar interface, we delegate the QueryInterface to the inner
// object's controlling IUnknown.
hr = m_pUnkCar->QueryInterface(riid, ppv);
}
return (hr);
}
The IID_IUnknown and IID_ICruise cases are familiar. If QueryInterface passes those pointers by request, it must call AddRef on their interface. The IID_ICruise interface is exposed as a native interface and is implemented in the COCruiseCar class, so it is easy to pass a pointer to it. For the IID_ICar interface, however, the request must be passed to the aggregated object that owns it. The cached object pointer m_pUnkCar is used to delegate the QueryInterface call to the inner COCar object, which will delegate the call to its outermost controlling IUnknown, the present COCruiseCar object.
For information on the COCruiseCar::Release method, see the code comments below.
STDMETHODIMP_(ULONG) COCruiseCar::Release(void)
{
ULONG ulCount = --m_cRefs;
LOGF1("D: COCruiseCar::Release. New cRefs=%i.",
m_cRefs);
if (0 == m_cRefs)
{
// We artificially bump the main ref count. This fulfills one
// of the rules of aggregated objects and ensures that an
// indirect recursive call to this release won't occur because
// of other delegating releases that might happen in our own
// destructor.
m_cRefs++;
delete this;
}
return ulCount;
}
The COCruiseCar::CImpICruise::CImpICruise constructor uses the following logic to assign its own IUnknown delegation pointer.
if (NULL == pUnkOuter)
{
m_pUnkOuter = pBackObj;
LOG("D: COCruiseCar::CImpICruise Constructor.
Non-Aggregating.");
}
else
{
m_pUnkOuter = pUnkOuter;
LOG("D: COCruiseCar::CImpICruise Constructor.
Aggregating.");
}
If the COCruiseCar object is aggregated, the ICruise interface's m_pUnkOuter delegation pointer is assigned the pointer to the outermost controlling IUnknown (constructor parameter pUnkOuter). If the COCruiseCar object is not aggregated, any IUnknown calls received by the ICruise interface are delegated to the IUnknown of the COCruiseCar object using the value of pBackObj. The pBackObj pointer is passed to the CImpICruise constructor as the COCruiseCar object's 'this' pointer. This parameter to the interface implementation constructor was achieved through the use of member initializers in COCruiseCar's constructor. For more details on this special use of member initializers, see the discussion of the COCruiseCar::Init method earlier in this text file.
The CImpICruise interface's IUnknown methods are simple outward delegations using the value of m_pUnkOuter assigned. This assignment of the m_pUnkOuter IUnknown pointer in the CImpICruise constructor requires no call to AddRef, because the CImpICruise lifetime is guaranteed by the lifetime of the parent object in which CImpICruise is nested. This convenience is one of the benefits of nested interface class implementations.
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