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COMPONENT

OBJECT MODELING(COM)

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Binary standard for windows objects

Components made by different software vendorsto be combined into a variety of applications.

COM defines a standard for componentinteroperability

Programming language independent

Is available on multiple platforms,

Is extensible.

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COM provides mechanisms for the following:

Communications between components, evenacross process and network boundaries

Shared memory management betweencomponents

Error and status reporting

Dynamic loading of components

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COM Fundamentals

Binary Standard

COM defines a standard way to

Lay out virtual function tables (vtables) in memory

and a standard way to call functions through the

vtables.

Any language that can call functions viapointers all can be used to write components

that can interoperate with other componentswritten to the same binary standard.

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The double indirection

(the client holds a pointer to a pointer to avtable)

allows for vtable sharing among multipleinstances of the same object class.

On a system with hundreds of objectinstances,

vtable sharing can reduce memoryrequirements considerably.

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What This Means

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Versioning mechanism allows one system component to be updated

without requiring updates to all the other componentsin the system.

Versioning in COM is implemented using interfaces andIUnknown::QueryInterface

. COM completely eliminates the need for version

repositories

When a software module is updated, it is generally to add new functionality or to improve existing functionality.

In COM, you add new functionality to your component

object by adding support for new interfaces.

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Objects and components

In COM, an object is some piece of compiledcode

that provides some service to the rest of the system.

Component objects usually have someassociated data,

A given component object will never have directaccess to another component object in itsentirety.

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Architectural feature of COM

Component objects alwaysaccess othercomponent objects through interfacepointers.

It allows COM to completely preserveencapsulation of data and processing,

It also allows for transparent remoting(cross-process or cross-network calling)

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COM Interfaces

Object interact through interface COM Interfaces consists of group of

logically related,

well defined functions/methods That use known parameters and return types

An Interface consists only of

methods/functions There is no way to access the data with in

an interface

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An interface is not a class

A class can be instantiated to form acomponent object,

An interface cannot be instantiated by itselfbecause it carries no implementation.

A component object must implement thatinterface

Thus the basic principle of polymorphism fully

applies to component objects. .

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Clients only interact with pointers tointerfaces.

Component objects can implement multipleinterfaces.

Interfaces are strongly typed.

Every interface has its own interface identifier, aglobally unique ID (GUID)

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A typical picture of a component object

that supports three interfaces A, B, and C

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Interfaces extend toward the clients

connected to them.

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Each interface contains a list of methods that itexposes

In DLL these functions must physically resides in DLL

COM components must be logged in system registry

Client that use COM component must get a pointer to

an interface to access COM object

For COM object at least one interface IUnknown is

always implemented

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IUnknown

Base interface All other COM interfaces are derived from it

It contain three methods to

Allow you to get pointer to an objects interface

Manage the objects interface pointers

1. QueryInterface

2. AddRef3. Release.

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QueryInterface

Mechanism that allows clients to dynamicallydiscover (at run time)

whether or not an interface is supported by acomponent object;

If supported a pointer is returned

Mechanism that a client uses to get an interfacepointer from a component object

It takes an interface ID and returns a pointer tothat interface

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AddRef:

Increments count of number of clients using

the interface

Release

Called in place of delete operator

Decrements the count of number of clientsusing the interface

When count is zero The object knows that it is no longer

And remove itself from memory

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COM identifiers

COM object is accessed through use ofUUID/GUID

A GUID is a 128-bit value

Classification of GUIDs

IIDs (interface identifier)

COM defines IID for each interface

IID is used to manipulate interface

CLSIDs (Class identifiers)

Each class of COM object has it

CLSID is used when creating objects of given class

Unique CLSID for a COM class is stored in the WindowsRegistry

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Role of registry

Window registry stores the CLSIDs of allCOM classes available on the system

Use function CLSIDFromProgID() to find entry in registry

Get the CLSIDs for an object class

This function reads the registry andTranslates a string in to class ID number

Regedit.exe