Interface in COM

8/8/2019 Interface in COM

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Presented by:

Satyajit Deshmukh(S.Y.I.F.)

Under Guidance:

Prof. Sandeep Sukdhev

Sir


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What is COM ?

Component Object Model (COM) is a binary

interface standard for software componentry

introduced by Microsoft in 1993.

It is used to enable interprocess communication

and dynamic object creation in a large range of

programming languages.

Casting between different interfaces of an object

is achieved through the QueryInterface() function.


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What is COM ?

For some applications, COM has been replaced at least to

some extent by the Microsoft .NET framework, and support

for Web Services through the Windows Communication

Foundation (WCF).

COM objects can still be used with all .NET languages

without problems.

COM was originally designed for suitable development anddeployment of desktop applications.

The preferred method of inheritance within COM is the

creation of sub-objects to which method calls are delegated.


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Technical details

COM programmers build their software using COM-aware

components.

Different component types are identified by class IDs,which

are Globally Unique Identifiers.

Each COM component exposes its functionality through one

or more interfaces.

COM interfaces have bindings in several languages, such

as C, C++, Visual Basic, and several of the scripting

languages. All access to components is done through the methods of

the interfaces. This allows techniques such as inter-

process, or even inter-computer programming


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What is Interface?

An interface is a named table of function pointers (methods)

An interface is not a class

An interface is not a COM component

COM clients only interact with pointers to interfaces

COM components can implement multiple interfaces


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What is Interface?

Interfaces are strongly typed

Interfaces are immutable

An interface consists of a pointer to a

virtual function table


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Interface Definition Language

IDL is a generic term for a language that lets a program or

object written in one language communicate with another

program written in an unknown language.

An IDL file is what COM provides that allows developers to

define object-oriented classes, interfaces, structures,

enumerations and other user-defined types in a language

independent manner.

(IDL) is a standard language for defining function and method

interfaces for distributed or component-based applications.


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IDL

It is same as c/c++ in addition of keywords like Interface &

library for defining interfaces and collections of classes,

respectively.

The IDL file is compiled by the MIDL compiler into a type

library (.TLB file).

In distributed object technology, it's important that new

objects be able to be sent to any platform environment anddiscover how to run in that environment.


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Difference between declaration

In IDL language

Ex. a function used to

calculate sales tax

float calculate_tax ( in floattaxable_amount );

In C/C++ language

Ex. a function used to

calculate sales tax

interface order {float calculate_tax ([in] float

taxable_amount);

float calculate_total([in]

item_list items);

bool place_order([in,out]item_list items);

}


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Why is it so ?

Function declarations are collected into interfaces, which

represent all the defined methods for a class.

Once youve defined an interface, it can be used as a

parameter type for function declarations and may be

extended by other interfaces using the C extension

operator (:).

In my example, I assume youve also created an item_list

interface, which you would use as the sole parameter of

the calculate_total and place_order functions.


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COM Interfaces

An interface is a named set of pure method prototypes.

The interface name typically reflects the functionality of its

methods and, by convention, begins with the capital letterI.

Thus, IMalloc might represent an interface that allocates,

frees,and manages memory.

Similarly, ISem might be an interface that encapsulates the

functionality of a semaphore.


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COM Interfaces

As a developer, you typically define your own custom

interfaces.

the COM interface becomes a contractual agreementbetween client and server, and enables the communication

protocol to function properly.

The contract between client and server is to provide a

service; but it is not a guarantee how that service isimplemented. The implementation details are hidden in the

CoClass.


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Pointers in Interface & COM

interface In the COM model, the communication protocol between

COM clients and servers in handled using pointers to the

COM interfaces.

The interface pointers are used to pass data between COM

clients and COM servers.

An interface pointer is a 32-bit pointer to an object instancethat points, in turn, to the implementation of each method in

the interface.


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Obtaining the first COM interface

pointer

All interaction with a remote COM server is done via its

interface pointers and the most common way to obtain thefirst interface pointer is using the function create-instance.

This takes the CLSID of the server and returns an interface

pointer for the i-unknown interface unless another interface

name is specified.


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How it works??


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All of you take tea I will go for ride

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Interface in COM