Component Object Model (COM)

Component Object Model (COM)

Microsoft has introduced this standard in 1993. Microsoft has developed this technology by which one object can talk to another object present at different location using its methods. This technology is known as COM.

The COM is software architecture that allows application to be build from binary standard. The operating system creates the object when they are required and Delete the object when they are not referred. It is responsibility of an OS to support the communication between two object located at different place across the network.

Features of COM:

                                i.            This standard is language independent.

                              ii.            It controls another exe on the same computer.

                            iii.            It provides the support for the new protocol such as OLE DB.

                           iv.            It allows dynamic loading and unloading of components.

                             v.            It is capable to communicate with operating systems.

                           vi.            It allows shared memory management between the components.

                         vii.            It can be provided on multiple platforms such as Windows XP, Windows NT, Windows 7, Windows 8, UNIX and MACINTOSH.

Basic Terminologies:

1.     Components (Control):

Components can be referred as objects. This is small code that provides specific service to rest of the system.

2.     VTable (Virtual function table):

COM defines the virtual function table in the memory. COM use VTable to call the function.

3.     Interface:

The interface is the collection of function through which application can interact with each other. The name of the interface begins with prefix “I”. All interfaces in COM are defined from “IUnknown” Interface. This interface has a pure virtual member function called “Query Interface”.

For Ex:

Struct IUnknown

{

Virtual Bool QueryInterface(int nIid,void ppvobj)=0;

}

Containment and Aggregation Vs Inheritance:

Containment:

In containment, the component is used as a part of implementation of new component. That means in containment, the internal copy of contained object is created so that the internal object is never exposed to the client.

Aggregation:

When an outer object aggregates an interface of an inner object, it does not re-implement the interface. It passes the inner object interface pointer directly to client.

Consider again that the interface IMul need to be accessed by the client then instead of containing object to call the contained object. The interface of contained object is directly executed. The inner IMul Interface can directly accessed by the client.