Presented By Justas Raslanas COM/COM+ Presented By Justas Raslanas

Table of contents What is COM? Why COM? Technical details COM+

What is COM? Component Object Model (COM) is a binary-interface standard for software components introduced by Microsoft in 1993. COM is the basis for several other Microsoft technologies and frameworks, including OLE, ActiveX, COM+ and DCOM.

Why COM? In the early days of programming, programmers kept their function libraries in separate files called object files, which contained the compiled version of the code. There were disadvantages, like waste of storage space, because each executable had a copy of the function library embedded in it. There was one more serious limitation -- client applications written in different programming languages as the function libraries could not be used together. Components built in any language according to COM specification can be used with a client in any other language.

Technical details Different component types are identified by class IDs (CLSIDs), which are Globally Unique Identifiers (GUIDs).  Each COM component exposes its functionality through one or more interfaces. COM interfaces have bindings in several languages, such as C, C++, Visual Basic, Delphi, Python. All access to components is done through the methods of the interfaces.

Interfaces All COM components implement the IUnknown (custom) interface, which exposes methods for reference counting and type conversion (casting).

Classes A COM class ("coclass") is a concrete implementation of one or more interfaces, and closely resembles classes in object-oriented programming languages.

Programming COM is a language agnostic binary standard that can be developed in any programming language capable of understanding and implementing its binary defined data types and interfaces.

What is COM+? COM+ is a better version of COM. COM+ combines enhancements to traditional COM-based development with many useful programming and administrative services. COM+ improvements are significant, including the following: COM+ implements a threading model called neutral apartment threading. COM+ supports components with a special environment called a context. COM+ provides role-based security, asynchronous object execution, and a built-in moniker that represents a reference to an object instance running on an out-of-process server.

Reference https://docs.microsoft.com/en-us/windows/desktop/api/combaseapi/nf-combaseapi- cocreateinstance https://docs.microsoft.com/en-us/windows/desktop/cossdk/com--programming-overview https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFPlu gIns/Concepts/com.html#//apple_ref/doc/uid/20001158-CJBEJBHH https://www.techopedia.com/definition/1294/component-object-model-com https://www.cs.umd.edu/~pugh/com/

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