The Component Object Model
The Component Object Model (COM) is the base technology of OLE, a broad set of
object-oriented technology standards. OLE includes, besides COM, object design
standards at a higher level. Among these are standards for OLE Structured
Storage, OLE Compound Documents, and OLE Controls. COM is the binary standard that
defines the means for applications to interact within these technology
To understand COM (and therefore all OLE technologies), it is crucial to bear
in mind that it is not an object-oriented language, but a standard. Nor does
COM specify how an application should be structured. Language, structure, and
implementation details are left to the application programmer. COM does specify an
object model and programming requirements that enable COM objects (also called
OLE Components, or sometimes simply objects
) to interact with other objects. These objects can be within a single
process, in other processes, even on remote machines. They can have been written in
other languages, and may be structurally quite dissimilar. That is why COM is
referred to as a binary standard
it is a standard that applies after a program has been translated to binary
The only language requirement for COM is that code is generated in a language
that can create structures of pointers and, either explicitly or implicitly,
call functions through pointers. Object-oriented languages such as C++ and
Smalltalk provide programming mechanisms that simplify the implementation of COM
objects, but languages such as C, Pascal, Ada, and even BASIC programming
environments can create and use OLE objects.
COM defines the essential nature of an OLE Component. In general, a software
object is made up of a set of data and the functions that manipulate the data.
An OLE Component is one in which access to an object's data is achieved
exclusively through one or more sets of related functions. These function sets are
, and the functions of an interface are called methods.
Further, COM requires that the only way to gain access to the methods of an
interface is through a pointer to the interface.
Besides specifying the basic binary object standard, COM defines certain basic
interfaces that provide function common to all COM-based technologies. It also
provides a small number of API functions that all components require.
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