Seminarium on Component-based Software Engineering Feraaz Imami LIACS – Leiden University Fall 2005 Component Object Model (COM)

11/11/2005Seminar Component Based Software Engineering 2 Introduction: COM  Binary software components  Foundation for higher-level software services Example OLE,  Compound documents, custom controls, inter-application scripting, data transfer etc.  Com, developing and supporting program component objects.  Dynamic Link library  Interoperability and reusability

11/11/2005Seminar Component Based Software Engineering 3 COM  Com is a binary standard. defines an interface,  pointer to an interface node  Basis for interoperability. Point of contact is the runtime binary represent.,  C, C++, SmallTalk, Ada, and BASIC.

11/11/2005Seminar Component Based Software Engineering 4 COM – Multiple Interfaces  Com component is free to contain implementations for any number of interfaces. Multiple interfaces Avoid versioning problem; Logical immutable IDL ex. page 333

11/11/2005Seminar Component Based Software Engineering 5 QueryInterface  How does a client access the other interfaces on that same object?  IID – Interface Identifier  IUnknown QueryInterface AddRef Release.  AddRef, Release used in reference counting

11/11/2005Seminar Component Based Software Engineering 6 COM object reuse  Containment/Delegation: The outer object behaves like an object client to the inner object.  Aggregation: The outer object wishes to expose interfaces from the inner object as if they where implemented on the outer object itself.

11/11/2005Seminar Component Based Software Engineering 7 Versioning  A good versioning mechanism allows system component be upgraded without requiring all the system components to be upgraded.  QueryInterface allows an object to make more interfaces (that is, new groups of functions) available to new clients while at the same time retaining complete binary compatibility with existing client code.

11/11/2005Seminar Component Based Software Engineering 8 COM object creation  COM, Class Instance, GUID, CLSID.  The Com library provides a way for clients to start an object server.

11/11/2005Seminar Component Based Software Engineering 9 Monikers, storage and persistence  Structured storage and persistence: Com objects need a way to store their data when they are running.  Monikers: Clients often require a way to allow them to connect to the exact same object instance with the exact same state at a later point in time.

11/11/2005Seminar Component Based Software Engineering 10 COM services  Uniform data transfer: UDT is a important mechanism in any component-based software. Ex. OLE  Dispatch Interfaces: Includes the method invoke that can invoke other methods.  Dual interfaces: Access methods as either dispInterface methods or vtables methods.  Connectable Objects: This enables two-way communication between the client and the component.

11/11/2005Seminar Component Based Software Engineering 11 Compound Documents and OLE  Documents that contain a document from a different application within itself. Excel spreadsheet that is contained within a larger Word document  OLE, Object Linking and Embedding Specially designed for compound documents. OLE builds on COM  provide services such as object "linking" and "embedding" that are used in the creation of compound document

11/11/2005Seminar Component Based Software Engineering 12 Questions ?