Plugins are code components that a host application loads and uses to enhance its default functionality. Plugins implement a plugin interface and object factory and are built as shared libraries. The application host loads the plugin, activates it using the object factory, and uses its services.

Host/Plugin Requirements:

In order to implement a host and plugin:

1. The host declares a plugin interface defining functions it knows how to call and an object factory to create instances of the plugin class(es).
2. Optionally, the host provides a registration interfact (implemented by the host) that allows multiple plugins to register with the host for activation and use.
3. If the host provides a registration interface, then the plugin interface must contain a function for the host to pass a pointer to its registration interface to the plugin to use for registration.
4. The plugin component implements the interface declared by the host.
5. The plugin is built as a shared library to that it can be loaded when the host starts or later when the host needs its services.

Interface and Object Factory Example Code:

• Widget Interface and Object Factory Widget Factory code
• Supporting multiple interfaces - class Diagram, vsd, Multiple interfaces code example

Plugin Sample Code:

• Basic Plugin Demo
• Dynamic Plugin Demo

Shared Library References:

• Shared library tutorial
• Building shared libraries
• Dynamically loading shared libraries
• More Dynamically loading
• Simple tutorial on the basics
• Setting shared library search path
• shared library search paths

Plugin Architecture References:

• Plugin Architecture - stackoverflow
  
• Plugin Architecture - Cygon's Blog
  
• Plugins - Mac Developer Library
  
• Plug-in (computing) - Wikipedia
  
• Building Your Own Plugin Framework: Part 1 - Dr.Dobb's
  
• Plugin System - alternative to GetProcAddress and interfaces - CodeProject
• Widget Interface and Object Factory - class Diagram, vsd, Widget Factory Code