This article is the first in a series of articles that discuss the architecture, development, and deployment of a C/C++ cross-platform plugin framework. This first installment explores the terrain, surveys (briefly) several existing plugin/component libraries, delves into the binary compatibility problem, and presents some desirable properties of the solution.
Subsequent articles showcase an industrial strength plugin framework that you can use on Windows, Linux, Mac OS X, and easily port to other operating systems. The plugin framework has some unique properties compared to other alternatives and it is designed to be flexible, efficient, easy for application developers, easy for plugin writers, supports both C and C++, and provides multiple deployment options (as dynamic or static libraries).
I will develop as a sample application a simple role-playing game that lets you add non-player characters (NPCs) plugins. The game engine loads the plugins and seamlessly integrates their contents. The game demonstrates the concepts and shows concrete running code.
Who Needs Plugins?
Plugins are the way to go if you want to develop a successful and dynamic system. Plugin-based extensibility is the current best practice to extend and evolve systems in a safe manner. Plugins let third-party developers add value to systems and let in-house developers add functionality without risk of destabilizing the core functionality. Plugins promote separation of concerns, guaranty implementation details hiding, isolated testing, and many other best practices.
Platforms like Eclipse are actually bare-bones frameworks where all the functionality is provided by plugins. The Eclipse IDE itself (including the UI and the Java Development Environment) is just a set of plugins hooked into the core framework.
C++ is notoriously non-accommodating when it comes to plugins. C++ is extremely platform-specific and compiler-specific. The C++ standard doesn't specify an Application Binary Interface (ABI), which means that C++ libraries from different compilers or even different versions of the same compiler are incompatible. Add to that the fact that C++ has no concept of dynamic loading and each platform provide its own solution (incompatible with others) and you get the picture. There are a few heavyweight solutions that try to address more than just plugins and rely on some additional runtime support. Still, C/C++ is often the only practical option when it comes to high-performance systems.
What's Out There?
Before embarking on a brand new framework, it's worth checking out existing libraries or frameworks. I found that there are either heavyweight solutions like Microsoft's COM and Mozilla's XPCOM (Cross-platform COM), or pretty basic offerings like Qt's plugins and a few articles about creating C++ plugins. One interesting library is DynObj that claims to solve the binary compatibility problem (with some restrictions). There is also a proposal for adding plugins to C++ as a native concept by Daveed Vandervoorde. It's an interesting read, but it feels strange.
None of the basic solutions address the myriad issues associated with creating an industrial strength plugin-based system like error handling, data types, versioning, separation between framework code, and application code. Before diving into the solution, let's understand the problem.