Implementing A Plugin System in C or C++ [closed]

Answer 1

I have had some success using a fairly naive system:

• Create API Specification for plug-ins
• Use a singleton plug-in manager
• Use LoadLibrary/GetProcAddress based run time dynamic linking
• Implement Inversion of control based event handling for notifying the plug-ins

Answer 2

The best platform and language neutral advice I can give is this:

Design your entire app around the plugin SDK.

IMO, a plugin SDK should not be an afterthought. If you design your app to basically be an empty shell which loads plugins, then the core features are implemented in your own SDK, you get the following benefits:

• High modularity of components, and clear separation of purpose (it kind of forces your architecture to be good)
• It forces your SDK to be really good
• It allows other third party developers to make extremely powerful, core-level features as well
• New developers/hires can easily start work on a major new feature without having to touch the main app - they can do all their work in a plugin (which prevents them screwing up anything else)

In C/C++, you probably use dynamic link libraries and either function pointers (C) or interfaces (classes solely consisting of pure virtual methods, for C++). However even if you use Javascript, I'd still recommend the above architecture.

Answer 3

Look at Poco Class Loader, it can be interesting for you.

Answer 4

I have written a plugin library Pugg that loads C++ classes from dll files and here is the logic I used:

User exports a c function from dll that has a unique name. This name has to be unique enough as functions cannot be distinguished using their arguments while loading from dlls.

C function registers one or several factory classes called "Driver". Every Driver class is associated with a string. When the main application wants to create a class, it gathers the related factory class using the associated string. I also implemented a version checking system to not load old plugins.

Dll loading is accomplished using the LoadLibraryA and GetProcAddress functions (Pugg currently works on windows).

One thing worth mentioning is that main application and dlls should be compiled using the same compiler and using the same compilation options (release/debug modes, optimization settings, stl versions etc...). Otherwise there might be issues with mapping of classes.

Answer 5

Qt provides QPluginLoader: http://qt-project.org/doc/qt-4.8/qpluginloader.html

If you need/want more fine grained control, Qt also provides a means to load libraries on the fly with QLibrary: http://qt-project.org/doc/qt-4.8/qlibrary.html

Even better, these are portable across platforms.