What's safe for a C++ plug-in system?

Dr Dobb's Journal has an article Building Your Own Plugin Framework: Part 1 which is pretty good reading on the subject. It is the start of a series of articles which covers the architecture, development, and deployment of a C/C++ cross-platform plugin framework.

You might also want to consider replacing the conventional plugin interface by a scripting interface. There are some very good bindings for several scripting languages in C/C++ that have already solved your problem. It might not be a bad idea to build on top of them. For example, have a look at Boost.Python.

Qt has a very nice system for plugins that I've used in the past. It uses Qt's meta-object system to overcome many of the problems typically found when trying to develop C++ plugins.

One example is how Q_DECLARE_INTERFACE works, to prevent you from using an incompatible plugin. Another is the build key, to make sure you load the correct plugin for your architecture, OS, compiler. If you don't use Qt's plugin system, these are things you will have to worry about and invent solutions for on your own. It's not necessarily rocket science, and I'm not saying you'd fail at it, but the guys at Trolltech are pretty smart and have spent a while thinking about it, and I'd rather use what they created than reinvent the wheel myself.

Another example is that RTTI typically doesn't work across DLL boundaries, but when using Qt, things like qobject_cast which rely on the meta-object system do work across DLL boundaries.

I think you are safe creating a plugin system based on:

• Packaging of plugin functionality into library (.dll, .so, etc.)
• Requiring that the plugin expose key C-language exports.
• Requiring that the plugin implement (and return a pointer/reference to) an abstract C++ interface.

Probably the most successful C++ plugin system: good old Adobe Photoshop. And if not that, one of the virtual synth formats such as VSTi etc.