Plug-in systems in C++ are hard because the ABI is not properly defined, and each compiler (or version thereof) follows its own rules. However, COM on Windows shows that it’s possible to create a minimal plug-in system that allows programmers with different compilers to create plug-ins for a host application using a simple interface.
Let’s be practical, and leave the C++ standard, which is not very helpful in this respect, aside for a minute. If I want to write an app for Windows and Mac (and optionally Linux) that supports C++ plug-ins, and if I want to give plug-in authors a reasonably large choice of compilers (say less than 2 year old versions of Visual C++, GCC or Intel’s C++ compiler), what features of C++ could I count on?
Of course, I assume that plug-ins would be written for a specific platform.
Off the top of my head, here are some C++ features I can think of, with what I think is the answer:
- vtable layout, to use objects through abstract classes? (yes)
- built-in types, pointers? (yes)
- structs, unions? (yes)
- exceptions? (no)
- extern ‘C’ functions? (yes)
- stdcall non-extern ‘C’ functions with built-in parameter types? (yes)
- non-stdcall non-extern ‘C’ functions with user-defined parameter types? (no)
I would appreciate any experience you have in that area that you could share. If you know of any moderately successful app that has a C++ plug-in system, that’s cool too.
Carl
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.