Preparing a library for plugin support

I searched for this particular question but could not开发者_高级运维 come up with any results, neither here nor on-line in general (maybe because it is a little harder to phrase for me). If it has already been asked, please point me in the right direction.

I am at a point where I would like my libraries/software to be pluggable. I see all these various libraries and systems where plugins are used extensively and the authors boastfully point out (in a good way!) that their software has plugin support.

So my question is, where do I start? Are there any books/on-line-resources that break the ice and may guide one on the do's and dont's of making your library pluggable, define best practices etc.?

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You have to understand some things before starting :

1. There is no support for modules (static or dynamic) in standard C++. Nope. Not yet. Maybe in 2015.
2. Dlls (or .so on unix-like systems) are dynamically loaded libraries that are compiler/os dependant. So it's a pragmatic solution that fill the need.

So, you'll have to use shared libraries (whatever the file extension, it's the keyword for searches about this subject) as plugin binaries. If your plugin should contain more than runtime code, like graphic resources, you can include your graphic resources in the binary, or have a file format or compressed archibe that contain the binary file.

Whatever the way you setup your plugin files, in C++ the problem is about the interface. Depending on wich compiler you use, you'll have different ways to "tag" functions or classes as exported/imported (meaning your plugin source code export the code and the user of the plugin should import the code).

Setup clean and clear interface in C++ for the modules, with no templates (because they are compiler and compiler configuration dependant). Those interfaces should be function declarations and class declarations with no inline code and marked exported/imported.

Now, once you've got this, you can use OS-specific API to load/unload dynamic library binaries while the application is running. Once it's done, you can get pointers to functions, again using the OS-specific API. I let you search for it.

Now, there are libraries that provide ways to abstract this in a cross-platform way. I didn't use them yet and they are known to be unperfect because of lack of definitions in the C++ standard, but they could be useful if you're planning to have your application cross-platform:

• boost::extension : it's not yet a boost library, nor even proposed yet, and it's developpement is in pause (until some new standard C++ implementations are done) so it might be a bad idea but a lot of people say they use it with success.
• POCO libraries have a library for shared libraries that would be the equivalent of boost::extension. Again lot of people say it's useful so I guess it's good enough to be used.

The other alternative, that is easy to setup if you don't need to support tons of target platforms, is to just write some wrapper code around OS-Specific APIs. That's what I did before knowing about boost::extension for example.